## An unpolarised light is incident on a plate of refractive index √3 and the reflected light is found to be completely …..

Q. An unpolarised light is incident on a plate of refractive index √3 and the reflected light is found to be completely plane polarised. The angels of incidence and refraction are respectively
(a) 60°, 30°
(b) 30°, 60°
(c) Sin-1 (1/√3) , 45°
(d) tan -1 (√3/2) , 30°
Ans: A
Sol:
According to Brewster Law

μ = tanip

√3 = tanip

ip = 60°

Since , ip + r = 90°

60° + r = 90°

r = 30°

## Un polarised light passes through a polariser and analyser which are at an angle of 45° with respect to each other…..

Q. Un polarised light passes through a polariser and analyser which are at an angle of 45° with respect to each other. The intensity of polarised light coming from analyser is 5W/m². The intensity of unpolarised light incident on polariser is
(a) 5√3 W/m²

(b) 10W/m²

(c) 20W/m²

(d) 5 √3/4 W/m²

Ans: C

Sol: According to Malus Law

I2 = I1cos2 θ

I2 = I1cos2 45°

$\displaystyle I_2 = \frac{I_0}{2}cos^2 45$

$\displaystyle 5 = \frac{I_0}{2}\times \frac{1}{2}$

I0 = 20 W/m2

## A slit of width ‘d’ is placed infront of a lens of focal length 0.5m and is illuminated normally with light of wavelength…..

Q.A slit of width ‘d’ is placed infront of a lens of focal length 0.5m and is illuminated normally with light of wavelength 5.89 × 10-7 m. The first diffraction minima on either side of the central diffraction maximum are separated by 2 × 10-3 m. The width of the slit is

(a) 1.47 ×  10-4 m

(b) 2.94 × 10-4 m

(c) 1.47 × 10-7 m

(d) 2.92 × 10-7 m

Ans: B

Soli: Width of central Maxima = $\displaystyle \frac{2 \lambda D}{d}$

$\displaystyle 2\times 10^{-3} = \frac{2 \lambda D}{d}$

$\displaystyle d = \frac{2 \times 5.89\times {10}^{-7} \times 0.5}{2\times 10^{-3}}$

= 2.94 × 10-4 m

## In Young’s experiment using monochromatic light, the fringe pattern shifts by a certain distance on the screen….

Q. In Young’s experiment using monochromatic light, the fringe pattern shifts by a certain distance on the screen when a mica sheet of refractive index 1.6 and thickness 2 micron is introduced in the path of one of the interfering waves. The mica sheet is then removed and the distance between the slits and the screen is doubled. It is found that the distance between successive maxima now is the same as the observed fringe shift upon the introduction of the mica sheet. The wavelength of light is
(a) 5762 Å
(b) 5825Å
(c) 6000Å
(d) 6500Å
Ans: C

## A mixture of light, consisting of wavelength 590nm and an unknown wavelength, illuminates Young’s double slit…..

Q. A mixture of light, consisting of wavelength 590nm and an unknown wavelength, illuminates Young’s double slit and gives rise to two overlapping interference patterns on the screen. The central maximum of both light coincide. Further, it is observed that the third bright fringe of known light coincides with the 4th bright fringe of the unknown light. The wavelength of the unknown light is
(a) 393.4 nm
(b) 885.0 nm
(c) 442.5 nm
(d) 776.8 nm
Ans: C