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DIFFRACTIONDr Spencer WheatonDept. of Physics University of Cape Town

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Huygens’ Principle:Every point on a wave front acts as a source of tiny wavelets that move forward with the same speed as the wave; the wave front at a later instant is the surface that is tangent

to the wavelets

1. Huygens’ Principle

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Huygens’ Principle:Plane Waves

Huygens’ Principle:Circular Waves

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2. DiffractionDiffraction is the bending of waves around

obstacles or the edges of an opening:

Diffraction of water waves

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In fact, the intensity of the sound varies with position (see later)

Diffraction of sound waves

No Diffraction:

Diffraction:

Can hear around corners!

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Diffraction is an interference effect

explained by Huygens’ Principle

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Question: Why is it that you hear sound around corners but don’t see around

corners?Answer: Diffraction is only appreciable when the opening or obstacle is of comparable size

to wavelength of wave (light has a tiny wavelength)

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Almost no diffraction

Little diffraction

Large diffraction

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Diffraction of light

Light is bent around obstacles and openings too!

Intensity varies with position

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Bright and Dark Bands(interference)

3. Diffraction of Light by Single-Slit

Without DiffractionWith Diffraction

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Central Bright Fringe

Constructive interference

Waves in phase

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First Dark Fringe

Destructive interference

Waves out of phase

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Second Dark Fringe

Destructive interference

Waves out of phase