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Downloaded from https:// www.studiestoday.com, , , , Refraction of Light, , , , _ ACTIVITY | 1. Put a straw into a glass. Then, pour some water into the glass. The appearance of the straw will change., The part dipped in water will seem bent., , 2. Put acoin into a bowl, and slowly pour water into the bowl. The coin will appear to rise or come closer., , , , Fig. 2.1 Refraction makes the straw look bent. Fig. 2.2 Refraction makes the coin appear closer., , These things happen because light bends, or the direction of light changes, when it moves from, one medium into another. Anything through which light can travel is called a medium. Glass, air,, water and vacuum are all mediums through which light can travel. The bending of light as it passes, from one medium to another is called the refraction of light. This is in contrast to reflection, in which, light bounces back into the same medium., , You can actually see the refraction of light by this simple activity. Fill part of a glass bottle or jar with water., Add a few drops of milk to the water (this will help you see a ray of light travelling in it). Insert a lit joss stick, (agarbatti) into the bottle for a short while, to trap some smoke, and then close the bottle. This will help you, see the ray in the air above the water. Let a ray of light from a laser pointer or a powerful torch pass through, the bottle, as shown in Figure 2.3(a). The ray will bend at the surface of the water. Change the angle at, which the ray strikes the surface of the water. You will notice two things., , 1. How much the ray bends depends on the angle at which it strikes the water., , 2. When the ray is perpendicular to the surface, it does not bend., , Li,, , Downloaded from https:// www.studiestoday.com

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Downloaded from https:// www.studiestoday.com, , 18 ICSE Physics for Class 8, , , , (b), , Fig. 2.3. (a) Light bends when it strikes the surface obliquely, (b) but not when it is perpendicular to the surface., , HOW REFRACTION OCCURS, , Why does refraction occur, and in what manner does light bend when it travels from one medium to, another? These are some of the questions we will explore in this section., , Light bends when it passes from one medium to another because it moves with different speeds in, different mediums. It moves the fastest through vacuum, with a speed of 299,792,458 m/s (about, 300,000 km/s). It moves a bit slower through air, much slower through water, and slower still, through glass. When light travels from one medium to another, it is this change in its speed that, makes it change its direction. This is similar to the way a strolley or pram turns when one of its wheels, , moves from a paved path to, say, a lawn. The wheel that moves on to the lawn slows down, so the, strolley or pram turns., , Ray Diagrams, , As in the case of reflection, the path taken by light before and after re, refraction is represented with the help of ray diagrams. Figure 2.4 A :, , is a ray diagram showing the path taken by a ray of light as it, passes from air to glass., , , , e PQ is the boundary or surface of separation between the P 0 Q, two mediums., , e AO, the ray travelling towards the boundary, is the :, incident ray. Glass D. 8, , e CD, which is perpendicular to the boundary at the point of Fig. 2.4, , incidence, is the normal to the boundary., e OB, the ray moving away from the boundary after refraction, is the refracted ray., e i, the angle between the incident ray and the normal, is the angle of incidence., , e r, the angle between the refracted ray and the normal, is the angle of refraction., , Refractive Index, , The amount of refraction that occurs when light passes from one medium into another depends on, , Downloaded from https:// www.studiestoday.com

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Downloaded from https:// www.studiestoday.com, , Refraction of Light 19, , the nature of the two mediums. To be more specific, it depends on the speeds with which light travels, in the two mediums. For example, light bends more while passing from air to glass than while, passing from air to water. This is because the speed of light in glass is less than the speed of light, in water., , A ratio called the refractive index determines the degree to which light bends while passing from, one medium to another. It is denoted by n, and defined as follows., , meek oadenicen — speed of light in the medium from which light travels, , , , speed of light in the medium into which light travels, , Thus, the refractive index of water with respect to air would be, , speed of light in air, , , , air!4water —, , speed of light in water, , EXAMPLE 1. The speed of light in air is 3x 108 m/s and that in glass is 2 x 10° m/s. Find the refractive index of, glass with respect to air., , To calculate the refractive index of glass with respect to air, we consider light to be travelling, from air to glass., , speed of light in medium from which light travels, , , , Refractive index = 2 —— _——speed of light in medium into which light travels, , speed of lightinair _ 3x 10° m/s =15, , , , air!!glass ~ speed of light in glass ~ 2x108 m/s, , 4, EXAMPLE 2. The speed of light in air is 3 x 108 m/s. The refractive index of water with respect to air is 3" Find, the speed of light in water., speed oflightinair 4, , ee speed of light in water oe even):, d of light in ai, speed of light in water = eofehanctSa -ielaar, air water, 108 :, 3, , EXAMPLE 3. The speed of light in glass is 2 x 108 m/s and that in water is 2.25x 108 m/s. Find the refractive, index of glass with respect to water., , 1 _. -SPeed of light in water, water! glass ~ speed of light in glass, , 8, g25x10 m/s _ 2:25 _ 44195,, 2x10° m/s, , , , Downloaded from https:// www.studiestoday.com

