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Diffraction

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# Diffraction - PowerPoint PPT Presentation

Diffraction – The bending of a wave around a barrier, such as an obstacle or the edges of an opening. Diffraction. Diffraction. Waves spread as they pass an obstacle or through an opening. Diffraction.

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Presentation Transcript
Diffraction – The bending of a wave around a barrier, such as an obstacle or the edges of an opening.
Diffraction

Waves spread as they pass an obstacle or through an opening

Diffraction

Diffraction is most when the opening or obstacle is similar in size to the wavelength of the wave

Diffraction

Diffraction is most when the opening or obstacle is similar in size to the wavelength of the wave

Diffraction

That’s why we can hear people around a wall but not see them!

Huygen’s Principle – Every point on any wave front can be regarded as a new point source of secondary waves.

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Single Slit Diffraction

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• When light passes through an aperture (the pupil) it is diffracted.
• If we look at two point sources there are two diffraction patterns and these diffraction patterns overlap.
• The diameter of the aperture, the wavelength of light and the source separation determine the amount of overlap.
• If the two diffraction patterns overlap too much the two point sources will not be resolved. The will appear as one.
• Example distance headlights from a car.

Rayleigh criterion – If the first minimum of one image coincides with the central maximum of the other then the two sources will just be resolved.

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θ is measured in radians

The Doppler Effect is a change in frequency (pitch) due to the relative motion of the sound source and observer.
• As the sound and listener approach each other the frequency is higher.
• As the sound and listener move away from each other the frequency is lower.
• The Doppler effect also occurs with light producing the red and green shift of distant stars.
• Doppler radar is used to track weather systems
Example

A driver honks a car horn as he approaches a listener with a speed of 25 m/s. What frequency does the listener hear as the car approaches and after it has passed him? The frequency as heard by the driver of the car is 220Hz.

Example

A sound wave of frequency 300 Hz is emitted towards an approaching car. The wave is reflected from the car and is then received back at the emitter with a frequency of 315 Hz. What is the speed of the car? (Take the speed of sound to be 340 m.s-1).

The car acts as a receiver:

Next, due to reflection the car acts as a source: