Interactions of Light and Matter

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Physics notes –
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 particles, we would expect to see two bright bands on the screen.

Destructive interference
results in smaller amplitude
Constructive interference is possible when the difference in distances from the two slits to the screen (i.e. path difference) is zero or equal to an integral multiple of the wavelength.

But Young observed many bright and dark bands.
I
Bright band
Dark band

i
i
i
i
i
i

P Bright
band
S1
Central
bright band
S2

Path difference pd = nλ, i.e. S2P – S1P = nλ, where n = 0, 1, 2, ...
Physics notes – Interactions of light and matter

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1

For destructive interference the path difference is an odd
multiple of half a wavelength.

i
i
i

S1

i

S2

P Dark
band

λ 
λ 
Path difference pd = n  , i.e. S2P – S1P = n  , where 2
2
n = 1, 3, 5,.....
1

Alternatively, pd =  n − λ , n = 1, 2, 3……
2

The path difference can be calculated if the separation d
between the slits and θ are known.

i
i
i
i
i

S1

x

θ

d

At high temperature (>1000 K) objects glow, e.g. the Sun, light bulbs, electric stove burners etc. The light emitted from these objects contains a wide spectrum of frequencies. In a heated object the electrons absorb thermal energy and move to

different excited states, they then return to the stable state by emitting lights of different frequencies. Due to the difference in frequencies, the emitted lights can never be in phase and the light source (the heated object) is said to be incoherent. Laser is a coherent source because all emissions have the same

frequency and occur at the same time.
Two separate sources can also be described in terms of
coherence. In Young’s experiment the two slits act as if they were coherent sources of radiation. They are described as
coherent because the light waves leaving them bear the same
phase relationship to each other at all times, e.g. if a crest is leaving...
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