Waves

Question 1

How out of phase is this wave?

Answer 1

¼ of a cycle

• 90˚
• π/2 radians
• λ/4 wavelengths

Question 2

How out of phase is this wave?

Answer 2

½ of a cycle

• 180˚
• π radians
• λ/2 wavelengths

Question 3

What are wavefronts?

Answer 3

Lines connecting waves on the points on the wave that are in exactly the same position (e.g. at a peak/trough)

Question 4

What is it meant when two waves are coherent and what can they then do if they are coherent?

Answer 4

• They have the same frequency.
• They have a constant pase relationship.

Interference can therefore be observed.

Question 5

What is the principle of superposition?

Answer 5

• When two waves meet, the total displacement at a point is equal to the sum of the displacement on each wave at that point.

Question 6

How is a standing wave formed?

Answer 6

• 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.

Question 7

What happens to the energy transfer in a standing wave?

Answer 7

No energy is passed along the wave, as it does not move along, it only oscillates.

Question 8

What do the letters A and B represent on the diagram?

Answer 8

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.

Question 9

What are modes/harmonics on a standing wave?

Answer 9

The different modes are when the wave has different numbers of nodes/antinodes (1, 2, 3, etc.).

Question 10

What is the fundamental frequency?

Answer 10

The frequency at which the first mode occurs.

Question 11

How many nodes and antinodes are there at the fundamental frequency?

Answer 11

2 nodes and 1 antinode

Question 12

How can you work out the length of the string (when it is a standing wave) from the harmonic and wavelength?

Answer 12

length = harmonic x (λ/2)

Question 13

Describe, with a diagram, how you could produce a stationary wave on a string (3)

Answer 13

• 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

Question 14

Explain how you could use a stationary wave to determine the speed of travelling waves on a string (4)

Answer 14

• 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).

Question 15

At a node, are the waves in the same phase?

Answer 15

No, they are in antiphase.

Question 16

At an antinode, are the waves in the same phase?

Answer 16

Yes.

Question 17

What is the law of reflection?

Answer 17

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

θ_i = θr

Question 18

What is refraction?

Answer 18

When a wave changes its speed when passing from one medium to another.

Question 19

What properties of the wave change during refraction?

Answer 19

Wave speed, and therefore wavelength (its frequency is a constant).

Direction may also change.

Question 20

When light enters a denser medium, what happens to its speed?

Answer 20

It decreases.

Question 21

What are the conditions for total internal reflection?

Answer 21

1. The light is travelling from a more dense to a less dense medium.
2. The angle of incidence is greater than the critical angle.

θ_i > θ_c

Question 22

If a lense is convex, what do the rays do?

Answer 22

They converge.

Question 23

If a lense is concave, what do the rays do?

Answer 23

They diverge.

Question 24

What happens in a converging lens?

Answer 24

The rays come together at a point, called the focal point.

Question 25

What happens in a diverging lens?

Answer 25

The rays spread apart, so they will not meet at a point.

Question 26

How can you find the focal point with a diverging lens?

Answer 26

By back-tracing the rays to find a virtual focus.

Question 27

What is the focal length?

Answer 27

The distance from the lense to the focal point.

Question 28

What is a real image?

Answer 28

When light rays converge at their focul point and actually meet, the image they form is called a real image.

Question 29

What is a virtual image?

Answer 29

When light rays diverge they form a virtual image, as the light rays meet at a point on the same side of the lens as the object.

Question 30

What is polarisation?

Answer 30

The orientation of the plane of oscillation of a transverse wave. If the wave is polarised, all of its oscillations will occur in one single plane.

Question 31

What are diffraction gratings?

Answer 31

A