Introduction

In the 17th century, Scientists divided into two groups, one group believing that light is a stream of particles and one group believing that light is a wave.

In this set of exercises, you will explore the wave nature of light .

Waves on the surface of a pond are a sequence of peaks and troughs that move along in the shape of a sine wave. If the peaks of the two waves coincide, we say that the waves are in phase, and then the two waves reinforce each other. But if the peaks of one wave coincide with the troughs of the other the waves are said to be out of phase,and they cancel each other.

The movie above shows the interference of water waves in a pond.

Taking your own notes, explain why this pattern is formed. Use the words destructive, constructive, in phase, out of phase, crests, troughs in your answer.

• In your notes, describe the pattern you see in the 'screen' section.

• Alter the slit separation d from 2mm to 5mm. 

Describe how the fringe pattern observed on the screen is altered as the slit separation is changed.

• Alter the wavelength (Lambda) of the light from 420nm to 720nm .  It changes from blue to red. 

What happens to the separation of the fringes as the wavelength of the light increases?

• Alter the distance L between the slit and the screen from 2m to 5m. 

What happens to the separation of the fringes?

• Write notes on the three factors found experimentally to affect the fringe separation:

a)

b)

c)

The way the fringes are produced can be explained by considering two rays from the slits S1 and S2

The ray from S1 has travelled a shorter distance to the screen than the distance travelled by the ray from slit S2.

The difference between these distances is called the path difference.

If the path difference is equal to zero, or a whole number of wavelengths, the waves will arrive in phase and constructive interference will occur, producing a bright fringe.

For a path difference of a whole number of wavelengths, a bright fringe (maximum

brightness) is formed.

Path difference = nλ for maxima

(n is a zero or an integer, λ is the wavelength of the wave)

Dark bands in an interference pattern are formed by destructive interference between the two different slits S1 and S2.

This occurs when two waves are out of phase and this occurs when the path

difference between the two rays is equal to an odd number of half wavelengths.

For a path difference of an odd number of half wavelengths, a dark band (minimum

brightness) is formed.

Path difference= (n+1/2)λ for minima where n is zero or an integer.