Interference

Thomas Young

Isaac Newton did not believe that Light could be a wave, but later aPhysicist called Thomas young showed that light travels in waves.In fact, the interference and diffraction of light cannot be explained any other way

What is Interference?

Interference is adding two waves are superposed are in the same place at the same

time

• The resultant displacement at any point is found by adding the displacement of each separate wave .

What is Interference?

Constructive Interference

Adding to waves that are in phaseFor example, if the crests (or troughs) of two light waves

are coincident, they combine together to create an amplified wave in what is known as constructive

interference.

Distractive Interference

Adding two waves that are in anti-phase In the opposite scenario, where the crests of one

wave are aligned with the troughs of another, they cancel each other out and the light

disappears. This is destructive interference.

Guitar Strings

Guitar Strings

A combination wave composed of the 1st harmonic and the third harmonic.

Standing WavesThe term standing wave is often applied to a resonant mode of an extended vibrating object. The resonance is created by constructive interference of two waves which travel in opposite directions in the medium, but the visual effect is that of an entire system moving in simple harmonic motion. The sketches illustrate the fundamental and second harmonic standing waves for a stretched string.

Standing Waves

Standing Waves

Adding reflected waves

Nodes and Antinodes

This diagram shows what the string is doing :At the Nodes, the string does not move at all, and you can see the string quite distinctly.

At the Anti Nodes the string oscillates with maximum amplitude, and so it appears as blur.

Between the nodes and anti nodes, the string would oscillate, but with less amplitude than at the anti nodes.The appearance is of a set of blurry loops.These are called stationary waves, or standing waves,Because the pattern does not move along the string

Vibrating StringThe fundamental vibrational mode of a stretched string is such that the wavelength is twice the length of the string.

Applying the basic wave relationship gives an expression for the fundamental frequency:

Calculation

Since the wave velocity is given by , the frequency expression

can be put in the form:

can be put in the form:

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