Waves

Question 1

Define the term progressive wave.

Answer 1

A wave which transfers energy from one place to another with a wavefront which travels through the material (in contrast to stationary waves which don’t appear to move).

Question 2

What is a longitudinal wave?

Answer 2

Waves which the particles oscillate in the same direction as energy propagation.
There are rarefactions (areas of low pressure) and compressions (areas of high pressure).

Question 3

What is a transverse wave? Give examples.

Answer 3

Waves where the particles oscillations are perpendicular to the energy propagation.
For example: electromagnetic waves.

Question 4

Define frequency and give its units.

Answer 4

The number of waves passing through a point per second.
Units of Hertz (Hz) or s-1

Question 5

Define wavelength.

Answer 5

The distance between two adjacent corresponding points (eg peaks) on a wave.

Question 6

Define amplitude.

Answer 6

The maximum displacement of the wave from its equilibrium position.

Question 7

How can you find out the time period of a wave using its frequency?

Answer 7

T=1/f

Question 8

What is the phase difference of a wave and what is it measured in?

Answer 8

The amoun one wave lags behind another as a proportion of the wavelength. Measured in radians or degrees.

Question 9

What do each of the ‘axes’ on an oscilloscope measure?

Answer 9

Vertical divisions = voltage/amplitude of the wave.
Horizontal divisions = time (can be used to find time period and frequency).

Question 10

Define refraction.

Answer 10

Refraction is when a wave bends at a boundary between two materials due to the difference in density causing it to speed up or slow down.

Question 11

True of false: A wave can either be refracted or reflected at a boundary, but never both.

Answer 11

False.
At low angle of incidence most will be refracted, but some will reflect.

Question 12

True or false: Diffraction is most noticeable when the wavelength is much larger than the gap the waves is travelling through.

Answer 12

False.
The most diffraction is seen when the gap and the wavelength are the same size.
If the wavelength is much bigger the waves will be mostly reflected.

Question 13

Can all waves be polarised?

Answer 13

No. Only transverse waves.

Question 14

What is the difference between a polarised and unplarised wave?

Answer 14

Polarised waves only contains waves oscillating along one axis.
Unpolarised waves can be oscillating in any direction perpendicular to the axis of propogation.

Question 15

Describe how a ripple tank might be used to investigate diffraction.

Answer 15

*Creat water waves in the tank.
*Vary the size of a gap for them to pass through.
*Note how the direction of the waves passing through changes.

Question 16

Polarised light is being passed through a rotating polarisation filter. What would happen to the intensity of the light passing through?

Answer 16

It would vary from a maximum (all light passes through) when the axis of polarisation and the axis of the filter line up to a minimum (no light passes through) when the axes are perpendicular.

Question 17

How is intensity defined in terms of power?

Answer 17

Intensity if power/area.

Question 18

How are intensity and amplitude related?

Answer 18

Intensity is proportion to amplitude squared

Question 19

True of false: All electromagnetic waves have the same time period.

Answer 19

False.
They all travel at the same speed - but their wavelengths, frequencies and time periods vary.

Question 20

How fast do electromagnetic waves travel in a vacuum?

Answer 20

3 x 10 ^ 8 m/s

Question 21

True of False? The magnetic field and electric field in an electromagnetic wave are parallel to each other.

Answer 21

False. The electric and magnetic field are at right angles to each other.

Question 22

Put the following in order from highest to lowest frequency:

Answer 22

X-rays, Radio, Microwaves, UV, Visible

Question 23

Put the following in order from highest to lowest frequency: X-rays, Radio, Microwaves, UV, Visible

Answer 23

*Highest: X-rays
*UV
*Visible
*Microwaves
*Lowest: Radio

Question 24

Which type of EM radiation has a wavelength of approximately 1μm.

Answer 24

Infra-red is between 700nm and 1mm.

Question 25

What is the range of wavelengths commonly known as 'visible light'?

Answer 25

300-700nm

Question 26

True of false: Microwaves can be polarised using a metal grid rather than a polarising filter.

Answer 26

True - this is because the wavelength of microwaves is sufficiently large that the grid works as a polarising filter.

Question 27

What is meant by the refractive index of a material? What equation can be used to find it?

Answer 27

The refractive index is a measure of how fast light travels in a material compared to its speed in a vacuum. It is found using n=c/v Where n = refractive index, c = speed of light, v = velocity in the material

Question 28

A beam of light is shone at a boundary between air and glass. As the angle of incidence is increased form 0 to 90, what would you see?

Answer 28

At 0° all of the light would pass into the material along the normal. The light would then be seen to be refracted (the angle to the normal in the glass woudl be larger than the incident angle). Eventually the light would bend so much it would start to be reflected back.

