Interactions of light and matter
Young was able to explain this result as a wave-interference phenomenon – the double-slit interference pattern demonstrates the wave-like nature of light.
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Explaining the interference pattern using the wave model
Light has been described both as a particle and as a wave.
Isaac Newton (~1665) made up a particle model of light to explain many of the known behaviours of light at that time. He was able to explain
- straight line propagation of light
- the intensity of light
- the reflection of light from flat and curved surfaces
- the refraction of light as it crosses the interface between two media.
The single slit provides the double slits with coherent light waves (Refer to page 2).
The bright bands are formed when light waves from the two slits arrive at the screen in phase, i.e. wave crest combines with crest and wave trough combines with trough. This is known as constructive interference. The following diagram shows the sum of the two waves as a function of time at a bright band.
Amplitude
He was unable to explain
- partial reflection and partial transmission of light at an interface - the existence of Newton’s rings and other related phenomena due to the interference of light.
0
Time
Christiaan Huygens (~1678) considered light as a wave. Using a wave model he was able to explain all the known phenomena of light mentioned above as well as interference and diffraction of light.
Constructive interference results in greater amplitude
The dark bands are formed when the two waves arriving at the screen are half a cycle out of phase, i.e. wave crest combines with wave trough. This is called destructive interference.
The following diagram shows the sum of the two waves at a dark band.
Young’s double-slit experiment, 1801
Screen
Amplitude
Sunlight
0
Time
If light consists of