NEET Physics Wave Optics Questions Solved

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(a) Why does unpolarised light from a source show a variation in intensity when viewed through a polaroid which is rotated? Show with the help of a diagram, how unpolarised light from sun gets linearly polarised by scattering.

(b) Three identical polaroid sheets P1, P2 and P3 are oriented so that the pass axis of P2 and P3 are inclined at angles 60° and 90° respectively with the pass axis of P1.

A monochromatic source S of unpolarized light of intensity I0 is kept in front of the polaroid sheet P1 as shown in the figure.

Determine the intensities of light as observed by the observer at O, when Polaroid P3 is rotated with respect to P2 at angles θ=30° and 60°

(a) As per Malu's law 

Transmitted intensity I=I0cos2θ

 The transmitted intensity will show a variation as per cos2θ             (1/2) marks

The electric field,of the incident wave, makes the electrons of the air molecules, acquire both components of motion ( as well as.)                                                    (1/2) marks

Charges accelerating parallel to , do not radiate energy towards the observer.

Hence the radiation , scattered towards the observer gets linearly polarised.

(b) We have, as per Malu's law 

I=I0cos2θ                                                             (1/2) marks

 If the intensity of light, incident on P1 is I0, we have 

I1= Intensity transmitted through P1=I02

I2= Intensity transmitted through P2=I02cos260°=I08                  (1/2) marks

For θ=30°, we have

Angle between pass axis of P2 and P3

=(30°+30°)=60°or (30°-30°)=0°

I3 can be either I032 or I08                                                        (1) marks

For θ=60° , we have 

Angle between pass axis of  P2 and P3

(30°+60°)=90°or (30°-60°)=-30°

I3 can be either 0 or 3I032                                                          (1/2) marks   


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