# Wave interference - P6

Bullet pointed information on wave interference

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## Disturbance

• When two waves meet, they both cause their own disturbances and their effects combine (interference)
• Two waves arriving in step means they disturb in the same direction and so reinforce each other (constructive interference)
• Two waves arriving out of step will disturb in the opposite direction, and cancel out, known as destructive interference
• Total amplitude of the waves at a point is the sum of the individual displacements at that point, taking into account direction
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## Interference of light

• Can be observed by diffracting two identical light beams through a pair of narrow slits, a fraction of a millimetre apart
• You will see several bands of light and dark bits on the screen if you do this experiment in a dark room
• At certain points on the screen, the light waves arrive from the two slits in phase, and so you get constructive interference
• During constructive interference, the amplitude of the waves is doubled, getting a bright band of light
• Constructive interference occur when the distance travelled by the waves from both slits is either the same or different by a whole number of wavelengths
• At certain other points, the light waves will be exactly out of phase, causing destructive interference
• During destructive, the waves cancel out and you see a dark band
• Destructive occrus when the difference in the distance travelled by the waves is 0.5 wavelength, 1.5 wavelengths etc
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## Light and sound waves

• These must travel as waves
• Light and sound can both diffract and interfer, showing that they have wave properties
• If light and sound acted like little particles, the particles would pass straight through a narrow gap
• If they were blocked by an object, they wouldn't be able to 'bend' around it and would either be absorbed, transmitted or reflected back
• If they just acted like particles, there wouldn't be any positive or negative disturbances to interfere with one another, and so you wouldn't get interference patterns either
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