Lec 3 Physics, Fall 2022/2023 PDF

Physics The nature of light by Dr. A Ahhmed Yassin Fall 2022/2023 Topics X What is light ?...

Physics The nature of light by Dr. A Ahhmed Yassin Fall 2022/2023 Topics X What is light ? X The field of Optics. X The dual nature of light. X Reflection. X Refraction. Dr./ Ahmed Yassin..... Physics What is light ? X Light is a transverse, electromagnetic wave that can be seen by the typical human. X Like all electromagnetic waves, light can travel through a vacuum. Dr./ Ahmed Yassin..... Physics X The speed of light = 3 × 108 m/s. X From a biological standpoint, light is energy that activates the human eye and brain. It is the combination of the brain, eye and light that provides sight. X In physics, there are two theories by which ligh light ht ca can an bbee an defined: the first theory defines light ight as part particles ticle icless and ndd the second theory as waves. Dr.// Ah D Ahmed d Yassin Y i..... Physics Ph i ¾ Light can only be seen when looking either directly at a luminous object, such as a lamp, or when looking at illuminated objects which reflect light from a source. ¾ Light exhibits properties such as; reflection, refraction, diffraction and interference. Dr./ Ahmed Yassin..... Physics The field of Optics Dr./ Ahmed Yassin..... Physics is the science that study of light and its interaction with matter, where …….. X allows us to understand the nature of colors colors. X facilitates to design optical devices such as; microscopes, telescopes, lenses, cameras, ……... X make a development in some applications such as; laser, optical fibers,......  The dual nature of light Dr./ Ahmed Yassin..... Physics X Dual nature of light means light has two different nature, sometimes it behaves like a particle sometimes it behaves like a wave. X Particle theory (Corpuscular theory of light) X Isaac Newton stated in his Hypothesis of Light of 1675 that light was composed of corpuscles (tiny particles of matter) which were emitted in all straight directions from a source. X These tiny particles is called photons. The dual nature of light Dr./ Ahmed Yassin..... Physics X Wave theory X Christiaan Huygens (1629–1695) proposed that light was emitted in all directions as a series of waves in a medium called the Luminiferous ether. The dual nature of light Dr./ Ahmed Yassin..... Physics X Electromagnetic theory X James Clerk Maxwell (1862) discovered that self- propagating electromagnetic waves would travel through space at a constant speed, which is equal to the previously measured speed of light. X From this, Maxwell concluded that light was a form of electromagnetic radiation. X Quantum theory X In 1900 Max Planck, suggested that although light was a wave, these waves could gain or lose energy only in finite amounts related to their frequency. X Physicists have noted that electromagnetic radiation tends to behave more like a classical wave at lower frequencies, but more like a classical particle at higher frequencies, but never completely loses all qualities of one or the other. Dr./ Ahmed Yassin..... Physics Reflection The change in direction of a wavefront at an interface between two different media, where the reflected ray returns into the medium from which it originated. Dr./ Ahmed Yassin..... Physics Laws of Reflection ™ The incident ray, the reflected ray and the normal all lie in the same plane. ™ The angle of incidence = Angle of reflection. Dr./ Ahmed Yassin..... Physics  Types of Reflection of Light 1. Regular/ Specular Reflection ™ It refers to a clear and sharp reflection, like the ones you get in a mirror. ™ This reflective surface reflects almost all the light incident on it uniformly. Dr./ Ahmed Yassin..... Physics Types of Reflection of Light 2. Diffused Reflection ™ Reflective surface other than mirrors, in general, has a very rough finish. This may be due to wear and tear such as scratches and dents or dirt on the surface. ™ This type of reflection is what enables us to see non-shiny objects. Dr./ Ahmed Yassin..... Physics  3. Multiple Reflection ™ A single image is formed when an object is placed in front of a mirror. What happens if we use two mirrors? ™ Since reflective surfaces such as mirrors are very good at preserving the intensity of light in a reflection, a single source of light can be reflected multiple times. This multiple reflection is possible Dr./ Ahmed Yassin..... Physics until the intensity of light becomes low. 3. Multiple Reflection ™ The variation of the number of images of an object placed between two mirrors with the angle between the mirrors can be described by a simple formula: ° = − Dr./ A Ah Ahmed hme ed Yassin............. P Physics ™ As we go on decreasing the angle between the mirrors, the number of images goes on increasing. Some definitions of reflection ™ The angle of incidence: is the angle between incident ray on a surface and the line perpendicular to the surface at the point of incidence, called the normal. Dr./ Ahmed Yassin..... Physics 3 ™ The angle of reflection: is the angle between a reflected ray and the normal drawn at the point of incidence to a reflecting surface. Refraction 9 is the change in direction of a wave passing from one medium to another. Dr./ Ahmed Yassin..... Physics 9 The bending of a wave when it enters a medium where its speed is different. Refractive Index of a material Dr./ Ahmed Yassin..... Physics 9 The ratio between the speed of light in vacuum and the speed in the material. = 9 The wave speed is inversely proportional to the index of refraction. 