A Level Physics: Waves

The oscillation of particles or fields

What is a wave?

... energy from one place to another without transferring any material

A progressive wave carries ...

One complete vibration of the wave

What is a cycle?

How far a point on the wave has moved from its undisturbed position

What is displacement?

The maximum magnitude of displacement

What is amplitude?

The length of one whole wave cycle

What is wavelength?

The time taken for a whole cycle to complete

What is a period?

The number of cycles per secon passing a given point

What is frequency?

A measurement of the position of a certain point along the wave cycle

What is phase?

The amount one wave lags behind another

What is phase difference?

Angles and fractions of a cycle

In which two ways are phase and phase difference measured?

A wave is bounced back when it hits a boundary

What happens in reflection?

A wave changes direction as it enters a different medium

What happens in refraction?

It slows down or speeds up

Why does a wave change direction in refraction?

1 / period

Frequency \=

Hz and s^-1

What two ways is frequency measured in?

wavelength x frequency

Speed of wave \=

Transverse waves vibrate ...

... at right angles to the direction of energy transfer

What type of wave are all EM waves?

Transverse

Give 3 examples of transverse waves.

EM waves, ripples on water and waves on strings

On which two types of graphs can transverse waves be shown?

Displacement-distance and displacement-time

Longitudinal waves vibrate ...

... along the direction of energy transfer

Give an example of longitudinal waves.

Sound waves

A sound wave consists of alternate ...

... compressions and rarefactions of the medium it's travelling through

Why can't sound go through a vacuum?

Longitudinal waves must vibrate the medium it's travelling through which can't happen if there's no medium

On what type of graph are longitudinal waves often plotted on?

Displacement-time

Polarisation can only happen for ...

... transverse waves

In the past, what type of wave was light thought to be?

Longitudinal

What did Etienne-Louis Malus discover in 1808?

That light was polarised by reflection

What did Young suggest in 1817?

That light was a transverse wave of vibrating electric and magnetic fields at right angles to the transfer of energy

What do polarising filters do?

Only transmit vibrations in one direction

Ordinary light waves are a mixture of ...

... different directions of vibration

When reflected from some surfaces, light ...

... becomes partially polarised- some of it vibrates in the same direction

Give 3 examples of household items that use polarisation.

Polaroid sunglasses, television aerials and radio aerials

Superposition happens when ...

... two or more waves pass through each other

What is the principle of superposition?

When two or more waves cross, the resultant displaccement equals the vector sum of the individual displacements

When two crests meet and a supercrest is formed, what type of interference is this?

Constructive

When two troughs meet and a supertrough is formed, what type of interference is this?

Constructive

When a trough and a crest meet and cancel each other out, what type of interference is this?

Destructive

For interference to be noticeable ...

... the two amplitudes should be nearly equal

What type of interference occurs when two waves are in phase?

Constructive

What does it mean when two waves are in phase?

They are at the same point in the wave cycle

Points in phase have the same ...

... displacement and velocity

If two points have a phase difference of an odd-number multiple of 180­°, they are ...

... exactly out of phase

In practice, what is the main reason why two in phase?

They are from the same oscillator

In order to get clear interference patterns, the two or more sources must be ...

... coherent and in phase

What does it mean when two waves are coherent?

They have the same wavelength and frequency and a fixed phase difference between them

What is the path difference?

The difference in distances from two coherent sources to an interference fringe

At any point an equal distance from two coherent and in phase sources, you will get ...

... constructive interference

Constructive interference occurs when path difference \=

nλ

Destructive interference occurs when path difference \=

(n + 1/2)λ

What is a stationary wave?

The superposition of two progressive waves with the same frequency, moving in the opposite direction

Progressive waves reflected at a boundary can create a ...

... stationary wave

Unlike progressive waves, what is not transmitted in a stationary wave?

Energy

How could you demonstrate a stationary wave?

Setting up a driving oscillator at one end of a stretched string with the other end fixed. The wave generated by the oscillator is reflected back and forth

For most frequencies, what is the resultant pattern of a stretched string in a driving oscillator?

Jumbled

How would the oscillator cause the original and reflected waves to reinforce each other?

