How out of phase is this wave?
¼ of a cycle
- 90˚
- π/2 radians
- λ/4 wavelengths
How out of phase is this wave?
½ of a cycle
- 180˚
- π radians
- λ/2 wavelengths
What are wavefronts?
Lines connecting waves on the points on the wave that are in exactly the same position (e.g. at a peak/trough)
What is it meant when two waves are coherent and what can they then do if they are coherent?
- They have the same frequency.
- They have a constant pase relationship.
Interference can therefore be observed.
What is the principle of superposition?
- When two waves meet, the total displacement at a point is equal to the sum of the displacement on each wave at that point.
How is a standing wave formed?
- When two continuous waves travelling in opposite directions superpose continuously, this can set up a standing wave (or stationary wave).
- This happens whem the waves are of the same speed and frequency, and have similar amplitudes, and a constant phase relationship.
What happens to the energy transfer in a standing wave?
No energy is passed along the wave, as it does not move along, it only oscillates.
What do the letters A and B represent on the diagram?
A) Node - a point on the wave where there is no oscillation.
B) Antinode - a point on the wave where there is continuous vibrations with maximum amplitude.
What are modes/harmonics on a standing wave?
The different modes are when the wave has different numbers of nodes/antinodes (1, 2, 3, etc.).
What is the fundamental frequency?
The frequency at which the first mode occurs.
How many nodes and antinodes are there at the fundamental frequency?
2 nodes and 1 antinode
How can you work out the length of the string (when it is a standing wave) from the harmonic and wavelength?
length = harmonic x (λ/2)
Describe, with a diagram, how you could produce a stationary wave on a string (3)
- By applying tension to the string, the vibrations from the signal generator would produce a stationary wave.
- The frequency/tension/length could then be varied until a stationary wave occurs
Explain how you could use a stationary wave to determine the speed of travelling waves on a string (4)
- My measuring the spacing in between the nodes and doubling it, you would have the λ.
- By recording the frequency from the signal generator, you would know the frequency.
- The equation v = fλ would allow you to find the speed (in ms-1).
At a node, are the waves in the same phase?
No, they are in antiphase.
At an antinode, are the waves in the same phase?
Yes.
What is the law of reflection?
A light ray incident on a reflective surface will be reflected at the same angle to the normal to the surface.
angle of incidence = angle of reflection
θi = θr
What is refraction?
When a wave changes its speed when passing from one medium to another.
What properties of the wave change during refraction?
Wave speed, and therefore wavelength (its frequency is a constant).
Direction may also change.
When light enters a denser medium, what happens to its speed?
It decreases.
What are the conditions for total internal reflection?
- The light is travelling from a more dense to a less dense medium.
- The angle of incidence is greater than the critical angle.
θi > θc
If a lense is convex, what do the rays do?
They converge.
If a lense is concave, what do the rays do?
They diverge.
What happens in a converging lens?
The rays come together at a point, called the focal point.
