Optics - Light and Reflection PDF

Light and Optics Optics and Reflection Refraction Mirrors, Lenses, and Images  Light Light is one form of Electromagnetic radiation that travels in the form of transverse waves. Light can spread without a medium, so light can spread in a vacuum. Light rays...

Light and Optics Optics and Reflection Refraction Mirrors, Lenses, and Images  Light Light is one form of Electromagnetic radiation that travels in the form of transverse waves. Light can spread without a medium, so light can spread in a vacuum. Light rays travel in straight lines from the light source. When it hits an opaque object, some light is absorbed, and the rest reflects off. If the object is transparent, light rays pass through it easily. If the object is translucent, some light can pass through, but the rest of the light will be reflected. Optics Optics is the study of how light behaves. Explaining how magnification occurs is part of the science of optics. Diagrams of light use one or more imaginary lines called light rays to show how light travels. A ray diagram is an accurately-drawn sketch showing how light rays interact with mirrors, lenses, and other optical devices.  Reflection and Refraction A lens is an optical device that is used to bend light in a specific way. A converging lens bends light so that the light rays come together to a point. A diverging lens bends light so it spreads light apart instead of coming together. Mirrors reflect light and allow us to see ourselves. A prism is another optical device that can cause light to change directions. A prism is a solid piece of glass with flat polished surfaces.  Reflection occurs when light bounces off a surface. Images appear in mirrors because of how light is reflected by mirrors. The incident ray follows the light falling onto the mirror. The reflected ray follows the light bouncing off the mirror. Reflection In specular reflection each incident ray bounces off in a single direction. A surface that is not shiny creates diffuse reflection. In diffuse reflection, a single ray of light scatters into many directions.  Reflection in a Plane Mirror Normal Incident Reflected Angle Angle ray ray of of inciden reflecti ce on Plane mirror Laws of reflection: The angle of incidence is equal to the angle of reflection. The incident ray, the reflected ray and the normal all lie in the same plane (i.e. the two rays and the normal can all be drawn on a single sheet of flat paper).  Where is the image in a Plane Mirror? Dotted lines show the construction of the virtual image) The image in the mirror looks the same as the object, but it is The image formed is laterally inverted (back to front). upright, but it is a virtual image (doesn’t really exist).  Where is the image in a Plane Mirror? Normal view from the front. Same view as seen in the rear view mirror of a car.  Refraction Refraction is the Light passing through a glass block at right angles to the surface will not be bending of light when it refracted. The rays will pass straight travels from one through. medium to another. A material which light Air passes through, such as glass or air, is known as a medium Glass block why is light refracted? the “bending,” or change of direction of light rays when light moves from one medium to a different one—takes place because light travels at different speeds in different media.  Refraction – labelling If andiagrams incident ray enters glass at an angle, then it i1 is refracted, and bends normal incident towards the normal. ray air The angle of incidence (i) glass is greater than the angle i2 of refraction (r). r1 When the light leaves the glass, the opposite happens: it bends away from the normal. normal r2 A material which light passes through, refracted such as glass or air, is known as a medium. ray  What is the ‘refractive index’? The speed of light in a vacuum is c = 3x108 m/s. The index of refraction of a material is defined by The refractive index of a medium (glass, water) is defined as the speed of light in a vacuum divided by the speed of lightcin the medium n= , v where c is the speed of light in a vacuum and v is the speed of light in the material. The speed and wavelength of light change when it passes from one medium to another, but not the frequency, so c  v= and n =. n n  What direction does bend light? A light ray going from a low index of refraction into a higher index bends toward the normal line. A light ray going from a high index of refraction to a low index bends away from the normal line. Refractive re Medium index fr ac te in d ra b Vacuum 1.0000 ci de y air (nb) nt Air 1.0003 ra y a air (na) water (na) Water 1.3333 a inc water (nb) Glass 1.5200 ide b nt r ref Diamond 2.4170 ay rac na>nb Perspex 1.4900 ted nb>na ray Because light never travels faster than c, n  1.