If the oscillator happens to produce the exact number of waves in the time it takes for a wave to get to the end and back again

What is the name of the frequency you get when the original and reflected wave reinforce each other perfectly?

Resonant frequency

In stationary waves, in what direction to the string do the particles vibrate?

At right angles

What is a node?

Where the amplitude is zero

What is an antinode?

Where there is maximum amplitude

At resonante frequencies, an exact number of ...

... half wavelengths fit onto the string

Describe the first harmonic.

A stationary wave vibrating at the lowest possible resonant frequency with one "loop" and a node at each end

Describe the second harmonic.

A stationary wave vibrating twice the frequency of the first harmonic with two "loops" and a node in the middle an at each end

Describe the third harmonic.

A stationary wave vibrating three times the frequency of the first harmonic with 1.5 wavelengths fitting on the string

The longer the string, the lower the resonant frequency ...

... the half wavelength at the resonant frequency is longer

The heavier the string, the lower the resonant frequency because ...

... waves travel more slowly down the string

The looser the string, the lower the resonant frequency because ...

... waves travel more slowly down a loose string

What is diffraction?

The way waves spread out as they come through a narrow gap or go round obstacles

What is the amount of diffraction dependent on?

The wavelength of the wave compared to the size of the gap

When the gap is a lot bigger than the wavelength, diffraction is ...

... unnoticeable

When the gap is several wavelengths wide, diffraction is ...

... noticeable

When happens when the gap is the same size as the wavelength?

The most diffraction occurs

What happens if the gap is smaller than the wavelength?

The waves are mostly reflected back

What kind of light source is needed to observe a clear diffraction pattern?

Monochromatic and coherent

Give an example of a light source that would be appropriate for light diffraction observation.

A laser

What is the name of the central bright fringe in a diffraction pattern?

The central maximum

What are the dark and bright fringes in a diffraction pattern caused by?

Destructive and constructive interference of light waves

What is white light?

A mixture of different colours, each with different wavelengths

What happens to white light when shone through a single narrow slit?

All the different wavelengths are diffracted by different amounts, so you get a spectra of colours instead of clear fringes

Describe the diffraction pattern of white light.

A bright white central maximum and the fringes have red on the outside and violet on the inside

Why is the central maximum the brightest part of a single slit light diffraction pattern?

Because the intensity of light is highest in the centre

What is intensity?

Power per unit area

For monochromatic light, all photons have the same energy, so an increase in intensity means ...

... an increase in the number of photons per second

In terms of photons, why is the central maximum the brightest?

There are more photons per unit area hitting the central maximum per second than the other bright fringes

If slit width is increased, what happens to the amount of diffraction?

It decreases

If slit width is increased, what happens to the central maximum?

It gets narrower and intensity gets higher

If wavelength is increased, what happens to the amount of diffraction?

It increases

If wavelength is increased, what happens to the central maximum?

It gets wider and intensity gets lower

To demonstrate two-source intereference in water and sound, what is needed?

A coherent source from the same oscillator which drives both sources

How could you demonstrate two-source interference in water?

One vibrator driving two dippers

How could you demonstrate two-source interference with sound?

One oscillator connected to two loudspeakers

How could you demonstrate two-source interference with light?

Two coherent light sources, or Young's double-slit experiment

Describe Young's double-slit experiment.

Shine a coherent and monochromatic laser through two slits onto a screen. The slits must be the same size as the wavelength so the light is diffracted, then the slits will act as two coherent point sources

What was different between the current double-slit experiment and Thomas Young's original one?

He used a lamp rather than a laser

Why is laser light dangerous?

It is a very powerful and direct beam of light and looking at it directly can cause permanent damage to your retina

Name 5 safety measures when using lasers.

Never shine the laser towards a person, wear laser safety goggles, avoid shining on a reflective surface, have a warning sign displayed, turn off the laser when unused

Fringe spacing, w \=

λD/s

How did Young's double-slit experiment prove the wave nature of light?

It showed waves could diffract and interfere, which were both properties of waves

What is the difference between performing the diffraction experiment with a double-slit and with a diffraction grating?

With a diffraction grating, the bright bands are brighter and the dark bands are darker