31.

Two mirrors at an angle 0° produce 5 images of a point. The number of images produced when θ is decreased to θ − 30° is

• 9

• 10

• 11

• 12

The radius of the light circle observed by a fish at a depth of 12 m is (refractive index of water = 4/3)

• $36\sqrt{7}$

• $\frac{36}{\sqrt{7}}$

• $36\sqrt{5}$

• $4\sqrt{5}$

In Young's double slit experiment, the fringe width is β. If the entire arrangement is placed in a liquid of refractive index n, the fringe width becomes

• nβ

• $\frac{\mathrm{\beta }}{\mathrm{n} + 1}$

• $\frac{\mathrm{\beta }}{\mathrm{n} - 1}$

• $\frac{\mathrm{\beta }}{\mathrm{n}}$

A plano-convex lens (f = 20 cm) is silvered at plane surface. Now focal length will be

• 20 cm

• 40 cm

• 30 cm

• 10 cm

The light beams of intensities in the ratio of 9 : 1 are allowed to interfere. What will be the ratio of the intensities of maxima and minima ?

• 3 : 1

• 4 : 1

• 25 : 9

• 81 : 1

If x₁ be the size of the magnified image and x₂ the size of the diminished image in Lens Displacement Method, then the size of the object is

• $\sqrt{{\mathrm{x}}_1{\mathrm{x}}_2}$

• x₁x₂

• x₁²x₂

• x₁x₂²

The decimal equivalent of the binary number (11010.101)₂ is 

• 9.625

• 25.265

• 26.625

• 26.265

In a common emitter configuration, a transistor has β = 50 and input resistance 1 kΩ. If the peak value of AC input is 0.01 V, then the peak value of collector current is

• 0.01 µA

• 0.25 µA

• 100 µA

• 500 µA

Half-life of a radioactive substance is 20 min. The time between 20% and 80% decay will be

• 20 min

• 30 min

• 40 min

• 25 min

The energy released by the fission of one uranium atom is 200 MeV. The number of fissions per second required to produce 3.2 W of power is (Take, 1eV = 1.6 x 10^-19 J)

• 10⁷

• 10¹⁰

• 10¹⁵

• 10¹¹