SNC2D Light & Geometric Optics PDF

Subject: SNC2D Unit: Light & Geometric Optics Lesson: 2.11 Topics: Materials: General Applications of Optics Nelson: Science Perspectives 10 Homework: A) Intro: Lasers…what more can I...

Subject: SNC2D Unit: Light & Geometric Optics Lesson: 2.11 Topics: Materials: General Applications of Optics Nelson: Science Perspectives 10 Homework: A) Intro: Lasers…what more can I say?! B) Light Amplification by Stimulated Emission of Radiation (Laser): Construction Applications: Medical: Laser eye surgery, hair removal, biophotonic cancer treatments, tattoo removal, cutting Industrial: Cutting, bar code readers, measurement, levels Security: Communications, trips Military: Guiding missles, rifle sights, destroying targets, communication Communications: Fiber Optics Entertainment: Laser light shows Holograms: Creation of Notes: Homework:  Subject: SNC2D Unit: Light & Geometric Optics Lesson: 2.10 Topics: Materials: The Laser Nelson: Science Perspectives 10 Homework: A) Intro: Lasers…what more can I say?! B) Light Amplification by Stimulated Emission of Radiation (Laser): Construction o A lasing material is typically in the form of a rod, with something around it / attached to it, to excite the material. There are mirrors on either end of the rod, one 100% reflective and the other mostly reflective 98-99% Photon Interaction o Photons interact with matter in that as they strike it, they are absorbed, reflected or transmitted. o Photon’s that are absorbed supply energy to the material. o If the energy supplied is at the right level, it can cause an electron to jump from its ground state to higher energy level (excited state). It’s called resonance absorption. Emission o When the electron drops back to the ground state it emits a photon of a specific energy (equal to the energy level difference. o The emission of a photon happens in one of two ways: o Spontaneous Emission ▪ Electrons don’t like to be excited. As soon as they reach an excited state they want to jump back down. ▪ The photon released will be released with a random phase and is completely unrelated to the one that caused the electron to jump to the excited state. ▪ This emission is similar to the emission of photons from a light bulb. Each is of a different phase, different direction (typically 30cm long, lasting 10-8 sec) ▪ This form of emission leads to a rapid dissipation of energy. Not what we need for a laser o Stimulated Emission ▪ If an electron is in an excited state and then is hit by a photon, matching the energy step needed for it to jump down, it will “stimulate” the release of a photon, that exactly matches the striking one (2 identical (phase, direction)) leave. ▪ The emission is said to be coherent and correlated. ▪ Population Inversion o In order to achieve stimulated emission, a population inversion needs to be created o More electrons in the excited state compared to the ground state o This higher state must be metastable (lasting longer than normal before decay) to prevent spontaneous emissions. Lasing/Pumping o The act of energizing the material to produce this metastable state is called lasing or pumping (see below, on left ruby (chromium), on right HeNe) (Metastable) 20.66 eV – E N2 2.25 eV – E2 20.61 eV – EH1 (Metastable) State Stimulated Spontaneous State 18.70 eV – EN1 1.79 eV – E1 (Metastable) Spontaneous State Collision Stimulated E0 EH0 EN0 Types Ruby Rod, HeNe, CO2, Nd:Yg, compare with LED Notes: Homework: Recall: Phase – How much of the wave has occurred at a point, relative to the reference point. Coherent – Constant relative phase Correlated – All points match Monochromatic – Single colour/wavelength/frequency Applications: Medical: Laser eye surgery, hair removal, biophotonic cancer treatments, tattoo removal, cutting Industrial: Cutting, bar code readers, measurement, levels Security: Communications, trips Military: Guiding missles, rifle sights, destroying targets, communication Communications: Fiber Optics Entertainment: Laser light shows Holograms: Creation of  Subject: SNC2D Unit: Light & Geometric Optics Lesson: 2.09 Topics: Materials: The Eye Nelson: Science Perspectives 10 Homework: A) Intro: We’ve looked at lenses, but now let’s look at our eyes and the lenses/mechanics involved in human sight. B) Basics: Light enters through the cornea (protective/transparent part of the eyeball or sclera) and passes through the pupil (its size is controlled by the iris) and is focused by the lens that can have its shape (ie focal length) altered by the ciliary muscles (accommodation) onto the retina. The eye is filled with a watery substance between the pupil and cornea, and a jelly like substance inside (aqueous humour and vitreous humour). The retina has 2 basic cell types. Rod cells (majority at 120 million) which are low light sensors but are black and white, and Cone cells (7 million) which are able to detect colour. There are red, green and blue cone cells, each detects “its” colour by means of pigment that absorbs the light (breaks up and then later recombines) and emits electrical energy to the optic nerve. The macula is a very sensitive part of the retina that is responsible for detail imaging (looking straight ahead). The one point that cannot detect light is our blind spot where the optical nerve attaches Stereoscopic vision is required for