That is why, the fish seems to be nearer than it really is. Answer: (i) Quartz prism (ii) Heat radiation. The critical angle for glass-air interface is 45° for yellow light. Answer: (i) (ii) When the incident ray is undergoing minimum deviation the angle of incidence is equal to angle of emergence, i.e., ∠i – ∠e. Question 10: A ray of light PQ is incident normally on the hypotenuse of a right angled prism ABC as shown in the diagram given below: (i) Copy the diagram and complete the path of the ray PQ till it emerges from the prism. (i) Will the image of all the letters be in the same place? How is the critical angle related to the refractive index of a medium? Answer: During sunset or sunrise, the rays of light from sun travel through a long distance through air. If the refractive index of water is 4/3, find the actual depth of the pond. (d)No the light will not suffer minimum deviation. In Fig., 4.66 a ray of light PQ is incident normally on the hypotenuse of an isosceles right angle prism ABC. (b) Denser medium has a higher refractive index and therefore the speed of light in such medium is lower in comparison to the speed of light in a medium which has a lower refractive index. Question 29: How is the refractive index of a medium related to its real depth and apparent depth? Name two factors which affect the critical angle for a given pair of media. The wavelength of light in air is  , the speed of light in air is c and in glass is v. If the refractive index of glass is 1.5, write down (a) the relationship between c and v, (b) the wavelength of light in glass. Question 8: (i) A ray of light passes from water to air. Hence, an apparent image of the star is seen. The temperature of the liquid is then slowly altered. A stick partially immersed in water in a glass container appears bent or raised as shown in figure above. Changes in the angle of deviation as we increase. Find the sine of angle of, refraction inside the glass block. Hence, the refractive index of glass for red light will decrease as compared to that for blue light. In glass, red colour has the higher speed than violet colour, so red will be raised to the maximum while violet will be at the bottom and the other colours will be raised in the order I-B-G-Y and orange. (ii) The refractive index of a medium is different for light of different colours. Question 20: State the factors on which the angle of deviation depends. The critical angle is related to the refractive index of a medium by the relation, State the approximate value of the critical angle for. (ii) The refractive index of medium I is equal to the refractive index of medium II. Answer: Question 3: The diagram shows the path of a ray of light through a rectangular glass block placed in a liquid of uniform density. A small air bubble in a glass block when seen from above appears to be raised because of: Hint: When a ray of light travels from denser to rarer medium, it moves away from the normal. (a) For which colour of white light, is the refractive index of a transparent medium (i) the least (ii) the most? (a)In the diagram show the approximate direction of these rays as they emerge out of the block. The critical angle for water-air surface is 48°. (ii) If the angle of incidence is changed to (a) 30o, (b) 60o, state whether the angle of deviation will be equal to less than or more than 36o. (i) As the ray is suffering minimum deviation in an equilateral glass prism so, (ii) If the angle of incidence is changed to. When a ray of light from air enters a denser medium, it: The ray of light bends towards the normal. Question 41: In what condition a prism is said to be in the position of minimum deviation? Answer: This is because, in total internal reflection 100% of incident light gets reflected. Label your diagrams. The following diagram can make it further clear. With R as centre and convenient radius, draw a circle cutting the incident ray at Q and the refracted ray at Q’. Case (ii) When angle of incidence is 45o. How is the critical angle of a material related to its refractive index? (ii) Mark in your diagram the two angles on the surface of separation when the ray of light moves out from the liquid to air. Answer: Red colour is used as sign of danger due to its greater wavelength and lesser deviation (scattering). The change in the direction of the path of light, when it passes from one transparent medium to another transparent medium, is called refraction of light. (d)The colour or wavelength ( ) of light used. Refractive index ( ) of glass is related to the angles as, (c)Angle of emergence (e) is marked in part (a), The two angles are related to each other by the relation. What is the angle of refraction for the ray? Will the ray have different speed in the other medium? Since the point B is very close to point A, i.e. Looking through the other side of the block, fix two pins, P3 and P4 vertically on the white sheet, such that all the four pins P1, P2, P3 and P4 appear to be in a straight line as seen through the glass block. Let two rays OA and OL from a source O are incident on the prism. So at the second face of the prism, the angle of incidence will be 60o. A ray of light strikes the surface at a rectangular glass slab such that the angle of incidence is (i) 0o, (ii) 45o. This is because, in a prism, refraction takes place at two inclined surfaces while in a glass block refraction takes place at two parallel surfaces. ii.The angle of incidence must be greater than the critical angle for the pair of media. (i) Violet colour will deviate the most and (ii) Red colour will deviate the least. In the figure, 4.63   PQ and PR are the two light rays emerging from an object P. The ray PQ is refracted as QS. Fill in the blanks to complete the following sentences: (a) When light travels from a rarer to a denser medium, its speed …………………. Answer: First the angle of deviation produced by prism decreases with the increasing angle of incidence but on further increasing the angle of incidence the angle of deviation increases. =1.5, =1.33) – Smaller the speed of light in a medium relative to air, higher is the refractive index of that medium. At any point R on the edge AB, draw a normal NN’. TIR only takes place when both of the following two conditions are met: In our introduction to TIR, we used the example of light traveling through water towards the boundary with a less dense material such as air. (iii) Name the phenomenon that occurs if the angle of incidence ∠PQN is increased still further. Answer: In reflection of light from a plane mirror. Also indicate the angle of deviation by the letter δ. In total internal reflection through a prism there occurs 100% reflection of light.


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