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Downloaded from https:// www.studiestoday.com, , 20 ICSE Physics for Class 8, , Denser and rarer mediums, , Since refractive index is a ratio of the speed of light in two mediums, we must always mention the, mediums whenever we refer to it. To make things easier, vacuum is chosen as the medium with, which all other mediums are compared. This is because light travels the fastest in vacuum. The, refractive index of a medium is then defined as the ratio of the speed of light in vacuum to the speed of, light in the medium., , speed of light in vacuum, , , , Refractive index of a medium =, ietmipitebas hak de Tes speed of light in the medium, , This is written in symbols as tee, , where c is the speed of light in vacuum and 1 is the speed of light in the medium., , , , It follows that v=, , A look at Table 2.1 will tell you that the refractive index of Table 2.1 Some refractive indexes, air is very close to 1. This is because the speed of light in air is Medium obicietn teins, very nearly the same as that in vacuum. Hence, for all, practical purposes air is chosen as the medium of reference. Air 1.0003 (almost), Thus, instead of saying “the refractive index of glass with Water 1.33, respect to air is 1.5”, we simply say “the refractive index of Alcohol 1.36, glass is 1.5”, or “Nejass 18 1.5%. pd bee 1.42, , When we compare transparent mediums, we call those Types of glass is47, with higher values of refractive index optically denser than ee 242, , those with lower values of refractive index. For example, —, water is optically denser than air but optically rarer than glass. Diamond is optically denser than most, transparent mediums. The degree to which light bends while passing from one medium to another, depends on the optical densities of the mediums., , , , EXAMPLE 4. Estimate the speed of light in diamond from the data available to you so far., , Speed of light in vacuum =3x10° m/s., From Table 2:1, ngissgadis Dae, , 3x108 m/s, , =1.24x108 m/s., aa <0 mys, , diamond =, , Downloaded from https:// www.studiestoday.com ‘, . _. cieadaiaa

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Downloaded from https:// www.studiestoday.com, , Refraction of Light 21, , Rules of Refraction, , Let us go back to the activity with the glass bottle. You already know one of the rules of refraction., , 1. Light bends while travelling from one medium to another only if it is incident on the boundary, , at an angle to the normal. It does not bend if it is incident perpendicular to the boundary, or, along the normal., , 2. If you imagine a normal to the surface of the water where the beam of light strikes it, you will, realise that the ray of light bends towards the normal as it passes into the water. This, observation holds whenever light travels from an optically rarer to an optically denser medium., Light bends towards the normal when it travels from an optically rarer to a denser medium., , , , (a) (b), , Fig. 2.5 (a) Light travelling from a rarer to a denser medium bends towards the normal. (b) Light travelling from a denser to a, rarer medium bends away from the normal., , 3. If you shine the light at the bottom of the bottle, you will notice that the ray bends away from the, normal as it travels from water to air. This too is a general rule. Light bends away from the, normal while travelling from an optically denser to a rarer medium., , 4. There is another rule that will become clear as you read on. If light follows a certain path while, travelling from a point A in one medium to a point B in another medium then it will follow the, same path while travelling from B to A. In other words, it will retrace its path. Thus, when you, draw a ray diagram to trace the path of light from one medium to another, all you have to do is to, reverse the direction of the arrows to show the path of light from the second medium to the first., In other words, ray diagrams are reversible. They remain valid if all the arrowheads are reversed., , Air Air, , , , , , Glass B Glass, (a) (b), , Fig. 2.6 Light retraces its path while travelling from one medium to another and back., , Downloaded from https:// www.studiestoday.com