INTERFERENCE BY THIN FILM

A ray of light incident on a thin film of thickness ‘t’ gets partially reflected and refracted at A at surface I and thereafter it gets reflected and refracted at B of surface II. The rays after emerging in the first medium interfere. Now the inference will depend upon the path difference between AD and ABC, as beyond CD path difference is zero

                                                  294_interference by thin lens.jpg

Putting (IV) and (V) in (III) we obtain

    Δ x = 2 μt cosr

AD as reflected at a denser medium it suffers an additional path difference λ/2
Total path difference the taken place is

2 μ t cosr –  λ/2

For constructive interference

2μt cos r - λ/2 = μn

>> 2 μt cosr = nλ +λ /2 = (2n + 1) λ/2 maxima

For normal Incidence r =0 >> 2 μt = (2n+1) λ/2 = μn

2 μt cosr - λ/2 = (2n+1) λ/2 >> 2 μt cosr = nλ

For normal incidence 2 μt = nλ

Illustration 6:    White light is incident normally on a glass plate of thickness 0.50 x 10-6 m and index of refraction 1.50. Which wavelengths in the visible region (400 nm - 700 nm) are strongly reflected by the plate?

Solution :    The light of wavelength λ is strongly reflected if the light rays reflected are interfering constructively.
    As we know the condition for constructive interference

1036_solution 2010.jpg

 

Contact askiitians experts to get answers to your queries by filling up the form given below:

We promise that your information will be our little secret. To know more please see our Privacy Policy
We promise that your information will be our little secret. To know more please see our Privacy Policy

OR

Sign Up with Facebook

Signing up with Facebook allows you to connect with friends and classmates already using askIItians. It’s an easier way as well. “Relax, we won’t flood your facebook news feed!”

Related Resources
Diffraction and Polarization

DIFFRACTION a) Diffraction is the bending or...

Wave Nature of Light

WAVE NATURE OF LIGHT Huygens’ Wave Theory...

Youngs Double Slit Experiment

YOUNG’S DOUBLE SLIT EXPERIMENT A train of...

Polarisation and Brewsters Law

POLARISATION Polarisation of two interfering wave...

Displacement of Fringes in Youngs Double Slit Experiment

Displacement of Fringes When a film of...

Questions and Solutions part 2

Questions and Solutions part 2 11. Angular width...

Questions and Solutions part 3

Questions and Solutions part 3 1. When light is...

Questions and Solutions part 1

Questions and Solutions Part 1 1. In Young's...