IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

# Diffraction from a CD and a DVD

## Objective

Diffraction from a CD and a DVD

## Introduction

You must have seen the bright colours on a CD/DVD surface when viewed in bright light. However, much more physics can be learnt from diffraction of a monochromatic light beam from CD/DVD surface. A CD has a large number of concentric tracks.

## Apparatus

A CD, a DVD, a laser source, a white screen

## Procedure

1. Put the CD on a horizontal surface and fix the laser torch to send the light beam obliquely on the surface. See the pattern on the screen.
• Question: What kind of pattern do you observe if the plane of incidence is parallel to the circular tracks and perpendicular to the track?
2. You will see separate bright spots when diffraction pattern is formed. Observe the separation between consecutive bright spots. This is inversely proportional to the separation between successive pits along a circular track on the CD.
3. Now send the beam on inner circles. Look at the separation between the bright spots on the screen. Shift the CD to gradually go to outer circles.
• Question: As you go towards outer circles, does the fringe separation increase or decrease?
• Question: What do you conclude about the number of pits per millimeter for inner and outer tracks. (you can makes calculation to check whether the number of tracks are equal on all tracks)
4. Now use a DVD in place of CD and do the same.
• Question: For the tracks at the same radial distance, does DVD diffraction gives narrower fringes or wider fringes as compared to the CD diffraction pattern?
• Question: Is the separation between consecutive pits on DVD larger or smaller as compared to that on the CD?

## Note

Figure to be made