9 The greater the index of refraction in a material, the slower the wave speed in that material. Snell’s Law Dr./ Ahmed Yassin..... Physics 9 Snell's law is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through a boundary between two different media, such as water, glass, or air. = = = =  Snell’s Law ¾ If n1 > n2 , the beam bends away from the normal. ¾ If n1 < n2 , the beam bends towards the normal. ¾ If the beam is incident perpendicularly, it goes unrefracted. n1 < n2 n1 > n2 Dr./ Ahmed Yassin..... Physics Example (1): Dr./ Ahmed Yassin..... Physics ¾ Calculate the incidence angle for the interface between water (n1 = 1. 33) and air (n2 = 1), where refractive angle = ? = = (1.33) = ° =. °.  Wave Optics ¾ The wave nature of light is needed to explain various phenomena such as; 9 Interference. 9 Diffraction. 9 Polarization. Dr./ Ahmed Yassin..... Physics Interference Dr./ Ahmed Yassin..... Physics ¾ Light waves interfere with each other much like mechanical waves do. ¾ Interference occurs when the electric and magnetic fields of two or more light waves interact. ¾ The electric and magnetic fields of the two waves add together to give you a new wave.  Conditions for Interference ¾ For sustained interference between two sources of light to be observed, there are Dr./ Ahmed Yassin..... Physics two conditions which must be met: 1) The sources must be coherent, i. e., they must maintain a constant phase with respect to each other. 2) The waves must have identical wavelengths Dr./ Ahmed Yassin..... Physics Coherent sources ¾ Two sources are said to be coherent if they emit light waves of the same frequency, amplitude and are always in phase with each other. Producing Coherent Sources Dr./ Ahmed Yassin..... Physics ¾ Light from a monochromatic source is allowed to pass through a narrow slit. ¾ The light from the single slit is allowed to fall on a screen containing two narrow slits. ¾ Recently, laser is commonly used as a coherent source. ¾ The laser produces an intense, coherent, monochromatic beam, which can be used to illuminate multiple slits directly. Constructive interference Dr./ Ahmed Yassin..... Physics ¾ When two light waves collide peak-to-peak and trough-to-trough. ¾ The resulting light wave is stronger than either of its two components. Destructive interference Dr./ Ahmed Yassin..... Physics ¾ The waves are meeting peak-to-trough and trough-to-peak. ¾ The two waves cancel each other out. ¾ They are opposites pp of each other,, and when they y meet,, the result is Zero. Young’s Double Slit Experiment X Thomas Young first demonstrated interference in light waves from two sources in 1801. X Light is incident on a screen with a narrow slit, So X The light waves emerging from this slit Dr./ Ahmed Yassin..... Physics 3 arrive at a second screen that contains two narrow, parallel slits, S1 and S2.  Dr./ Ahmed Yassin..... Physics 3 X The narrow slits, S1 and S2 act as coherent sources of waves. X The waves emerging from the slits originate from the same wave front and therefore are always in phase. Resulting Interference Pattern Dr./ Ahmed Yassin..... Physics 3 X The light from the two slits form a visible pattern on a screen X The pattern consists of a series of bright and dark parallel bands called fringes. X Constructive interference occurs where a bright fringe appears. X Destructive interference results in a dark fringe.  Dr./ Ahmed Yassin..... Physics 3 X Repeating pattern of dark and bright fringes, known as interference pattern. S1 S S2 X Superposition of waves from S1 and S2 produces interference. - constructive interference (bright fringes). - destructive interference (dark fringes). X The distance between any two Dr./ Ahmed Yassin..... Physi Physics ysics 3 consecutive bright or dark fringes x is known as fringe width or fringe separation, given by: a Fringe separation x OD a = , D = + Where, n = 0, 1, 2, 3, ….. Changes in the interference pattern 1) Change in the space between S and double slit: What happen if we move slit S and light source toward the double slit? S1 S - bright fringes become brighter (intensity of light through double slit is S2 greater) - fringe separation (x ) unchanged. Dr./ Ahmed Yassin..... Physics 3 Changes in the interference pattern x 2) Change in a : Fringe separation x OD a a a increases, x