good 3D vision. Each eye sees the same image slightly differently and can hence detect depth. loose colour (poor quality film) In order to see, we must have an image reduced in size, focused on the retina, curved to match the retina’s surface. When all three conditions are not met we have a vision problem. C) Vision Problems and Treatment (have students read and fill in) Term (medical) Layman's Term Correction Problem Image in front of retina, or lens- Myopia Nearsighted Divergent lenses (slightly) retina distance too great Myopia Image behind retina, or lens retina Hyperopia Farsighted Convergent lenses (slightly) distance too small Farsighted / Loss in Presbyopia Loss of elasticity in lenses Convergent lenses, bifocals Accommodation Non perfect spherical lenses or Astigmatism Lenses with different radii cornea (different focal planes) Hyperopia Damage to optic nerve often from Glaucoma intraocular pressure Cataracts Opaque/cloudy area on lens Removal of lens D) Lens Limitations: All lenses have problems: Spherical Aberration – Parallel rays not focused to a single point (spherical lens) Chromatic Aberration – Result of Dispersion Coma – Off-axis images at focal plane (blurry tails, i.e. comets) Distortion – Square curve in/out Cost – Manually intensive Notes: Homework: Homework Sheet – P09 Extension Simple Magnifier: The closest point the eye can clearly focus on is called the near point (typically about 25cm) and the farthest point that the eye can see is called the far point and is typically set to infinity. Glasses use converging or diverging lenses to accommodate our personal near and far points o The near point is the closest point rays can diverge from and be focused by the eye o The far point is where parallel rays are focused to form an image (infinite distance) Far Sighted (Hyperopia) Person cannot see things up close (they see “far away” things just fine. Often the eyeball is too short but regardless, the eye has difficulty focusing light onto the retina (it wants to focus behind the retina). As objects come closer, more power/accommodation is required from the eye to focus on the retina (rays are becoming more divergent). Convergent lenses are needed to help the eye focus the light in a shorter space. So their near point is not at 25cm but some larger distance away. If their near point is 100cm find the focal length and power of the lens required to let them see like a normal person. do is the object distance or near point, and di is the image distance or far point. 1 1 1 = + f do di The eye’s near point is 100cm so we need the lens to create an image of a near object at a 1 = 1 + 1 point 100cm ahead of the lens in order for them to be able to see it clearly. For most f 25cm − 100cm 1 4 1 people the near distance is about 25cm, so if we put the object at 25cm and its image is at = − the eye’s near point of 100cm (negative since it’s a virtual image) on the near side of the f 100 100 1 1 lens, so they can see it.   P = = 3D 33 f Presbyopia is a similar condition (effect) brought about by a different mechanism: loss of accommodation as the lens ages and becomes stiffer. It is treated in the same manner. Near Sighted (Myopia) Person cannot see things at a distance but can see things up close (often closer than the normal near point) Here the eye’s optical power is too great and focuses the image ahead of the retina. Their far point is not at infinity. If their far point is at 20cm find the focal length and power of the lens required to let them see like a normal person. 1 1 1 = + f do di 1 1 1 Objects at an infinite distance from the eye must have an image at the eye’s far point of = + 20cm. The image is on the near side of the lens, so its negative. For most of this the f − 20  1 −1 distance is about 25cm. so we put the object at 25cm and the far point as 100cm (negative) = +0 since it’s a virtual image on the near side of the lens. f 20 f = −20cm f = − 0.2 m 1 P= = −5 D − 0.2  Adjustable eye glasses developed by Professor Joshua D Silver a UK physicist for the third world allows glasses to be adjusted to need (presbyopia and hyperopia) using liquid (oil) flexing the lens. Red-eye: The appearance of eyes being red in photographs. Excess light entering the light (from the flash illuminates the retina (which is flesh or red). Cameras can use a series of flashes (red-eye reduction) to shrink the pupil to reduce this effect. Pictures showing white were used to diagnose eye cancer, but digital photographs can show this effect when no cancer is present (unsure why). Retinoblastoma is a childhood eye cancer typically affecting infants and children up to 5 years of age. It accounts for approximately 3% of pediatric cancers and affects 1 in 15,000 live births.  