81.

The figure shows variation of photocurrent with anode potential for a photo- sensitive surface for three different radiations. Let I_a , l_b and I_c be the intensities and v_a , v_b and v_c , be the frequencies for the curves a, b and c respectively. Then

• v_a = v_b and I_a ≠ I_b

• v_a = v_c and I_a = I_c

• v_a = v_b and I_a = I_b

• v_b = v_c and I_b = I_c

A plane mirror produces a magnification of

• zero

• - 1

• + 1

• between 0 and + 1

In a pond of water, a flame is held 2 m above the surface of water. A fish is at depth of 4 m from water surface.Refractive index of warer i $\frac{4}{3}$ . The apparent height of the lame from the eyes of fish is

• 5.5 m

• 6 m

• $\frac{8}{3} \mathrm{m}$

• $\frac{20}{3} \mathrm{m}$

White light is incident on the interface of glass and air, as shown in the figure. If green light is just totally and internally reflected then the emerging ray in air contains

• all colours

• yellow, orange and red

• violet, indigo and blue

• all colours except green

A convex lens made up of a material of refractive index µ₁ is immersed in a medium of refractive index µ₂ as shown in the figure. The relation between µ₁ and µ₂ is 

• µ₁ < µ₂

• µ₁ > µ₂

• µ₁ = µ₂

• ${\mathrm{\mu }}_1 = \sqrt{{\mathrm{\mu }}_2}$

If a flint lens glass of dispersive power 0.066 renders achromatic to a convex lens of crown glass of focal length  60 cm and dispersive power 0.033, then its focal length is

• − 60 cm

• + 60 cm

• − 120 cm

• + 120 cm

Two coherent light beams of intensity I and 4I are  superposed. The maximum and minimum possible intensities in the resulting beam are

• 5 I and I

• 5 I and 3 I

• 9 I and I

• 9 I and 3 I

If Young's double slit experiment is performed in water instead of air, then

• no fringes would be seen

• fringe width would decrease

• fringe width would increase

• fringe width would remain unchanged

In Young's double-slit experiment, the angular width of fringe formed on a distance screen is 0.1°. If wavelength of light is 6000 $\stackrel{\circ }{\mathrm{A}}$, then spacing between the slits is

• 3.4 × 10^-4 m

• 4.3 × 10^-4 m

• 5.4 × 10^-4 m

• 6.3 × 10^-4 m

A person uses spectacles of power + 2 D. He is suffering from

• myopia

• presbyopia

• astigmatism

• hypermetropia