51.

A prism of refractive index √2 has a refracting angle of 60°. At what angle a ray must be incident on it so that it suffers a minimum deviation?

• 45°

• 60°

• 90°

• 180°

A transparent cube of 15 cm edge contains a small air bubble. Its apparent depth, when viewed through one face, is 6 cm and when viewed through the opposite face is 4 cm. Then the refractive index of the material of the cube is:

• 2.0

• 2.5

• 1.6

• 1.5

The radius of the convex surface of the plano-convex lens is 20 cm and the refractive index of the material of the lens is 1.5. The focal length is :

• 30 cm

• 50 cm

• 20 cm

• 40 cm

If the work function of a metal is 4.2 eV, the cut-off wavelength is :

• 8000 Å

• 7000 Å

• 1472 Å

• 2950 Å

Regarding a semiconductor which one of the following is wrong?

• There are no free electrons at room temperature

• There are no free electrons at 0K

• The number of free electrons increases with the rise of temperature

• The charge carriers are electrons and holes

Nuclear fusion is common to the pairs :

• thermonuclear reactor, uranium based nuclear reactor

• energy production in sun, uranium-based nuclear reactor

• energy production in sun, hydrogen bomb

• the disintegration of heavy nuclei, hydrogen bomb

Which of the following statements is true for an n-type semi-conductor?

• The donor level lies closely below the bottom of the conduction band

• The donor level lies closely above the top of the valence band

• The donor level lies at the halfway mark of the forbidden energy gap

• None of the above

The refractive index of the material of the prism and liquid are 1.56 and 1.32 respectively. What will be the value of θ for the following refraction?

• $\mathrm{sin\theta } \ge \frac{13}{11}$

• $\mathrm{sin\theta } \ge \frac{11}{13}$

• $\mathrm{sin\theta } \ge \frac{\sqrt{3}}{2}$

• $\mathrm{sin\theta } \ge \frac{1}{\sqrt{2}}$

A person who can see things most clearly at a distance of 10 cm, requires spectacles to enable to see clearly things at a distance of 30 cm. What should be the focal length of the spectacles?

• 15 cm (concave)

• 15 cm (convex)

• 10 cm (concave)

• 0

In the given figure, what is the angle of the prism?

• A

• B

• C

• D