Subject: SNC2D Unit: Light & Geometric Optics Lesson: 2.08 Topics: Materials: Optical Instruments (1 to 2 classes) Nelson: Science Perspectives 10 Homework: A) The Basics: Multiple Lens / Microscope or Telescope: The objective magnifies the object and puts a real image ahead of the focal point of the eyepiece, which in turn generates a virtual image (magnified again). Note, rays diverge from a point (appear to ☺) and the object must be in a narrow range of positions from the objective lens (working distance) B) The Telescope: Object at infinity, so rays are parallel generates an image on focal plane of objective, just inside the focal length of the eyepiece so that the magnified image appears much larger. fe θ h θ' fo Telescope oo The Galilean telescope uses a double concave eyepiece for a similar effect The objective lens has a long focal length, and the eyepiece has a shorter focal length. The image is placed just inside the eyepiece’s focal length. A third converging lens can be used as in terrestrial (field) telescopes to provide an upright image Began in the 1600’s (Same time as microscopes) Basic types are refracting and reflecting Refracting Telescope o Uses lenses in a manner similar to microscopes (different lens configuration, focusing at infinity) Reflecting Telescope o Uses mirrors to reflect and form an image. C) The Microscope: Began in the 1600’s Basic one involves 2 lenses to make the “invisible – visible” Short focal length objective and longer focal length eyepiece/ocular Image is placed inside the focal length of the eyepiece to create a larger virtual image The objective lens creates an image and the ocular lens magnifies that real image into a larger virtual image. Many lenses can be combined (compounded) to achieve better magnification Light is good up to 1500x (0.2um resolution) but to look at smaller objects need an electron microscope. See next page Notes: Homework: Homework Sheet – P08  There are many types of microscopes and telescopes that use EM radiation other than visible light Regular optical microscope Electron Microscope Radio Telescope LIGO - Laser Interferometer Gravitational-Wave Observatory Technically, LIGO is not a telescope (or microscope) but it’s cool ☺ D) Magnification Calculations: Let’s recall the formation of images in general And the thin lens equation: 1 1 1 = + f do di Where f is the focal length of the lens, do is object distance (from the optical axis) and di is the image distance (from the optical axis) and measured negative if it’s on the same side as the object (see virtual image) Linear magnification is simply the ratio of the image size to the object size where: hi d M= =− i ho do With multiple lenses, there are other formulas. The first is simple and can be used when the magnification of the objective lens (closest to object) and the ocular or eyepiece (the one you look through) are known individually: hi h M Total = M O  M E =  eg. M T = (20 )(10 ) = 200  h ho Note in the middle the h’s are the intermediate image (the real image from the objective and object for the virtual image formed with the ocular lens) Using these relationships we can derive the following expressions: f For a single lens: M= f − do How much a lens changes the direction of the light (focuses) is called its optical power, dioptric power, refractive power, focusing power, or convergence power). It is measured in D, diopters [m-1] as: 1 P= f Angular Magnification In the above we are changing the angular magnification of the image rather than the actual image size. It is based on changing the angle subtended by the eye and the image: By using a converging lens we can bring the object much closer than normally possible (ahead of the near point) and still focus on the retina, with an apparently larger image. The f of the converging lens in smaller than the near point (xnp). By this we see that angular magnification is given by  M anglular = 0 We can derive an expression for this as follows: Using the diagram to the right we can use the basic trigonometric, tangent ratio to establish some relationships and the fact that for small angles tanθ=θ (approximately): y0 y y y tan  0 =  0 = 0 & tan  =  = L L L L Subbing into the angular magnification formula and solving yields the same value as linear magnification (in this case):  y d M= = and so M = i  0 y0 do And so for a telescope h h tan       &   fo fe  h fo M= = =− fe  h fo fe The length of the telescope is approximately Fo+FE  Subject: SNC2D Unit: Light & Geometric Optics Lesson: 2.07 Topics: Materials: Applications of Refraction (2 classes) Nelson: Science Perspectives 10 Homework: A) Intro: Recall that refraction happens as light changes mediums (at an angle) and hence changes speeds. There will be some light that is transmitted at a medium boundary (and refracted) and some that will be reflected back. B) Application of Refraction: Spear Fishing o Fish/water look shallower than actually are. Fish is also closer than it appears. o o Apparent Depth. Water is always deeper than it looks, sometimes depending on clarity etc, it can be much deeper. Derivation is an extension More generally: n DA = DR observer nobject Aquarium – Corners, how sticks look half submerged (demo if possible) Scuba mask – Water and Air refraction and need for different “prescription” Cameras on the moon – air versus vacuum Lens & Lensing – Any curved surface with a medium change Identification of materials – Diamond versus Cubic Zirconia, etc Continued next page Notes: Homework: Homework Sheet – P07 C) Total Internal Reflection: When light goes from a slower medium to a faster medium, the refracted light bends away from the normal. As the angle of incidence increases the light bends more and more until it bends 90 degrees and cannot escape (total internal reflection, angle of incidence is at the critical angle). While this is happening some light is always being reflected. The reflected light’s strength/intensity