# Light - Reflection and Refraction, Class X, CBSE, Science

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• 1. By- Devesh Saini
• 2. Refraction: Refraction is the bending of a wave when it enters a medium where its speed is different. Refraction of light: When light travels obliquely from one transparent medium into another it gets bent. This bending of light is called refraction of light.
• 3. When light travels from a rarer medium to a denser medium, it bends towards the normal. Denser medium Rarer medium Normal
• 4. When light travels from a denser medium to a rarer medium to a rarer medium, it bends away from the normal. Rarer medium Denser medium Normal
• 5. When a ray of light passes through a rectangular glass slab, it gets bent twice at the air- glass interface and at the glass- air interface. The emergent ray is parallel to the incident ray and is displaced through a distance.
• 6. i e Normal Incident ray Emergent ray Refracted ray Glass Air Normal r Glass Air Rectangular glass slab displacement Angle of emergence Angle of incidence Angle of refraction
• 7. 1. What is the cause of refraction of light? Ans:- Refraction is due to change in the speed of light as it enters from one transparent medium to another. 2. What is a Refractive Error? Ans:- Refractive errors are optical imperfections that prevent the eye from properly focusing light, causing blurred vision. The primary refractive errors are near-sightedness, farsightedness and astigmatism.
• 8. LIGHT The incident ray, the refracted ray and the normal at the point of incidence all lie in the same plane.
• 9. For two given media, the ratio sin i ÷ sin r is a constant, where i is the angle of incidence and r is the angle of refraction LIGHT i r air water Incident ray Refracted ray normal Refractive Index, n = sin i sin r
• 10. If i is the angle of incidence and r is the angle of refraction, then, sin 𝑖 sin 𝑟 = constant This constant value is called the refractive index of the second medium with respect to the first.
• 11. The refractive index can be linked to an important physical quantity, the relative speed of propagation of light in different media. It turns out that light propagates with different speeds in different media. Light travels the fastest in vacuum with the highest speed of 3×108 m s–1. In air, the speed of light is only marginally less, compared to that in vacuum. It reduces considerably in glass or water. The value of the refractive index for a given pair of media depends upon the speed of light in the two media
• 12.  Consider a ray of light travelling from medium 1 into medium 2, as shown in Fig.10.11. Let v1 be the speed of light in medium 1 and v2 be the speed of light in medium 2. The refractive index of medium 2 with respect to medium 1 is given by the ratio of the speed of light in medium 1 and the speed of light in medium 2. This is usually represented by the symbol n21 . This can be expressed in an equation form as  n21 = 𝑆𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑚𝑒𝑑𝑖𝑢𝑚 1 𝑆𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑚𝑒𝑑𝑖𝑢𝑚 2 = 𝑣1 𝑣2
• 13. By the same argument, the refractive index of medium 1 with respect to medium 2 is represented as n12. It is given by  n12 𝑆𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑚𝑒𝑑𝑖𝑢𝑚 2 𝑆𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑚𝑒𝑑𝑖𝑢𝑚 1 = 𝑣2 𝑣1 If medium 1 is vacuum or air, then the refractive index of medium 2 is considered with respect to vacuum. This is called the absolute refractive index of the medium. It is simply represented as n2. If c is the speed of light in air and v is the speed of light in the medium, then, the refractive index of the medium nm is given by ……. to be continued
• 14.  nm = 𝑆𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑎𝑖𝑟 𝑆𝑝𝑒𝑒𝑑 𝑜𝑓 𝑙𝑖𝑔ℎ𝑡 𝑖𝑛 𝑚𝑒𝑑𝑖𝑢𝑚 = 𝑐 𝑣 The absolute refractive index of a medium is simply called its refractive index. Example: The refractive index of water, nw = 1.33. This means that the ratio of the speed of light in air and the speed of light in water is equal to 1.33. Similarly, the refractive index of crown glass, ng =1.52.
• 15. Note: An optically denser medium may not possess greater mass density. For example, kerosene having higher refractive index, is optically denser than water, although its mass density is less than water.
• 16.  The ability of a medium to refract light is also expressed in terms of its optical density. Optical density has a definite connotation. It is not the same as mass density. We have been using the terms ‘rarer medium’ and ‘denser medium’ in this Chapter. It actually means ‘optically rarer medium’ and ‘optically denser medium’, respectively. When can we say that a medium is optically denser than the other? In comparing two media, the one with the larger refractive index is optically denser medium than the other. The other medium of lower refractive index is optically rarer. The speed of light is higher in a rarer medium than a denser medium. Thus, a ray of light travelling from a rarer medium to a denser medium slows down and bends towards the normal. When it travels from a denser medium to a rarer medium, it speeds up and bends away from the normal.
