Question and Answers on the Topic of Refraction of Light

Question and answers on the topic of refraction of light 1. An object is placed at a distance of 15 cm from a lens of focal length +10 cm. (a) Which type of lens is used here?(b) Calculate the image distance.(c) What is the nature of the image formed? Answer : a) Convex [ Because focal length is positive] b. u = - 15 cm, f = 10 cm v = uf / (u + f) v = (- 15 * 10) / (- 15 + 10) = 30cm c. Inverted, real and large. 2. (a) When a beam of light enters glass from the air, the angle of incidence is 'i' and the angle of refraction is 'r'. State Snell's law in this regard. (b) Imagine that when light enters glass from air, the velocity of light in air is 3 * 10 ^ 8 * m / s and the velocity of light in glass is 2 * 10 ^ 8 * m / s . Calculate the refractive index. Answer : a. The ratio of the sine of the angle of incidence (i) to the sine of the angle of refraction (r) is a constant. b. n = c / v n= (3x108)/(2x108) = 1.5 3. A ray of light travelling from glass to air grazes through the surface of glass as shown. (a) Name the angle of incidence in the: figure.(b) Can we reflect back the incident ray into the glass? Justify your answer. Answer: a. critical angle. b. Yes. If the angle of incidence is greater than the critical angle, the light will be completely reflected back to the glass. 4. Observe the figure. (a) Write the values of u and f by using the New Cartesian Sign convention. (b) Calculate the distance of the image from the lens. Answer : a. u = - 45cm , f = - 30cm b. v = uf / (u + f) v = (- 45 * (- 30)) / (- 45 - 30) v = 1350 / (- 75) v = 18 cm 5. Write the reason for the twinkling of stars in the sky. Answer : The reason for the twinkling of stars in the sky is atmospheric refraction. Here, the light undergoes successive refraction. 6. A ray of light falling obliquely from air to a glass slab is shown in the figure (a) Find out the angle of incidence. (b) Why is the angle of refraction less than the angle of incidence in this case? (c) How can you calculate the refractive index of the material of this slab? (Calculation not required) Answer : (a) The angle of incidence is 60°. (b) The angle of refraction is less than the angle of incidence because light is travelling from a rarer medium (air) to a denser medium (glass). (c) By using Snell's Law 7. The speed of light in certain media is given in the table below. Analyse the table and answer the following questions. Media Speed of light (m / s) Vacuum 3 * 10 ^ 8 Water 2.25 * 10 ^ 8 Diamond 1.25 * 10 ^ 8 Glass 2 * 10 ^ 8 (a) Re-arrange the given media in the increasing order of their optical density. (b) If light enters obliquely at the same angle of incidence from air to various media, which medium will result in the largest angle of refraction?(c) What do you mean by absolute refractive index? Answer : (a) Vacuum < Water < Glass < Diamond (b) Since vacuum has the largest speed of light, it will give the largest angle of refraction. (c) The refractive index of a medium with respect to vacuum is called absolute refractive index. It (c) The refractive index of a medium with respect to vacuum is called absolute refractive index. It is the ratio of the speed of light in a vacuum (c) to the speed of light in that medium (v)