increases (the refracted light decreases) as the angles increase until the critical angle at which total internal reflection occurs Mirrors o Typically 90% efficient at reflecting light (10% absorbed) o Silvered backed mirrors with glass on the front may produce multiple images (much less intense and not usually observed). For sensitive measurements use of silvered fronted (very fragile) o Prisms can reflect (total internal reflection) almost 100% of the light (used in quality binoculars etc) D) Mirages: When the air at the ground is hotter than the air above, it will refract the light causing a virtual image to be observed. As the temperature changes, so does the index of refraction (but it’s still air!) Something similar happens with water where the speed of a wave through water depends on the depth of that water. E) Dispersion and Rainbows: Dispersion is the apparent separation of light into its distinct colours (or more generally the spreading out of wavelengths of light from a combined state) Dispersion is the rainbow effect from light emerging from a prism. At first this was believed to be “contaminated light” from impurities in the glass. Newton and others were able to show that light is composed of those colours. Rainbows – refracted light passing through water droplets, with the red coming out the bottom, violet on top. Light must be “at your back” and at an angle of about 42 degrees between the incident rays and the reflected rays entering your eyes. This forms the primary rainbow (K=1) where K indicates the number of times it has gone through the water drop. Rainbow Extension Material Usually a secondary rainbow (K=2) can also be seen when the first is very bright. The secondary is about 43% as bright as the primary.  There are (but rarely seen) third and fourth rainbows. These form behind you around the sun and are normally too faint to see even if you look for them. (only 5 have been confirmed with photographs in history!) 5th and 6th rainbows can also form where the 5th forms just under the secondary (partially over lapping) and the 6 th below the primary (both very faint). In a lab they have produced rainbows of the order K=13 (single drop) and K=19 (stream of water). Supernumerary rainbows can sometimes also be seen with different colour patterns (pastels) just below the primary and secondary. These are formed by light leaving the drops at slightly different angles and the colours (pale green and pinks!) form from interference. In between the primary and secondary rainbow, the sky usually looks darker since the light has been refracted from the drops into the rainbows and so less light than normal is coming from that direction. It is known as Alexander’s dark band. A B A) The primary, secondary rainbows with several supernumerary arcs below the primary B) Several supernumerary arcs from a mist. A secondary rainbow can also be seen. F) Fibre Optics: Use of TIR within fiber (core and glass) to send information over long distances) As time permits Internet and fiber optics Multiplexing Thin film coatings vFibers – construction, multimode, single mode, uses and applications G) Retroreflectors: Retro-reflectors are optical devices capable of returning any incident light back in exactly the same direction from which it came. They are often used on signs, clothing, bike reflectors, on the moon (left by Apollo 11 astronauts). They are formed from the corner of cubes (then rounded off), of three perpendicular faces. H) Etc: Invisibility cloaks Copper rings invisible to microwaves (not fully understood) Flattened sun – coming through atmosphere at an angle Shimmering – temperature inversions (like a mirage) Aberrations – Spherical and Chromatic  Subject: SNC2D Unit: 3.0: Light & Geometric Optics Lesson: 2.06 Topics: Materials: Lenses and Refraction (2 classes) Homework: A) Refraction Recap: When light (a wave) changes mediums (what it’s travelling through), at an angle, it will bend or change direction. Pencil in water jar, fish tanks, etc. Light going through air then glass/plastic (a lens) then back into air, will have its direction changed. Lenses can be used, as mirrors were, to create images using refraction to change the path of the light. B) Lenses: Vocabulary Lens – Transparent device with at least one curved surface that changes the direction of light passing through it. Converging Lens – Lens that causes the light rays to come closer together and cross at a single point. Diverging Lens – Lens that causes parallel light rays to move apart so that they appear to emerge from a single point. Focal Point – Position where parallel incident rays meet, or appear to come from, after passing through a lens. Optical Centre (O)– The geometric centre of the lens. Optical Axis (OA) – Vertical line passing through the optical centre (perpendicular to the Principal Axis). Principal Axis (PA) – Horizontal line passing through the optical centre (perpendicular to the Optical Axis). Principal Focus (F) – The point on the PA where rays parallel to the PA refract (converge to). There are 2 F’s. F’ is on object side Focal Length (f) – The distance between the O and the F measured along the PA. Focal Plane – The plane perpendicular to the PA on which the Focal Points lie. Image (SALT) o Size/Magnification – The height of the object, ho, in relation to the height of