How Many Marks Required for IIT Delhi in JEE Advanced 2025

Thin Film Interference - Practice Questions & MCQ

Edited By admin | Updated on Sep 18, 2023 18:34 AM | #JEE Main

Quick Facts

  • Thin film interference is considered one the most difficult concept.

  • 7 Questions around this concept.

Solve by difficulty

What is the minimum thickness of a soap film needed for constructive interference in reflected light, if the light incident on the film is of \mathrm{750 \mathrm{~nm}}? Assume that the index for the film is \mathrm{\mu=1.33} :

What is the minimum thickness of a soap film needed for constructive interference in reflected light, if the light incident on the film is\mathrm{ 750 \mathrm{~nm}}? Assume that the index for the film is \mathrm{\mathrm{n}=1.33 :}

What is the minimum thickness of a soap film needed for constructive interference in reflected light, if the light incident on the film is 300 \mathrm{~nm} ? Assume that the index for the film is \mathrm{n}=1.33 :

What is the minimum thickness of a soap bubble needed for constructive interference in reflected height, if the light incident on the film is 1500 \mathrm{~nm} ? Assume that the refractive index for the film is \mathrm{n}=1.33 :

Concepts Covered - 1

Thin film interference

Interference effects are commonly observed in thin films when their thickness is comparable to the wavelength of incident light ( if it is too thin as compared to the wavelength of light it appears dark and if it is too thick, this will result in uniform illumination of the film). A thin layer of oil on the water surface and soap bubbles shows various colors in white light due to the interference of waves reflected from the two surfaces of the film.

In thin films, interference takes place between the waves reflected from its two surfaces and waves refracted through it

  • Interference in reflected light :

Net path difference between two consecutive waves in the reflected system $=$

$
\Delta x=2 \mu t \operatorname{cosr}-\frac{\lambda}{2}
$

(As the ray suffers reflection at the surface of a denser medium an additional phase difference of $\pi$ or a path $\underline{\lambda}$ difference of $\overline{2}$ is introduced.)
1. Condition of constructive interference (maximum intensity):

$
\begin{aligned}
& \Delta x=n \lambda \\
\Rightarrow & 2 \mu t \cos r+\frac{\lambda}{2}=n \lambda \\
\Rightarrow & 2 \mu t \cos r=\left(n-\frac{1}{2}\right) \lambda
\end{aligned}
$


For normal incidence, i.e $r=0$, so $2 \mu t=(2 n-1) \frac{\lambda}{2}$
4. Condition of destructive interference (minimum intensity):

$
\Delta x=2 \mu t \cos r=(2 n) \frac{\lambda}{2}
$


And For normal incidence $2 \mu t=n \lambda$
- Interference in refracted light:

Net path difference between two consecutive waves in the refracted system $=\Delta x=2 \mu t \operatorname{cosr}$
1. Condition of constructive interference (maximum intensity):

$
\Delta x=2 \mu t \cos r=(2 n) \frac{\lambda}{2}
$

and For normal incidence $2 \mu t=n \lambda$
2. Condition of destructive interference (minimum intensity):

$
\Delta x=2 \mu t \cos r=(2 n-1) \frac{\lambda}{2}
$
 

For normal incidence : $2 \mu t=(2 n-1) \frac{\lambda}{2}$

Study it with Videos

Thin film interference

"Stay in the loop. Receive exam news, study resources, and expert advice!"

Books

Reference Books

Thin film interference

Errorless Physics Vol 2

Page No. : 1677

Line : 55

E-books & Sample Papers

Get Answer to all your questions

Back to top