Reflection, Refraction & Dispersion of Light

Previous Years Examination Questions

1.A biconvex lens made of a transparent material of refractive index 1.25 is immersed in a water of refractive index 1.33. Will the lens behave as a converging or a diverging lens? Give reason. [All India 2014]

Ans. When a lens is placed in a liquid, where refractive index is more than that of the material of lens, then the nature of the lens changes. So, when a biconvex lens of refractive index 1.25 is immersed in water (refractive index 1.33), i.e. in the liquid of higher refractive index, its nature will change. So, biconvex lens will act as biconcave lens or diverging lens.

2.A biconvex lens made of a transparent material of refractive index 1.5 is immersed in a water of refractive index 1.33. Will the lens behave as a converging or a diverging lens? Give reason. [Ail India 2014]

Ans. A biconvex lens acts as a converging lens in air because the refractive index of air is less than that of the material of the lens. The refractive index of water is less than the refractive

index of the material of the lens (1.5). So, its nature will not change, it behaves as a converging lens.

3.A convex lens is placed in contact with a plane mirror. A point object at a distance of 20 cm on the axis of this combination has its image coinciding with itself. What is the focal length of the lens? [All India 2014]

Ans. The adjacent figure shows a convex lens L in contact with a plane mirror P is the point object kept in the front of this combination at a distance of 20 cm from it.

20 cm

Since, the image is coinciding with the object itself, the rays from the object after refraction from the lens fall normally on the mirror M and form an image coinciding with the object itself. So, the image is formed at the focus of the lens. So, focal length of the lens is 20 cm.

1. Write the relationship between angle of incidence i, angle of prism A and angle of minimum deviations from a triangular prism. [Delhi 2013]

Ans. The relation between the angle of incidence /', angle of prism, A and the angle of minimum deviation, A_m for a triangular prism is given by

<math display="block">i = \frac{A + \Delta_m}{2}.</math>

1. How does focal length of a lens change when red light incident on it is replaced by violet

light? Give reason for your answer. [Foreign 2012]

Ans. The refractive index of the material of a lens increases with the decrease in wavelength of the incident light. So, focal length will decrease with decrease in wavelength according to the formula.

<math>\frac{1}{f} = (\mu - 1) \left( \frac{1}{R_1} - \frac{1}{R_2} \right)</math>

Thus, when we replace red light with violet light then due to increase in wavelength the focal length of the lens will decrease

6.Name the physical quantity which remains same for microwaves of wavelength 1 mm and UV-radiation of 1600 A in vacuum. [Delhi 2012]

Ans. Both microwave and UV-rays are a part of the electromagnetic spectrum. Thus, the physical quantity that remains for both types of radiation will be their speeds equal to

1. <math>c = 3 \times 10^8 \text{ m/s}</math>

1. Under what condition, does a biconvex lens of glass having a certain refractive index act as

a plane glass sheet when immersed in a liquid? [Delhi 2012]