the image, hi, given as hi M =. Note that heights are positive if above the PA and negative below the PA ho o Attitude – The orientation of the image: upright or inverted o Location – Distance between the object and the lens, do, and the distance between the image and the lens, di. By convention, do is positive when on the side of the incident rays, and di is positive when on the opposite side. We also reference the position as either the object side or the image side in relation to the principal focus (F) o Type ▪ Real – Light rays converge to a point ▪ Virtual – Light rays do not pass through, but appear to diverge from When light enters the lens it refracts (air to glass) and it also refracts when it leaves the lens (glass to air). Rather than having to work out two refractions and determine the normal on the surface of the lens, we can simply change direction once using the optical access and saying that all direction change happens here. (See aren’t we nice☺) See next page Notes: Homework: Homework Sheet – P06 C) Converging Lenses: Mr. Parallel - Rays parallel to the PA refract through the F Ms. Focus - Rays that pass through the secondary F on their way to the lens, refract parallel to the PA Mr. Optical Centre - Rays passing through the O pass straight through without refraction Draw these 3 rays to find images (2 needed, 3rd is check) Similar to Concave mirrors: There are 5 cases for images to form (note: 2F relates to C in mirrors): Case Object Location Image Location Type Orientation Magnification 1 Beyond 2F’ Between F and 2F Real Inverted Smaller 2 At 2F’ At 2F Real Inverted Same Size 3 Between 2F’ and F’ Beyond 2F Real Inverted Larger 4 At F’ No Image 5 Between F’ and Lens Behind Object Virtual Upright Larger D) Diverging Lenses: Rays parallel to the PA refract so that they diverge from the F (note: F and F’ switch sides for divergent lenses) Rays that “would” pass through the secondary F’ on their way to the lens, refract parallel to the PA Rays passing through the O pass straight through without refraction Draw these 3 rays to find images (2 needed, 3rd is check) Similar for Convex Mirrors: For all positions of the object the image is Virtual, Upright and Smaller, between the O and F E) Thin Lens Equation: 1 1 1 and recall from mirrors hi = d i , hi − d i + = M= = do di f ho do ho do Extension: Thin Lens Equation Derivation Consider a converging lens with an object BC and its image DE. If B A we consider similar triangles we can develop two equations using D ΔBCO & ΔEDO as well as ΔAOF & ΔEDF. We also note that C F’ O F the focal length, f, is the line segment OF. E For AOF and EDF For BCO and EDO Sub  in to  ED DF DE DO di d = so = so −1 = i AO OF BC CO f do hi d i − f hi d i di di = =  = +1 ho f ho d o f do hi d i 1 1 1 = −1  = + ho f f do di  Subject: SNC2DE Unit: 3.0: Light & Geometric Optics Lesson: 2.04 Topics: Materials: 11.9, 11.10 Image Formation in Curved Mirrors (1-2 Classes) 2.04s Handouts Go over plane mirror handout A) Intro: In the previous class we looked at images formed from plane mirrors. This class will deal with Curved Mirrors and the images they can form B) Image Types: Images are “defined” sometimes as the optical counterpart of objects, formed by lenses and mirrors. Our eyes can’t actually tell the difference between an image and an object There are two types of images: Virtual and Real. o Virtual Images: These are formed from light entering the eye and “appearing” to diverge from some point, but the light never actually went there. It is arises from our eyes/brain believing that light only travels in straight lines. o Real Images: These are formed when the light rays actually converge to a point and recreate a semblance of the object (2D or 3D) In the plane mirrors only virtual images were possible because they do not cause the light rays to converge or diverge. In curved mirrors both real and virtual images can be formed and in certain cases no image. C) Concave (Converging) Mirrors: Show concave mirror, and talk about uses In order to find if and where an image is formed we will need to trace at least 2 light rays to see if they converge or appear to diverge from any point. Rays and their rules: Convex mirrors are capable of generating real and virtual images. Under specific conditions they will also not produce any image. First let’s get some terms out of the way: Principle Axis (PA), Focal Length (f), Focus (F), Centre of Curvature (C), Vertex (V), and rays Mr. Parallel , Ms. Focus, Mr. Vertex, Mr. Centre  See next page Notes: Homework: Homework Sheet – P04  There are 5 cases for a converging mirror (handout) C) Convex (Diverging) Mirrors: Show convex mirror, and talk about uses Rays and their rules: Convex mirrors can only produce virtual images and so only have 1 case: Note: similarity between case 5 of the concave mirror and the only case for the convex mirror  Note: We are assuming parabolic shaped mirrors, but in practice spherical mirrors are often used (cost less) but this leads to a problem known as spherical aberration: Spherical mirrors are still used, due to their lower cost, and as long as the C>>the length used i.e. close to the PA, there is no noticeable spherical aberration present.  