superposition and coherence

Simple defi of superposition

When waves meet, you must add their individual displacement together (vector sum)

• If two peaks (+2cm and +2cm) meet, they "combine" to make a super-peak of +4cm.

• If a peak (+2cm) meets a trough (-2cm), they "combine" to make 0cm (flat).

Define superposition

Where the displacements of two waves are combined as they pass each other, the resultant displacement is the vector sum of each waves displacement

What is phase difference

The angular distance (units: radians or degrees) one wave lags behind another wave

(Same as path difference just different units)

What is path difference

The difference in distance, in metres, travelled by two waves.

(How much one wave lags behind the other)

What is the equation linking phase and path difference and wave length

Phase difference (rad) = path difference (m)/wavelength (m) x2pi

What is a coherent light source?

A source that has the same frequency and wavelength and a fixed phase difference

What does monochromatic mean?

Same wavelength/ colour

What is an example of light which is coherent and monochromatic?

Lasers

Define interference.

The interaction where waves overlap

Define ‘in phase’

When two waves are the exact same point in their cycle at the same time

Peak aligns with peak; trough aligns with trough

Has a phase difference of 0 or an integer multiple of 360 degrees (so even multiple of 180 degrees)- e.g. 2 × 180=360 , 4×180=720

So if the phase difference is 720 degrees between two waves that means they are in phase

Constructive interference

Define ‘out of phase’

When two waves reach the same point in their cycle at different times- occurs when one wave is exactly half a cycle behind the other

Peak aligns with trough

Phase difference of 180 degrees or odd multiple of 180. (E.g. 1×180=180, 3×180=540)- just divide the phase difference (540) by 180 to find the multiple (3 therefore out of phase)

So if 2 waves have a phase difference of 540 that means they are out of phase

Destructive interference

Define constructive interference

When two waves reinforce each other, creating a combined wave with a larger amplitude

Define a destructive interference

When two waves cancel each other out, creating a combined wave with a smaller or 0 amplitude