Diffraction:

the spreading out of waves (as they pass through a gap)

Constructive Interference:

When waves from coherent sources meet in phase (or path diff is nλ), waves reinforce and the resultant wave has increased displacement/amplitude

Destructive Interference:

When waves from coherent sources meet in antiphase or 180° phase diff. (or path diff is (n+1/2λ), waves reinforce and the resultant wave has reduced or zero displacement/amplitude

Principle of superposition:

When two waves meet/interfere the sum of displacements (= resultant displacement)

Coherent source:

wave sources that have a constant phase difference

Phase:

the fraction of the wave cycle which has elapsed relative to the origin Phase denotes the particular point in the cycle of a waveform, measured as an angle in degrees.

Phase difference:

the angle and the shape difference between the two waves

Path difference:

difference in length between paths from the sources difference in length between detector and each wave source difference between the actual lengths traversed by the two waves in space. 

State what is meant by a standing wave and explain how it is formed.

It consists of nodes and antinodes / it does not transfer energy formed by two identical waves travelling in opposite directions (meeting/superposing) waves must be COHERENT e.g. (microwaves leaving transmitter) interfere (with reflected waves)

Suggest how the appearance of the interference pattern would change if coherent white light sources were used instead of the monochromatic sources.

central white fringe other fringes are colored (a spectrum is formed with blue and red ends)

Interference FAQ:

1. What is a phase shift, or phase difference?

Consider two identical waves. If the waves don’t exactly line up (for instance, crest aligned with crest in a sine wave), then there is a phase shift, or phase difference, between them.

Phase shift can be measured in radians, degrees, or fractions of a wavelength. A 360˚ phase shift (same as 2π radians) is a shift of one wavelength. Since this is the repeat distance of the wave, the waves are still aligned. 180˚ phase shift (same as π radians) is a shift of one-half wavelength. Now the crest of one wave lines up with the trough of the other.

2. What does phase difference have to do with interference?

If the crests of the waves line up (phase shift of 0, λ, 2λ, etc) the resulting wave is bigger, giving constructive interference. If a crest lines up with a trough (phase shift of λ/2, 3λ/2, etc), there is cancellation (destructive interference)

3. Why does a difference in path length cause a phase shift between the waves traveling those paths?

If two identical waves travel different distances to reach the same point, one will have progressed through more (repeat distances) wavelengths than the other. If the waves started out in phase, they will no longer be in phase at their destination. They will be out of phase by the (fractional) number of wavelengths that fit in the path length difference.