Subject: SNC2DE Unit: 3.0: Light & Geometric Optics Lesson: 2.03 Topics: Materials: 11.4, 11.5, 11.6 Image Formation and Plane Mirrors (2 Classes) 2.03s Plane Mirror Exercise A) Intro: So now we know what light is, where it comes from and the different types (including colours). Now we are going to look at images and how they are formed. B) Light as a Particle: Previously we looked at light as a wave, in particular an electromagnetic wave and one part of spectrum of EM waves that spanned radio to gamma rays. The nature of light has long been debated with two main camps: particle and wave. It’s beyond this course (more in grade 11 and 12 ☺) to discuss this here, suffice to say that it has both characteristics but this was not accepted until the 20th Century. We will now consider geometric optics in which light rays will behave as if they were particles: o Travelling in straight lines o Light ray is a line representing the direction and path light travels o Light consists of massless particles called photons Light rays travel from luminous objects and are incident (strike) objects (non-luminous typically). What happens depends on the material it strikes and geometry of the situation as we shall see. In general however, when a photon strikes an object (or changes mediums) it can do one of three things: o Absorbed – If the photon is absorbed and none make it through a substance we consider that substance, Opaque. This book is opaque, i.e. no light gets through. o Transmitted – If the photon passes through the material (or most of them☺), it’s either Translucent or Transparent : ▪ Translucent – Allows some light to pass through, but doesn’t allow you to clearly see objects behind it ▪ Transparent – Allows light to pass through easily (most of it makes it through) and lets you see clearly objects behind it o Reflected – If the photon rebounds or bounces off the object then it’s considered reflected. We will consider the first two more in future classes but for now let’s focus on the last case, reflection C) Reflection: Most of us have played pool, basketball, tennis etc. where a ball was bounced off a surface/wall. We instinctively know where to go to catch it. (Actually we learned about reflection as kids and use it every day, but now we need to reflect on that learning☺) As kids we in fact learned even more and we now “know” that light travels in straight lines. Extension: Can talk about standard model with gravity not affecting photons, and relativity where we see it actually does We’ll look at Reflection now – o i =  r Always. Additionally, these rays are coplanar Angles must be measured from the normal which is always perpendicular to the surface (at the point of contact). Light/waves will always reflect this way individually, but as a group their overall behaviour is usually classified in one of two ways: Specular Reflections from Diffuse Reflections Continued next page Notes: Homework: Homework Sheet – P03 (possibly in two lots)  Plane mirrors reflect light back to us using a silvered (shiny) surface usually protected with a layer of glass. Historically, the shiny surface was silver or aluminum (even bronze or tin). The glass/plastic layer (more modern addition) is to protect and enhance the image formed (nice and flat). There are other types of “mirrors” designed for EM waves other than visible light as well. There are some medical conditions where the refection of light causes difficulty – Dyslexia, Synesthesia D) Image Formation: To our eyes there is no difference between a real object and an image. Moreover, the eye (brain) considers light to always be (and have been) travelling in straight lines. Extension: The brain is well developed for projectile motion (parabolic path) but not for other curves In order to see this we first have to form the image and then see how our eyes receive the light. It’s important to note that if you can see something, it is because light has reflected off it (or been emitted by it) and that light has travelled to your eye. The fact that many people can see the same thing reinforces the idea that the light rays, from the same point, are emitted in many directions and there are very many of them. In order to form an image we draw an exact copy of the object (same size) on the other side of the mirror such that each point is equidistant from the mirror perpendicularly.. There are two methods of drawing the rays from the object to the eye o Diverging – typically when object is a single point o Converging – typically a shape (usually an arrow) Method: o Draw the image with ho = hi and do = di perpendicular to the surface o Now draw to rays either converging on th eye or diverging from the image (see above). These rays go directly/straight from the image to the eye. Where the ray is behind the mirror, they are typically dashed to show that the light never actually went there. o At the mirror interface/surface, the rays have to obey the rules for incidence and reflectance. No measuring is needed however if connecting lines from the object are drawn to this point (see figure) Plane mirrors create virtual images (we’ll talk about image types next class) We can classify images using SALT: o S – Size – Compared to the object is the image smaller, the same size or larger? o A – Attitude – Is the image erect (upright) or inverted (upside down) o L – Location – Where, relative to the mirror surface, is the image located o T – Type - Is the image real or virtual (more in later classes) Consider especially two plane mirrors at 90 degrees (perpendicular) to each other (see handout). Extension: 360 For two mirrors at an arbitrary angle to each other the number of images is: N images = −1   Subject: SNC2D Unit: 3.0: Light & Geometric Optics Lesson: 2.02 Topics: Materials: Sources of Light A) Intro: Now that we know a bit about light and what it is, we need to understand something of its sources. B) Creating Light: Objects that generate light (like the sun) are considered luminous and objects that merely reflect light (like the moon) are non-luminous. There are a number of ways of producing light but in reality all of them end up doing the same thing. Before we talk about that consider the following sources: o Incandescence – This is light produced by heating a substance, for example incandescent bulbs heat a tungsten wire. Heat results in IR radiation. As the radiation gains energy it moves to right along the EM spectrum until it enters the visible spectrum at the red end. o Electrical Discharge – Think of lightening or a spark. The electrical discharge (remember grade 9 and electrons, current etc.) when it passes through a gas produces light. o Chemiluminescence – When light is a by-product of a reaction. Typically this does not involve heat (cold light). Think of glow sticks. o Bioluminescence – Basically the same as chemiluminescence but when produced by biological or living organisms such as fireflies or some deep sea creatures etc. o Triboluminescence – This comes from the physical destruction of bonds in materials (typically crystalline) by crushing, breaking, rubbing, scratching, etc. Can be seen in eating candy (Certs) in the dark, unrolling duct tape, quartz crystals etc. Light can also be changed from the invisible to the visible. UV radiation can be converted in two ways to do this: o Fluorescence – Certain materials when excited by UV radiation will immediately re-radiate some energy (at a lower level i.e. in the visible spectrum). Highlighters work this way and some materials are “fluorescent” under black light. o Phosphorescence – These materials work similar to fluorescent materials but instead of immediately re-radiating the energy they absorb the energy and slow release it. These are the “glow in the dark” materials. o All of the above “sources” of light are just different means to affect the same process where an orbital electron in an atom is excited to a higher orbital and then drops back down to its normal orbital and releases a discrete amount of energy (photon) that is light. o Extension – E=hf o Each possible orbit change can produce a discrete amount of energy (frequency/colour) of light o All of the above “methods” cause electrons to become excited and move to a higher energy orbital and when the electron comes back down, light is emitted. See next page Notes: Homework: Homework Sheet - P02 C) Types of Light: Once light is produced, how it appears can be very different. Intensity is the measure of how “much” light there is (brightness). It can be measured in lums, power (Watts), dB, etc. But power isn’t always enough to really describe the light. o Normal light bulb – A normal light bulb will produce light that covers a wide range of wavelengths/frequencies (colours), the photons will emerge in random directions and with random phase. This is very diffuse light and is good for general lighting. o Laser light – lasers produce a very focused beam of light that covers a very narrow range of wavelengths/frequencies (monochromatic), coherent (in phase at all times), and travelling in the same direction. o In a 100W incandescent light bulb, only 5W of the power goes to producing light but if it were a laser, a 5W laser could easily set fire to things. o LED (Light-Emitting Diode), allows electric current to flow in only one direction (semiconductor) and emits light in the process. LED’s do require filaments and don’t produce much waste heat (they are very efficient). Total Cost Energy Cost Rated Life Capital Cost Cost / hr Power For 1 hr use for 1 hr Type Name [kW] For 1 hr [$ / [hr] [$] [$] [W] [kWh] running kWh] [$/hr] PHILIPS 13W BR30 Interior Flood, LED Dimmable, Soft 25000 40.00 0.0016 13 0.013 0.013 0.00176618 0.00336618 white (65W) - LED Bulb Philips 23W BR40 CFL Energy Saver 8000 15.00 0.001875 23 0.023 0.023 0.00312478 0.00499978 Reflector Flood DuraMax 65 Watt BR30 Incandscent Incandescent 2500 3.60 0.00144 65 0.065 0.065 0.0088309 0.0102709 Reflector Flood Light 3 PK oC Color 400 Red heat, visible in the dark 474 Red heat, visible in the twilight 525 Red heat, visible in the daylight 581 Red heat, visible in the sunlight 700 Dark red 800 Dull cherry-red 900 Cherry-red 1000 Bright cherry-red 1100 Orange-red  Subject: SNC2D Unit: 3.0: Light & Geometric Optics Lesson: 2.01 Topics: Materials: The Nature of Light A) Intro: We are now going to start our study of light and the field of geometric optics. First we will have to discuss the concept of waves. Light acts both as a wave and a particle under different circumstances. But what which is it? The debate began with Pythagoras (582-500 BC) who thought light was a particle and Aristotle (384-322 BC) who thought it was a wave and continued to the 19th century with Young’s double slit experiment, James Maxwell, Newton. With the discovery of quantum