131.

Light of wavelength 6000 A⁰ is reflected at nearly normal incidence from a soap film of refractive index 1.4. The least thickness of the film that will appear black is

• 200 A⁰

• 2000 A⁰

• 1000 A⁰

• Infinity

Light travels faster in air than that in glass .This is accordance with

• wave theory of light

• corpuscular theory of light

• Neither(a) nor(b)

• Both (a) and (b)

In Young's double slit experiment, light of wavelength 6000 A⁰ is used to produced fringes of width 0.8 mm at a distance of 2.5 m. If the whole appart is dropped in liquid of refractive index 1.6, then the fringe width will be

• 0.2mm

• 0.4mm

• 0.5mm

• 0.6mm

Interference was observed in an interference chamber when air was present. Now, the chamber is evacuated and if the same light is used, a careful observation will show

• no interference

• interference with bright band

• interference with dark bands

• interference in which breadth of the fringe will be slightly increased 

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

• 5I and I

• 5 I and 3 I

• 9 I and I

• 9 I and 3 I

The tip of a needle does not give a sharp image on a screen. This is due to

• polarisation

• interference

• diffraction

• None of these

Two coherent light beams of intensities I and 41 are superposed. The maximum and minimum possible  intensities in the resulting beam are

• 5I and I

• 5I and 3I

• 9I and I

• 9I and 3I

If prism angle α = 1^o , μ= 1.54, distance between screen and prism (D) = 0. 7 m, distance between prism and source $\mathrm{\alpha } = 0.3 \mathrm{m} , \mathrm{\lambda } = 180 \mathrm{\pi }$ nm, then in Fresnal biprism find the value of β (fringe width).

• 10^-4 m

• 10^-3 mm

• 10^-4 × $\mathrm{\pi }$m

• $\mathrm{\pi } \times {10}^{-3 }\mathrm{m}$

The magnification produced by a astronomical telescope for normal adjustment is 10 and the length of the telescope is 1.1 m. The magnification, when the image is formed atleast distance of distinct vision is

• 6

• 18

• 16

• 14

Two slits are separated by a distance of 0.5 mm and illuminated with light of wavelength λ = 6000 A^o. If the screen is placed at 2.5 m from the slits. The distance of the third bright fringe from the centre will be

• 1.5 mm

• 3 mm

• 6 mm

• 9 mm