# Conduct investigations to analyse quantitatively the interference of light using double slit apparatus and diffraction gratings

Thomas Young is generally credited for conduction the double slit experiment which shows evidence for wave nature of light through interference

The two slit interference can be investigated as follows:

• We would need a monochromatic(consists of only one frequency) light source, for example lasers.
• We set up the experiment such that the laser passes through two very thin adjacent slits.
• We record the the pattern formed on a screen

• When the light beams from the two slits interfere constructively, it forms a bright fringe on the optical screen.
• When the light beams from the two slits interfere destructively, it forms a dark fringe on the optical screen.

To understand the interference taking place in double slit experiment, let’s look at the following diagram :

• The distance between the two slits is d
• The two rays r1 and r2 , meet at P1 on the optical screen
• The angular displacement of the point P1 is θ
• If we consider the distance from the slits to optical screen >> d , then we can consider the rays r1 and r2 to be almost parallel
• Then the path difference (extra distance travelled) between the two rays is : d x sin θ
• For constructive interference , the path difference must be an integer multiple of the wavelength
• i.e.  , where m = 0, 1, 2, 3, 4, . . . . . .

• For destructive interference , the path difference must be
• i.e. , where m = 0, 1, 2, 3, 4, . . . . .

Extract from Physics Stage 6 Syllabus © 2017 NSW Education Standards Authority (NESA)