• 17. LIGHT Light slows down when it enters an optically denser medium. The refracted ray bends towards the normal when the second medium is optically more dense than the first. i r air water Incident ray Refracted ray normal
• 18. LIGHT Light speeds up when it enters an optically less dense medium. The refracted ray bends away from the normal when the second medium is optically less dense than the first. air water i r Incident ray Refracted ray normal
• 19. LIGHT Among the 3 transparent mediums (air, water and glass), glass has the highest optical density. air water i1 r1 Incident ray Refracted ray glass i2 r2 Refracted ray air water i1 r1 Incident ray glass i2 r2 Refracted ray Refracted ray
• 20. LIGHT Complete these ray diagrams. air glass glass water
• 21. LIGHT airwater glassair Complete these ray diagrams.
• 22. Spherical lenses: A spherical lens is a transparent material bounded by two surfaces one or both of which are spherical. Spherical lenses are of two types: Spherical lenses Convex Concave
• 23. The lens which have two spherical surfaces, bulging outwards, is called a double convex lens. It is simply called a convex lens. It is thicker at the middle as compared to the edges. Convex lens converges light rays. Hence convex lenses are called converging lenses.
• 24.  1. A convex lens is used as a reflector in street lamps. As a result, light from the lamp diverges over a large area.  2. A convex lens is used by drivers of automobiles as a rear view mirror. These mirror are fitted on the sides of the vehicle enabling the driver to see traffic behind for safe driving. For example: i) a convex mirror produces an erect image of the objects. ii) size of the image formed by a convex mirror is much smaller than the object. iii) Convex mirror have a wider field of view as they are curved outwards. Therefore, convex mirrors enable the driver to view much larger area than would be possible with a plane mirror.
• 25. A double concave lens is bounded by two spherical surfaces, curved inwards. It is thicker at the edges than at the middle. Such lenses diverge light rays. Such lenses are called diverging lenses. A double concave lens is simply called a concave lens.
• 26. i) A concave mirror is used as a reflector in torches, searches light, head lights of motor vehicles etc. to get powerful parallel beams of light. ii) A concave mirror is used as doctor’s head mirror to focus light on body parts like eyes, ears, nose, throat etc., to be examined. iii) A concave mirror is also used as a shaving/make up mirror, as it form erect and magnifies image of the face. iv) The dentists use concave mirrors to observe large images of the teeth of patients. v) Large concave mirrors are used to concentrate sunlight to produce heat in solar cookers, solar furnaces etc. vi) Large concave mirrors are also used in reflecting type telescopes.
• 27. A lens, either a convex lens or a concave lens, has two spherical surfaces. Each of these surfaces forms a part of a sphere. The centres of these spheres are called centres of curvature of the lens. The centre of curvature of a lens is usually represented by the letter C. Since there are two centres of curvature, we may represent them as C1 and C2. An imaginary straight line passing through the two centres of curvature of a lens is called its principal axis.
• 28. The central point of a lens is its optical centre. It is usually represented by the letter O. A ray of light through the optical centre of a lens passes without suffering any deviation. The effective diameter of the circular outline of a spherical lens is called its aperture.
• 29.  1. Aperture: The aperture of a lens is the diameter of the circular edge of the lens.  2. Centre of curvature: A lens whether concave or convex has two spherical surfaces. Each of these surfaces forms a part of a sphere. The centres of these spheres are called centres of curvature of the lens.  3. Principal axis: An imaginary straight line passing through centre of curvature of the two surfaces of the lens is called principal axis of the mirror.  4. Optical centre: The optical centre of a lens is a point on the principal axis of the lens, such that a ray of light passing through it goes undeviated.
• 30. When we hold a convex lens in our hand and direct it towards the Sun and focus the light of the sun on a piece of a paper, the paper begins to burn producing smoke. It may even catch fire after a while. REASON The light from the Sun constitutes parallel rays of light. These rays were converged by the lens at the sharp bright spot formed on the paper. In fact, the bright spot you got on the paper is a real image of the Sun. The concentration of the sunlight at a point generated heat. This caused the paper to burn.
