A convex lens of focal length 12 cm is made up of a glass of refractive index $\frac{3}{2}$.When it is immersed in a liquid of refractive index $\frac{5}{4}$, its focal length will be

• 15 cm

• 6 cm

• 30 cm

• 24 cm

The magnification of an astronomical telescope is 10 and the focal length of eye-piece is 20 cm. The focal length of objective lens is

• $\frac{1}{200} \mathrm{cm}$

• 2 cm

• 200 cm

• 100 cm

A diver, at a depth of 12 metre from the surface of the water, sees the sky within a cone whose half vertex angle is : $\left({\mathrm{a}}^{{\mathrm{\mu }}_{\mathrm{w}}}=\frac{4}{3}\right)$

• cos⁻¹$\left(\frac{4}{3}\right)$

• sin⁻¹$\left(\frac{4}{3}\right)$

• 90$°$

• sin⁻¹$\left(\frac{3}{4}\right)$

When a ray is incident on a medium of refractive index n at Brewster's angle,it gets

• totally reflected

• totally absorbed

• circularly polarised

• plane polarised

To get three images of a single object, one should have two plane mirrors at an angle of

• 60°

• 90°

• 120°

• 30°

156.

Consider telecommunication through optical fibres. Which of the following statements is not true?

• Optical fibres can be of graded refractive index

• Optical fibres are subjected to electromagnetic interference from outside

• Optical fibres have extremely low transmission loss

• Optical fibres may have homogeneous core with a suitable cladding

An astronomical telescope has a large aperture to

• reduce spherical aberration

• have high resolution

• increase span of observation

• have low dispersion

Two lenses have powers equal to +2 D and -4 D respectively, the power of combination will be

• -2D

• +2D

• -4D

• +4D

The colour of a red cloth in green light appears

• red

• green

• orange

• black

Focal length of a lens is +10 cm and its refractive index is 1.4, it is dipped in a liquid of refractive index 1.6, then its focal length becomes

• - 16 cm

• + 32 cm

• + 16 cm

• - 32 cm