Double Slit Experiment

This experiment proves that light undergoes INTERFERENCE, and hence LIGHT is a WAVE.

A thin rectangular piece of glass is painted with black paint, and TWO thin single parallel SLITS are made close to each other with a razor blade.
This allows TWO IDENTICAL (Coherent) light beams to pass through and emerge on the other side.
The light beams above are identical since they come from the same bulb.

Important Points

Coherent Source of Light
Two sources of light which maintain the same phase relationship with each other are called COHERENT.

White Light
A beam of white light is shone through the slits.

Observations
1) Diffraction occurs at EACH SLIT causing the waves to overlap.
2) The overlapping creates INTERFERENCE between the waves.
3) A band of EVENLY SPACED white and black lines are seen.

4.1) White lines are regions of Constructive Interference (antinodes)
4.2) Black lines are regions of Destructive Interference (nodes)


If RED light is used:

1) Red and Black EVENLY SPACED bands are observed.
2) Lines are more distinct


If BLUE light is used:

1) Blue and Black EVENLY SPACED bands are observed.
2) BANDS are CLOSER than in the case of RED.
This shows that BLUE has lesser diffraction and hence a lower wavelength than red.


IMPORTANT POINTS

1. What is the effect of the DISTANCE between the WIDTHS of the slits?
A smaller distance creates a wider interference pattern.

2. What does INTERFERENCE prove about light?
INTERFERENCE proves that LIGHT is a WAVE.

3. For what TYPE of wave does interference occur?
Interference occurs for BOTH TRANSVERSE and LONGITUDINAL WAVES.

4. What does INTERFERENCE of light NOT INDICATE about light?
Interference DOES NOT indicate whether LIGHT is transverse or longitudinal since interference occurs for both types of waves.