Ray Optics and Optical Instruments

A convex lens and a concave lens, each having same focal length of 25 cm, are put in contact to form a combination of lenses. The power in diopters of the combination is

1. zero
2. 25
3. 50
4. infinite.

The refractive index of the material of a prism is 2 and its refracting angle is 30 . One of the refracting surfaces of the prism is made a mirror inwards. A beam of monochromatic light entering the prism from the other face will retrace its path after reflection from the mirrored surface if its angle of incidence on the prism is

1. 45
2. 60
3. 0
4. 30

A beam of light composed of red and green rays is incident obliquely at a point on the face of a rectangular glass slab. When coming out on the opposite parallel face, the red and green rays emerge from

1. two points propagating in two different non-parallel directions
2. two points propagating in two different parallel directions
3. one point propagating in two different directions
4. one point propagating in the same directions

An equiconvex lens is cut into two halves along (i) XOX′ and (ii) YOY′ as shown in the figure. Let f, f', f'' be the focal lengths of the complete lens, of each half in case (i), and of each half in case (ii), respectively. Choose the correct statement from the following [figure]

1. f' = f, f'' = 2f
2. f' = 2f, f'' = f
3. f' = f, f'' = f
4. f' = 2f, f'' = 2f

A convex lens is dipped in a liquid whose refractive index is equal to the refractive index of the lens. Then its focal length will

1. become zero
2. become infinite
3. become small, but non-zero
4. remain unchanged

A bulb is located on a wall. Its image of the same size is to be obtained on a parallel wall with the help of a convex lens. The lens is placed at a distance d ahead of the second wall, then the required focal length will be

1. only d4
2. only d2
3. more than d4 but less than d2
4. less than d4

For the given incident ray as shown in the figure, for total internal reflection of this ray, the required refractive index of the prism will be [figure]

1. 3+12
2. 2+12
3. 32
4. 76

Optical fibres are based on

1. total internal reflection
2. less scattering
3. refraction
4. less absorption coefficient

A ray of light travelling in air has wavelength , frequency n, velocity v and intensity I. If this ray enters into water then these parameters are ', n', v' and I' respectively. Which relation is correct from following?

1. = '
2. n = n'
3. v = v'
4. I = I'

A disc is placed on a surface of pond which has refractive index 5/3. A source of light is placed 4 m below the surface of liquid. The minimum radius of disc needed so that light is not coming out is

2. 3 m
3. 6 m
4. 4 m