Question 29

What is the name given to the angle of incidence at which light will reflect off a boundary rather than refracting in the medium?

Answer 29

The critical angle

Question 30

What is the relationship between the critical angle and refractive index of a medium?

Answer 30

Sin C = 1/n Where C=critican angle, n = refractive index of the material being passed into

Question 31

What is the name given to what happens to light at angles greater than the critical angle?

Answer 31

Total Internal Reflection.

Question 32

What are the two conditions needed for total internal reflection?

Answer 32

1 Angle of incidence is greater than the critical angle 2 Wave is travelling from more dense to less dense material

Question 33

Waves can 'superpose' - what does this mean?

Answer 33

Two waves in the same place (ie one on top of the other) will combine

Question 34

What are the two types of interference?

Answer 34

Constructive and destructive.

Question 35

Describe an experiment to investigate the principle of superposition using sound.

Answer 35

*Use two speakers, a moderate distance apart, connected to the same signal generator to transmit sound waves. *Walk along a line perpendicular to the speakers - you should hear alternating loud and quiet points. *This is because in some places the waves from each speaker constuctively interfere (loud) and in some places it's destructive.

Question 36

Define coherence

Answer 36

Coherent waves have the same frequency and wavelength and a fixed phase difference (often zero in exam questions).

Question 37

It wo waves are in phase will they constructively or destructively interfere?

Answer 37

Constructively.

Question 38

True of false: Path difference and phase difference are two names for the same thing.

Answer 38

False. Path difference is the difference in distance that two waves have travelled in terms of the wavelength (units of length). Phas difference is the difference in the point in the cycle of two waves as a proportion of full wave cycle (units of degrees/radians).

Question 39

Why is a LASER usefuly in showing interference and diffraction?

Answer 39

It produces monochromatic (same wavelength/colour) light.

Question 40

What is Young's double-slit experiment?

Answer 40

A single source of light directed twoards a double slit, which creates two coherent beams of light. This interferes as it hits the screen and creates an interference pattern.

Question 41

Young's double slit - describe the interference pattern created using white light

Answer 41

The interference pattern would be a repeating coloured spectrum along the screeen, with a bright white point directly in front of the slit.

Question 42

Write down the equation linking the wavelength of the source to the fringe width in a double slit experiment. Define each term in the equation.

Answer 42

Question 43

Is the following a double slit pattern, single slit pattern or diffraction grating pattern?

Answer 43

Single slit

Question 44

What two properties of light can only be explained if it is a wave?

Answer 44

*Diffraction *Interference (as seen in Young's double slit experiments)

Question 45

When shining light through a diffraction grating there is a maximum number of fringes which would be produced. How would you find this maximum number?

Answer 45

nλ = D sinθ The max angle that would produce a fringe would be 89.999999999 (so call it 90!) Re-arrange the equation for n, using θ = 90

Question 46

What is a stationary wave?

Answer 46

Stationary waves consist of alternating fixed patter of nodes (points with zero amplitude) and antinodes (points with maximum amplitude). No energy is transferred across the wave.

Question 47

What is a node?

Answer 47

A point with no vibrations in which the resultant amplitude is 0.

Question 48

What is an antinode?

Answer 48

A point with maximum vibration in which the resultant amplitude is at maximum.

Question 49

What are the conditions for a stationary wave to be produced?

Answer 49

*The waves must be coherent. *The waves must be travelling in opposite directions. These conditions are often met when a wave is reflected back on itself.

Question 50

Give an example of an experiment you could do to show a stationary wave.

Answer 50

Use an oscillator to pass a wave along a string which is fixed at one end. The station wave will form when the progressive wave is reflected off the fixed end.

Question 51

Give a similarity and a difference between stationary waves and progressive waves.

Answer 51

Similarity: Both have wavelength, frequency, amplitude. Difference: Stationary waves do not transmit energy from one place to another.

Question 52

How could you use the formation of stationary waves in a resonance tube to find the speed of sound?

Answer 52

*Create a closed end pipe using a hollow pipe inside a measuring cylinder containing water. *Use a tuning fork (producing a known frequency) and hold it above the tube. *Move the tube up until you find the first position which causes resonance. *This length will be a quarter of the wavelength. *Use speed = frequency x wavelength.

Question 53

What is meant by 'harmonics'?

Answer 53

Harmonics are points where the stationary wave form doesn't change because the waves in each direction are reinforcing each other.

Question 54

A stationary wave on a string is made to oscillate at its fundamental frequency (1st harmonic) - how many nodes and antinodes would you see?

Answer 54

Nodes - 2 (1 at either end). Antinodes - 1 (in the middle).