# Unit 2 Section 3 Interference

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## Coherence and Path Difference

Coherent: Sources that have the same wavelength and frequency and a fixed phase difference between them are coherent. The troughs and crests line up - causing constructive interference and a very intense beam

Path Difference: The amount by which the path travelled by one wave is longer than the path travelled by another wave.

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## The Double-slit Formula

• w = landa x D / s
• w = fringe spacing, the distance between two adjacent maxima or two adjacement minima in m
• landa = wavelength in m
• D = distance between slits and screen in m
• s = distance between slits in m

Since the wavelength of light is so small you can see from the formula that a high ratio of D/s is needed to make the fringe spacing big enough to see.

Rearranging, you can use landa = (w x s) / D to calculate the wavelength of light.

The fringes are usually so tiny that it's very hard to get an accurate value of w.

It's easier to measure across several fringes then divide by teh number of fringe widths between them.

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## Laser Safety Precautions

if you looked at a laser beam directly, your eye lens would focus it onto your retina, which would be permanently damaged. To make sure you don't cause damage while using lasers make sure you:

• Never shine the laser towards a person.
• Are waring safety goggles.
• Avoid shining the laser beam at a reflective surface.
• Have a warning sign on display.
• Turn the laser off when not it's not needed.
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