4.4- Waves

Define the term progressive wave?

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

What is a longitudinal wave?

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).

What is a transverse wave? Give examples.

Waves where the particle oscillations are perpendicular to the direction of energy transfer.

For example: electromagnetic waves.

Define frequency and give its units

The number of waves passing through a point per second. (hertz) Hz

Wavelength

The distance between two corresponding parts of a wave

Define amplitude

The maximum displacement of the wave from the equilibrium position

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

T = 1/f

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

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

What do each of the 'axes' on an oscilloscope measure?

Vertical divisions = voltage/amplitude of the wave, Horizontal divisions = time

Define refraction

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.

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

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

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

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.

Can all waves be polarised?

No. Only transverse waves.

What is the difference between a polarised and unpolarised wave?

Polarized waves only contain waves oscillating along one axis.

Unpolarized waves can be oscillating in any direction perpendicular to the axis of propagation

Describe how a ripple tank might be used to investigate diffraction

Create water waves in tank

Vary the size of a gap for them to pass through

Note how the direction of the waves passing through changes

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

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

How is intensity defined in terms of power?

Intensity is power / area

How are intensity and amplitude related?

intensity is proportional to amplitude^2

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

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

How fast do electromagnetic waves travel in a vacuum?

3 x 10^8 (speed of light)

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

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

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

X - rays

UV

Visible

Microwaves

Radio waves

What type of EM radiation has a wavelength of approximately 1 x10 -6 m?

Infrared is between ~ 700nm and 1mm

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

300-700nm

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

True. This is because the wavelength of microwaves is sufficiently larger than the grid works as a polarizing filter.

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

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

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

At 0 degrees 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 would be smaller than the angle of incidence)

Eventually the light would start to bend so much it would reflect back.

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

The critical angle.

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

Total internal reflection.

Waves can 'superpose' - what does this mean?

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

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

Use 2 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 constructively inerfere (loud) and in some places its destructive (quiet)

Define coherence

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

if 2 waves are in phase will they constructively interfere or destructively?

Constructively.

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

False.

Path difference is the difference in distance that 2 waves have travelled in terms of wavelength (units of length)

Phase difference is the difference in the point of cycle of 2 waves as a proportion of a full wave cycle (degrees or radians)

Why is a laser useful in showing interference and diffraction?

It produces monochromatic light (same wavelength/color)

What is Young's double slit experiment?

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

Increasing the slit width increases the width of the central diffraction maximum. True or False?

True.

What equation relates the wavelength of light to the slit spacing and distance to the screen?

wavelength = ax/d

a = slit spacing, x = fringe spacing, d = distance to screen

Which 2 properties of light can only be explained if its a wave?

Diffraction

interference

What is a stationary wave?

Stationary waves consist of alternating fixed pattern of nodes (points with zero amplitude) and antinodes (points with maximum amplitude).

No energy is transferred across the wave.

What is a node?

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

What is an antinode?

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

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

Coherent waves Traveling in opposite directions

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

Use an oscillator to pass a wave along a string which is fixed at one end.

The stationary wave will form when the progressive wave is reflected off the fixed end.

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

Similarity: Both have wavelength, frequency and amplitude

Difference: Stationary waves don't transmit energy from one place to another

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

Create a closed end pipe using a hollow pipe inside a measuring cylinder containing water

Use a tuning fork 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

What is meant by 'harmonics'?

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

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

Nodes - 2 (1 at either end)

Anti-nodes - 1 (in the middle)