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Optics, IDC202 Lecture 5. Rejish Nath Contents Literature: 1. Optics, (Eugene Hecht and A. R. Ganesan) 2. Optical Physics, (A. Lipson, S. G. Lipson and H. Lipson) 3. Optics, (A. Ghatak) Ray optics screen slit w o d a h ical s r t e m Geo Geom etrica l shad ow If the wave length of the light is much small compared to the slit width or the radius of a circular aperture, we have well-defined boundaries. Ray optics screen slit w o d a h ical s r t e m Geo Geom etrica l shad ow What happened if I reduce the slit width??? What happened if the wavelength of a light is taken to be zero??? (Geometrical or ray optics) Ray optics In a homogeneous medium light ray travels in straight line. (Homogeneous medium means refractive index independent of position) • In general the path of the ray is determined by the Fermat’s principle. (Similar to principle of least action in Mechanics) “ The actual path between two points taken by a beam of light is the one which takes the least time” B D C A y Light at the interface The angles are defined with respect to y-axis. Incident ✓ ✓ 0 Reflected n1 x n2 z ✓=✓ 0 Law of Reflection Transmi/ed sin ✓ n2 = sin n1 Snell’s law of refraction Law of Reflection A B h1 n1 n2 ✓ ✓ x 0 h2 l x C l Time taken by light to travel from A to B is p p x2 + h21 (l x)2 + h22 t= + c c we can consider different such paths with x being the variable. Law of Reflection Fermat’s principle tells you that for the real path dt =0 dx t= c p x x2 + h21 p c (l sin ✓1 = sin ✓2 p l x2 + h21 + c x x)2 + h22 p =0 (l x)2 + h22 c Snell’s Law A h1 h2 ✓ x l x C B l t= p x2 + h21 + c/n1 p (l x)2 + h22 c/n2 Snell’s Law Fermat’s principle tells you that for the real path dt =0 dx p n1 x x2 + h21 p n2 (l (l x) x)2 + h22 =0 If angles are small n1 sin ✓ = n2 sin ✓ n2 = n1 Optical Systems • Combination of mirrors, lenses and so on. • We need to trace the light through this optical elements! • Just need to use the Law of reflection and Snell’s law. (sounds simple!!) • The principle of reversibility (We can trace back) • The angle of deviation: 𝛼 ✓ ↵ Color dispersion white light ✓ Glass Red Blue yellow • For typical materials the refractive index is larger for blue. Color dispersion • Dispersive power nB nR V = nY 1 • Dispersive index ⌫ = 1/V white light ✓ Glass Red Blue yellow • For typical materials the refractive index is larger for blue. Examples for tracing light: Plane parallel plate n1 n2 ✓ ✓ ????? Examples for tracing light: Plane parallel plate n1 t n2 ✓ d ✓ ✓ d = t sin ✓ 1 n1 cos ✓ n2 sin ◆ Home work Examples for tracing light: Prism ↵ ✓ monochromatic light Examples for tracing light: Prism =✓+✓ ↵ 0 ↵ ✓0 0 ✓ incident 0 sin ✓ n2 sin ✓ = = sin n1 sin 0 transmitted Angle of deviation Examples for tracing light: Prism 1 sin n2 2 (↵ + = n1 sin 12 ↵ ↵ ✓ ✓0 0 incident m Angle of minimum deviation transmitted Angle of deviation m) Convex and Concave spherical surfaces A F C Principal axis centre of curvature f • Primary focal point (F) is an axial point having the property that any ray coming from it or proceeding toward it travels parallel to the axis after refraction. • A plane perpendicular to the axis and passing through a focal point is called a focal plane. f is called the primary focal length. • Convex and Concave spherical surfaces n1 A n2 ′ F C secondary focal point f 0 secondary focal length • ʹ Secondary focal point (F ) is an axial point having the property that any incident ray travelling parallel to the axis will after ʹ refraction proceed toward or appears to come from F . f 6= f 0 f0 n2 = f n1 Convex and Concave spherical surfaces C A F centre of curvature f Principal axis Convex and Concave spherical surfaces A F C f 0 Principal axis Image formation: Reflection at a plane surface 👁 Q 🔦 • Virtual image Q’ the light appears to be coming from Q’. Refraction of light • Bending of light when it travels from one transparent medium to another. • Speed