* For water, n = 1.33 and for glass, n 1.5. Indices of refraction for several materials are listed in your text. Example: calculate the speed of light in diamond (n = 2.42). c v = n 3×108 m/s v = 2.42 v = 1.24×108 m/s 1. Light passes from air to water at an angle of incidence of 32 degrees. The angle of refraction is 23.5 degrees. What is the refractive index of the water? 2. Light travels from air into diamond (n = 2.42) at an angle of incidence of 20 degrees. What is the angle of refraction? 3. Light passes from air to perspex and refracts with an angle of 18 degrees. If the refractive index of perspex is 1.49, what was the angle of incidence? 4. Light travels from vacuum to glass and has an angle of incidence of 25 degrees. The speed of light in the glass is 1.97 x 10^8 m/s. What is the angle of  Dispersi The splitting up of white light into its seven constituent colors on on passing through a transparent called dispersion of light. medium (like a glass prism) is This effect is called dispersion, It happens because white is a mixture of all the colours in the rainbow Rainbow  Total Internal Reflection TOTAL INTERNAL REFLECTION :-When light travels from a denser medium to a rarer medium, if the angle of incidence is greater than the critical angle the ray return to the denser medium. This is known as total internal reflection Condition for total internal Reflection The light should travel from denser medium to rarer medium 2. Angle of incidence in the denser medium should be greater than the critical angle(Ɵ1>c)  Total Internal Reflection Recall Snell’s Law: n1 sin(q1)= n2 sin(q2) (n1 > n2  q2 > q1 ) q1 = sin-1(n2/n1) then q2 = 90 “critical angle” q2 Light incident at a larger angle will only n2 have reflection (qi = qr) qr n1 qi For water/air: qc n1=1.33, n2=1 q1 = sin-1(n2/n1) q1 normal = 48.80  Lenses and Refraction Convex lens Concave lens Converging lens Diverging lens Principal focus Principal focus Focal length Focal length  Lenses and Refraction What happens to light Convex lens as it passes through the lens? As light passes through the first face of the lens it bends towards the normal (refraction) As light passes through the second face of the lens it bends away from the normal (refraction)  Lenses and Images Rays from a distant object brought to focus on a screen by a convex lens. Object Convex lens Image Light rays from a distant object are considered to The image on the screen be parallel to each other, so the image passes is real and inverted through the principal focus. (upside-down) Converging Lens Principal Rays P.A.(principle F Image axis) Object F 1) Rays parallel to principal axis pass through focal point. 2) Rays through center of lens are not refracted. 3) Rays through F emerge parallel to principal axis. Image is: real, inverted and enlarged (in this case). Assumptions: monochromatic light incident on a thin lens.  Converging Lens All rays parallel to principal axis pass through focal point F. Double Convex P.A. F F nlens > noutside A beacon in a lighthouse produces a parallel beam of light. The beacon consists of a bulb and a converging lens. Where should the bulb be placed? At F F P.A. Inside F Outside F F 3 Cases for Converging Lenses Past 2F Inverted This could be used in Object Image Reduced a camera. Big object Real on small film Between Inverted This could be used F & 2F Image Enlarged as a projector. Small Object Real slide on big screen Inside F Upright This is a magnifying Enlarged Image Object glass Virtual  Uses of Convex Lenses 1. As a magnifying The rays appear to be coming from a position glass behind the lens. The image is upright and magnified, and it is called a virtual image because no rays actually meet to form it and the image cannot be formed on a screen. F1 F The image is Object virtual, between F1 upright and and lens 2. In projector magnified.  Converging Lens Which way should you move object so image is real and diminished? F P.A. Object F (1) Closer to lens (2) Further from lens (3) Converging lens can’t create real diminished image. Diverging Lens Principal Rays F P.A. Object F Image 1) Rays parallel to principal axis pass through focal point. 2) Rays through center of lens are not refracted. 3) Rays toward F emerge parallel to principal axis. Only 1 case for diverging lens: Image is always virtual, upright, and reduced.