• 31. Now, we shall consider rays of light parallel to the principal axis of a lens. Several rays of light parallel to the principal axis are falling on a convex lens. These rays, after refraction from the lens, are converging to a point on the principal axis. This point on the principal axis is called the principal focus of the convex lens.
• 32. Several rays of light parallel to the principal axis are falling on a concave lens. These rays, after refraction from the lens, are appearing to diverge from a point on the principal axis. This point on the principal axis is called the principal focus of the concave lens.
• 33. If you pass parallel rays from the opposite surface of the lens, you get another principal focus on the opposite side. Letter F is usually used to represent principal focus. However, a lens has two principal foci. They are represented by F1 and F2. The distance of the principal focus from the optical centre of a lens is called its focal length. The letter f is used to represent the focal length.
• 34.  We can represent image formation by lenses using ray diagrams. Ray diagrams will also help us to study the nature, position and relative size of the image formed by lenses. For drawing ray diagrams in lenses, alike of spherical mirrors, we consider any two of the following rays – (i) A ray of light from the object, parallel to the principal axis, after refraction from a convex lens, passes through the principal focus on the other side of the lens, as shown in Fig.(a)
• 35. In case of a concave lens, the ray appears to diverge from the principal focus located on the same side of the lens, as shown in Fig.(b)
• 36. (ii) A ray of light passing through a principal focus, after refraction from a convex lens, will emerge parallel to the principal axis. This is shown in Fig.(a). A ray of light appearing to meet at the principal focus of a concave lens, after refraction, will emerge parallel to the principal axis. This is shown in Fig.(b). (a) (b)
• 37. (iii) A ray of light passing through the optical centre of a lens will emerge without any deviation. This is illustrated in Fig.(a) and Fig. (b). (a) (b)
• 38. The ray diagrams for the image formation in a convex lens for a few positions of the object are shown in Fig. 10.1. The ray diagrams representing the image formation in a concave lens for various positions of the object are shown in Fig. 10.2.
• 39. Figure 10.1 The position, size and the nature of the image formed by a convex lens for various positions of the object
• 40. Figure 10.2 Nature, position and relative size of the image formed by a concave lens
• 41. For lenses, we follow sign conventions, similar to the one used for spherical mirrors. We apply the rules for signs of distances, except that all measurements are taken from the optical centre of the lens. According to the convention, the focal length of a convex lens is positive and that of a concave lens is negative. You must take care to apply appropriate signs for the values of u, v, f, object height h and image height h′.
• 42. As we have a formula for spherical mirrors, we also have formula for spherical lenses. This formula gives the relationship between object distance (u), image-distance (v) and the focal length (f ) . The lens formula is expressed as 1 𝑣 − 1 𝑢 = 1 𝑓 The lens formula given above is general and is valid in all situations for any spherical lens. Take proper care of the signs of different quantities, while putting numerical values for solving problems relating to lenses.
• 43. MAGNIFICATION  The magnification produced by a lens, similar to that for spherical mirrors, is defined as the ratio of the height of the image and the height of the object. It is represented by the letter m. If h is the height of the object and h′ is the height of the image given by a lens, then the magnification produced by the lens is given by, m = 𝐻𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑡ℎ𝑒 𝐼𝑚𝑎𝑔𝑒 𝐻𝑒𝑖𝑔ℎ𝑡 𝑜𝑓 𝑡ℎ𝑒 𝑜𝑏𝑗𝑒𝑐𝑡 = ℎ′ ℎ Magnification produced by a lens is also related to the object-distance u, and the image-distance v. This relationship is given by Magnification (m) = h’/h = v/u
• 44.  The degree of convergence or divergence of light rays achieved by a lens is expressed in terms of its power.  The power of a lens is defined as the reciprocal of its focal length. It is represented by the letter P. The power P of a lens of focal length f is given by P = 1 𝑓 .  The SI unit of power of a lens is ‘dioptre’. It is denoted by the letter D. If f is expressed in metres, then, power is expressed in dioptres. Thus, 1 dioptre is the power of a lens whose focal length is 1 metre. 1D = 1m–1. Power of a convex lens is positive and that of a concave lens is negative.  Opticians prescribe corrective lenses indicating their powers. Let us say the lens prescribed has power equal to + 2.0 D. This means the lens prescribed is convex. The focal length of the lens is + 0.50 m. Similarly, a lens of power – 2.5 D has a focal length of – 0.40 m. The lens is concave.
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