What does it mean to say two points on a wave are in phase?

They have the same displacement and are moving in the same direction at every instant, so their phase difference is zero or a whole number of cycles.

What does it mean to say two points are in antiphase?

They are exactly half a cycle apart, so when one is at maximum positive displacement the other is at maximum negative displacement and the two are moving in opposite directions.

State the equation linking phase difference to path difference, defining each symbol.

Δφ = 2πΔx/λ, where Δφ is the phase difference in radians, Δx is the path difference in metres and λ is the wavelength in metres.

CCEA A-Level Physics 1210

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Waves

Phase and phase difference

3 things to know
Unit AS 2

Overview

Recall questions on what in phase and in antiphase mean, the phase difference for one whole cycle in radians, the values for separations of a half and a quarter of a wavelength, the equation linking phase difference to path difference, worked conversions, and why phase difference is measured as an angle.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

What phase difference, in radians, corresponds to one whole cycle?

Key term

Two points on a wave are half a wavelength apart. State the phase difference between them.

Key term

Two points on a wave are a quarter of a wavelength apart. State the phase difference between them.

Questions from this subtopic

Each one with the model answer underneath.

1
What does it mean to say two points on a wave are in phase?
Model answer
They have the same displacement and are moving in the same direction at every instant, so their phase difference is zero or a whole number of cycles.
2
What does it mean to say two points are in antiphase?
Model answer
They are exactly half a cycle apart, so when one is at maximum positive displacement the other is at maximum negative displacement and the two are moving in opposite directions.
3
What phase difference, in radians, corresponds to one whole cycle?
Model answer
One whole cycle is a phase difference of 2π rad, which is 360°.
4
Two points on a wave are half a wavelength apart. State the phase difference between them.
Model answer
The phase difference is π rad, which is 180°, so the two points are in antiphase.
5
Two points on a wave are a quarter of a wavelength apart. State the phase difference between them.
Model answer
The phase difference is π/2 rad, which is 90°.
6
State the equation linking phase difference to path difference, defining each symbol.
Model answer
Δφ = 2πΔx/λ, where Δφ is the phase difference in radians, Δx is the path difference in metres and λ is the wavelength in metres.

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