of light changes entering a denser medium. Source: http://mserdelyiscience.weebly.com/uploads/1/3/8/6/13865601/phenomena_refraction.pdf Refraction of light: Apparent Depth Objects under the water looks nearer to the surface. Source: http://mserdelyiscience.weebly.com/uploads/1/3/8/6/13865601/phenomena_refraction.pdf Refraction of light: Mirage Refraction of light: Mirage Optical instruments: Human eye Human visual system Eyes +Path ways+ Part of the brain • Eyes behave like a camera. Capture lights and convert to neural messages. • Pathways transmit and modify light from eyes to the brain. • Brain does the complex image processing. Optical instruments: Human eye Human visual system Eyes +Path ways+ Part of the brain • The structure of the eye governs what we can see, and what we cannot see! (infrared light is not visible to us, we need to modify our vision) Optical instruments: Human eye Human visual system Eyes +Path ways+ Part of the brain • • The structure of eye varies from species to species. For some worms, eyes mean a patch of photo sensitive pigment on the surface of the skin, sense light which help them to distinguish between day and night. (may be the first version of the eyes) Optical instruments: Human eye View of a rabbit : View of a human Source highered.mheducation.com/sites/dl/free/0070579431/19282/02_Sekular_36882.pdf Optical instruments: Human eye Light Source:Wikipedia (Rhcastilhos) • • • • • • Cornea aqueous humor Iris Lens Vitreous humor Retina Optical instruments: Human eye Light • • • • • • Cornea aqueous humor Iris Lens Vitreous humor Retina • Cornea is transparent, and its transparency is very crucial for our vision, especially the quality of our vision. • Cornea is extremely sensitive to foreign objects, has protective measures like lid closing and tear production. Optical instruments: Human eye Light • • • • • • • Cornea aqueous humor Iris Lens Vitreous humor Retina Aqueous humor nourishes both the Cornea and lens. Optical instruments: Human eye Light • • • • • • Cornea aqueous humor Iris Lens Vitreous humor Retina • Iris has two layers, an outer layer with pigments and inner layer with blood cells. • Iris means Rainbow in Greek, provides the color to your eyes, depending how much it is pigmented. Optical instruments: Human eye Light • • • • • • • • • Cornea aqueous humor Iris Lens Vitreous humor Retina Lens, of all the body parts highest percentage of proteins. We need to keep it transparent for a clear vision. The opacity of the lens is called cataract, can be caused by the exposure of ultra violet rays. Optical instruments: Human eye Light • • • • • • • • • Cornea aqueous humor Iris Lens Vitreous humor Retina Vitreous humor covers 2-3 rd of the volume of the eye. This chamber is filled with transparent fluid called Vitreous. You will also see floaters. Optical instruments: Human eye Light • • • • • • Cornea aqueous humor Iris Lens Vitreous humor Retina • Retina, a complex layered organization. • Light must pass through a complex network of neural elements before reaching the photoreceptors which convert light energy into neural signals. Optical instruments: Human eye Light • • • • • • Cornea aqueous humor Iris Lens Vitreous humor Retina The neural signals generated by the photoreceptors pass through a network of cells (bipolar and amacrine cells) that collect and recombine the photoreceptor signals. These signals are passed on to the retinal ganglion cells, where biologically important information about the distribution of light over space and time is extracted and recoded. Optical instruments: Human eye Light • • • • • • Cornea aqueous humor Iris Lens Vitreous humor Retina The light interacts with the light-sensitive molecules called photopigments in the photoreceptors. It triggers a cascade of bio-chemical events, that generates photo signals with in the photoreceptors. It triggers the neural activity in neurons of retina. Eye defects Eye defects:corrections