51.

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

When a p-n junction diode is connected to a battery with its p-side to positive terminal and n-side to negative terminal, the width of the depletion layer

• decreases

• increases

• first decreases then increases

• first increases then decreases

Wave nature of light does not explain

• Interference

• Total internal reflection

• Photo-electric effect

• Refraction

In forward and reverse biasing of a p-n junction diode, the ratio of resistances is

• 10² : 1 

• 10^-2 : 1 

• 1 : 10⁴

• 1 : 10^-4

In a sample of radioactive substance, what percentage decays in one mean life time?

• 69.3 %

• 63 %

• 50 %

• 36 %

A p-type semiconductor is formed by adding 1 indium atom in to a sample of silicon per 5x10⁻⁷ silicon atoms. If the density of atoms in the sample is 5×10²⁸ atoms/metre³, the number of atoms accepted in per metre³ of silicon is

• 1.0 × 10¹³ atoms/cm³

• 2.5 × 10²⁰ atoms/cm³

• 2.5 × 10²⁵ atoms/cm³

• 1.0 × 10¹⁵ atoms/cm³

The ratio of intensities of two waves is 1: 9. The waves interference, what will be the ratio of maximum and minimum intensities?

• 1:4

• 4:1

• 3:2

• 2:3

The light of frequency v falls on a certain photoelectric substance of threshold  frequency v₀.  The work function of the substance is

• hv₀

• hv

• $\frac{\mathrm{h\gamma }}{{\mathrm{v}}_0}$

• h(v - v₀)

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$\left(\frac{3}{4}\right)$