mechanics in the early 20th century, we finally resolved the question, in that light is both☺. B) What is light?: Light is a form of energy visible to the human eye and it is a transverse wave. Waves are one means of transferring energy. Light (as we usually think about it) is actually only a small fraction of the electromagnetic (EM) spectrum which includes many things we wouldn’t call “light” since the “visible spectrum” is only the range of 400-700 nm (violet to red, ROYGBIV) / 4.3 x 1014 - 7.5 x 1014 Hz. The electromagnetic spectrum is simply the full range of radiation (energy being emitted as waves (light) or particles (photons) without the need for a medium). Photons are particles with zero mass. Electromagnetic waves (radiation), another term for light. Light waves are fluctuations of electric and magnetic fields in space. In fact, everything that has “heat” or energy (which is everything unless it’s at absolute zero (not even space is absolute zero, its 2.7K due to background microwave emissions)), radiates this energy or “glows”. Heat can also be transferred by convection and conduction but only radiation allows it to be sent across the vacuum of space because radiation needs no medium. For the heat emitted by something to be “visible” to us, it must be above approximately 540oC or higher (will look deep red). Energy increases as frequency increases (note wavelength gets smaller) What a wave “interacts” with depends on the relative size of the wavelength and object c is the speed of light, 299792458 m/s or 3108 ms (in a vacuum), and is a special speed Astronomers all types of light to view the universe: visible, radio, X-rays. Most “pictures” of space are composits of all types of light (colours assigned to the non-visible ones). Notes: o Newton is credited with discovering the visible spectrum colours ROYGBIV. o The existence of electromagnetic waves was proposed by James Maxwell who predicted they would involve rapidly alternating electricity and magnetism, would travel at “c”, and would require no medium to propagate. o The existence of EM waves was proven with William Roentgen discovering X-Rays and Heinrich Hertz radio waves. see next page Notes: http://imagine.gsfc.nasa.gov/docs/science/know_l1/emspectrum.html Homework: Homework Sheet P01 C) Colour Schemes: Earlier we mentioned that objects are either luminous or non-luminous. It depends on which they are, as to what colours we see. Color Wheel In color theory, we often talk about the color wheel. A color wheel is really just the spectrum twisted around so that the violet and red ends are joined. The color wheel is particularly useful for showing how the colors relate to each other and how you can create new colors by mixing two or more colors. Primary Colours Among the colors in the color wheel, there are three colors which are referred to as the primary colors. All other colors can be created by mixing these three colors. The primary colors are, as seen in the figure to the left, red, green and blue. Why red, Green and Blue? This might require a bit more explanation - There really is nothing intrinsically primary about red, green and blue, rather they are just points on the color wheel. In fact, in the print industry the primary colors are considered cyan, magenta and yellow. (commonly referred to as cmyk, where k stands for black). Additive Color System: If you look very closely at your computer screen or TV (any color source that emits the light itself) you will see that it is built up of tiny red, green and blue dots. This color system is commonly referred to as the Additive Color System. In the additative system, you get white when the three primary colours are present at 100%, as seen in the illustration to the left. > Subtractive Color System: As I said above, the primary colors in printing is considered cyan, magenta and yellow. This is also called the subtractive color system. The subtractive colour system is what comes to play when the color does not emit any light of its own, but reflects light from its surroundings. In the subtractive colour system, you get black when all colours are mixed. > Secondary Colors: The secondary colors are what you get when you mix any two adjacent primary colors. Red and green give yellow, red and blue give you magenta and a mix of green and blue result in a cyan color. The secondary colors are also the primary colors in the subtractive color system. Tertiary Colors: To complete the color wheel we need to add the tertiary colors. The tertiary colors are those which lie in between the primary and secondary colors. As you can see on the color wheel, they are a further blending of adjacent colors. The complete wheel Adding it all together, we get the complete color wheel. The color wheel is the foundation for much of color theory, and you would do well to remember what it looks like and where the colors are in relation to each other. http://www.colorsontheweb.com/colorinformation.asp Color Wavelength (nm) oC Color Red 650 - 800 400 Red heat, visible in the dark Orange 590 - 640 Yellow 550 - 580 474 Red heat, visible in the twilight Green 490 - 530 525 Red heat, visible in the daylight Blue 460 - 480 581 Red heat, visible in the sunlight Indigo 440 - 450 Violet 390 - 430 700 Dark red 800 Dull cherry-red 900 Cherry-red 1000 Bright cherry-red 1100 Orange-red