March 30, 2022
In this chapter of AS/A-level Physics, we will learn about waves in phase and introduce stationary waves .
Phases of Waves
In AS/A-level Physics, in phase means in step - two points in phase interfere constructively
- Two points on a wave are in phase if they are both at the same point in the wave cycle
- Points in phase have the same displacement and velocity (360° = 2π radians)
- A&B = In phase
- A&C = Out of phase
- Two points with a phase difference of 0 or a multiple of 360° (full cycle) are in phase
- Points with a phase difference of odd number – multiples of 180 (π radians, or a half cycle) are exactly out of phase
- Two different waves can be considered --> happens due to both waves coming from same oscillators therefore have a phase difference between each other
- In AS/A-level Physics, to get interference patterns the two sources must be coherent
- Can still happen if there’s a different wavelength and frequency, its all just a jumble --> two sources are coherent if they have the same WAVELENGTH, FREQUENCY and a FIXED PHASE DIFFERENCE
- Can get destructive interference when there is a p.d of half a wavelength, one and a half wavelength two and a half etc. = out of phase
- Amount by which the path travelled by one wave is longer than the path travelled
- In AS/A-level Physics, Constructive when P.D=n𝜆
Stationary Waves
In AS/A-level Physics, progressive waves reflected at a boundary --> stationary (standing) wave is the superposition of two progressive waves with the same frequency (wavelength), moving in opposite direction
- No energy is transmitted by a stationary wave --> demonstrated by setting up a driving oscillator at one and of a stretched string with the other end fixed --> wave generated but the oscillation is reflected back and forth
- Most frequency the resultant pattern is a jumble --> but if oscillators produce an exact number of waves in the time it takes for the wave to get to the end and back
- At these ‘resonant frequencies’ you get a stationary wave where the pattern does not move. It just bobs up and down
This is the end of the topic!