decreases D Smaller a Larger a Dr./ Ahmed Yassin..... Physics 3 Changes in the interference pattern 3) Change in source of light Use white light (different wavelengths) Dr./ Ahmed Yassin..... Physics 3 Changes in the interference pattern 4) Change in source of light Use two separate sources of light: - No interference pattern will be created because sources are not coherent) Dr./ Ahmed Yassin..... Physics 3  Example (1): Dr./ Ahmed Yassin..... Physics 3 A laser beam (λ = 632.8 nm) is incident on two slits 0.200 mm apart. How far apart are the bright interference fringes on a screen 5.00 m away from the double slits? Solution Wavelength = 632.8 ǘ 10–9 m Slit separation, a = 0.2 mm = 2 ǘ 10–4 m Distance, D = 5 m. × × = = =. =.. × Example (2): Dr./ Ahmed Yassin..... Physics 3 A Young’s interference experiment is performed with monochromatic light. The separation between the slits is 0.500 mm, and the interference pattern on a screen 3.30 m away shows the first side maximum 3.40 mm from the center of the pattern. What is the wavelength? Solution Wavelength = ?? m, x = 3.4 ǘ 10–3 m Slit separation, a = 0.5 mm = 5 ǘ 10–4 m Distance, D = 3.3 m = → = = 515 nm  Example (3): Dr./ Ahmed Yassin..... Physics 3 In a Young’s double slit experiment, the slit separation is 0.05 cm and the distance between the double slit and screen is 200 cm. When blue light is used, the distance of the first bright fringe from the center of the interference pattern is 0.13 cm. (i) Calculate the wavelength of the blue light used in the experiment. (ii) Calculate the distance of the fourth dark fringe from the center of the interference pattern. Solution Dr./ Ahmed Yassin..... Physics 3 Slit separation, a = 0.05 cm = 5.0 ǘ 10–4 m Distance, D = 200 cm = 2.00 m Fringe separation, X = 0.13 cm = 1.3 ǘ 10–3 m = → = = 3.25 ǘ 10–7 m The wavelength of the blue light, O = 3.25 ǘ 10–7 m.  Solution The distance of the fourth dark fringe from the center of the interference pattern is = X4th Dr./ Ahmed Yassin..... Physics 3 =3 = 3.5 × 1.3 × 10 = 4.55 ǘ 10–3 m Example (4): Dr./ Ahmed Yassin..... Physics 3 In a Young’s double slit experiment, 16 bright fringes are observed in a certain segment of the screen when light of wavelength 700 nm is used. If the wavelength of light is changed to 400 nm, the number of fringes observed in the same segment of the screen would be: a) 28. b) 24. c) 30. d) 18.  Solution Dr./ Ahmed Yassin..... Physics 3 = , For the first wavelength, λ1 = 700 nm, n = 16. × × = = For the second wavelength, λ2 = 400 nm, n = ??. × × = = The same segment of the screen means the same distance i.e., x. × × × × = → × = →∴ = Uses for Young’s Double Slit Experiment ‰ Young’s Double Slit Experiment provides a method for measuring wavelength of the light. ‰ This experiment gave the wave model of light a great deal of credibility, i.e., proved the wave nature of light. Dr./ Ahmed Yassin..... Physics 3  Diffraction of light: The phenomenon of bending of light around the corners and the encroachment of light within the geometrical shadow of the opaque obstacles is called diffraction. X X S S Slit Y Y Obstacle Screen Diffraction at a slit Diffraction at an obstacle Screen X & Y – Region of diffraction In another words, ™ When the light falls on the obstacle whose size is comparable with the wavelength of light then the light bends around the obstacle and enters in the geometrical shadow. This bending of light is called diffraction. Dr./ Ahmed Yassin..... Physics 3  Diffraction....... S...... New.. New Wavefront (Spherical). Wave- front. (Plane) (Wavelets - Red dots on the wavefront) Huygen’s principle requires that the waves spread out after they pass through slits. Each point on a wavefront acts as a fresh source of disturbance of light. Diffraction, 2 XA single slit placed between a distant light source and a screen produces a diffraction pattern XIt will have a broad, intense central band XThe central band will be flanked by a series of narrower, less intense secondary bands XCalled secondary maxima XThe central band will also be flanked by a series of dark bands XCalled minima  Diffraction, 3 X The results of the single slit cannot be explained by geometric optics XGeometric optics would say that light rays traveling in straight lines should cast a sharp image of the slit on the screen. Diffraction, 4 Difference between Interference and Diffraction: Interference Diffraction 1. Interference is due to the 1. Diffraction is due to the superposition of two different superposition of secondary wave trains coming from coherent wavelets from the different parts sources. of the same wavefront. 2. Fringe width is generally 2. Fringes are of varying width. constant. 3. The maxima are of varying 3. All the maxima have the same intensities. intensity. 4. There is a poor contrast 4. There is a good contrast between between the maxima and the maxima and minima. minima. Dr./ Ahmed Yassin..... Physics