A level physics OCR A Module 4 - Chapters 11 & 12 Waves

A level physics OCR A Module 4 - Chapters 11 & 12 Waves

What is a progressive wave?

A wave that carries energy from one place to another without transferring any material

What is a wave caused by?

Something making particles or fields or magnetic fields oscillate at a source

what do the oscillations of waves do?

They pass through the medium as the wave travels, carrying energy with them in the direction of travel

What is displacement measured in?

Metres

What is displacement?

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

What is amplitude measured in?

metres

What is amplitude?

The maximum magnitude of the displacement of a point of a wave from the the undisturbed position

What are the crests of a wave?

The positive amplitude

What are the troughs of a wave?

The negative amplitude

What is the symbol for wavelength?

λ

What is wavelength measured in?

Metres

What is the wavelength?

The length of one whole wave oscillation or wave cycle e.g the distance between two crests or troughs on a wave

What is the symbol for period?

T

What is period measured in?

Seconds

What is the period?

The time taken for one whole wave cycle

What is the symbol for frequency?

F

What are the units for frequency?

Hertz

What is the frequency of a wave?

The number of whole wave cycles per second passing a given point. Or the number of whole wave cycles given out from a source per second.

What is time phase?

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

What is phase difference?

The amount by which one wave lags behind another wave or the difference in phase between two points on a single wave

What can phase and phase difference be measured in?

Angles (degrees or radians) or in fractions of a cycle

What are the 2 types of wave?

Transverse or longitudinal

What are the oscillations like in transverse waves?

The oscillations of the particles or field is at right angles to the direction of energy transfer

Give some examples of transverse waves

All EM waves, Ripples on water, Waves on strings, S-waves

How can you work out what direction a point on a wave is going to move from a snapshot of a transverse wave?

Look at what direction the wave is travelling e.g left to right. Then look at the point and look at the wave on the side of the point closest to the source of the wave e.g the left of the point. See if the point would have to move up or down to be in that placement

What type of graphs can be drawn to show transverse waves?

They can be drawn as graphs of displacement against distance along the path of the wave or as graphs of displacement against time for a point as the wave passes

On a displacement-distance graph what does the separation between two crests show?

It gives you the wavelength of the wave

On a displacement-time graph what does the separation between two crests show?

it gives you the period of the wave

What are the oscillations like in longitudinal waves?

The oscillations are parallel to the direction of energy transfer

What are some examples of longitudinal waves?

Sounds waves, P-waves

f=1/T?

f=frequency hz(s^-1)
t=period (s)

What does a cathode ray oscilloscope produce?

A function of voltage over time from a signal generator. The displayed function is called a trace

What does the type of trace you get on an oscilloscope depend on?

The source it is connected to

If you plug an AC into an oscilloscope what trace do you get?

A trace that goes up and down in a regular pattern

On an oscilloscope what are the units for the vertical units?

Volts

What are the units for the horizontal axis?

Seconds also called the timebase

v=d/t?

v= wave speed in ms^-1
d=distance in m
t=time in s

How do you derive the wave speed equation?

Imagine how long it takes for the crest of a wave to move across a distance of one wavelength. The distance travelled is the wavelength. The time taken to travel one wavelength is the period of the wave, equal to 1/f. Then substitute these into v=d/t to get wavespeed=wavelength/1/f. This is the same as wave speed = wavelength x frequency "

v=fλ?

v= wavespeed in ms^-1
f=frequency in Hz (s^-1)
λ=wavelength in m "

What is the scientific measure of how loud something is or how bright something is?

intensity

What is intensity?

The rate of flow of energy per unit area at right angles to the direction of travel of the wave, measured in Wm^-2

I=P/A?

I=intensity in Wm^-2
P=power in W
A=area in m^2

What is intensity proportional to?

The amplitude of the wave squared

What are EM waves?

A group of transverse waves

Why are EM waves able to travel through a vacuum?

They consist of vibrating electric and magnetic fields, not matter, which means they are able to travel through a vacuum.

How do the electric and magnetic waves intersect?

They are always at right angles to each other and to the direction of travel

What are some connections between all EM waves?

they all travel at the same speed in a vacuum. They can be refracted, reflected, diffracted and can undergo interference. They can be polarised because they are transverse. They obey the wave equation: V=λf

What do all EM waves form?

A continuous series of waves called the electromagnetic spectrum

What are the EM waves from the lowest to highest wavelength?

Gamma rays, X-rays, Ultraviolet, Visible light, Infrared, Microwaves, Radio waves

What side of the electromagnetic spectrum has the highest energy?

Gamma rays

Which side of the electromagnetic spectrum has the lowest frequency?

Radio waves

Why does the energy increase as the frequency increases?

Energy of an EM wave is proportional to its frequency so gamma rays have the highest energy

At which end of the spectrum do waves become energetic enough to cause ionisation?

Towards the high end of the UV range

What is ionisation?

Where waves transfer so much energy to an atomic electron, that the electron is removed from the atom.

What happens if ionisation happens in the human body?

It can kill cells or cause cancer so the higher the frequency the more dangerous an EM wave is

What is the approximate wavelength of radio waves?

10^-1 - 10^6

What is the penetration of radio waves?

They can pass through matter

What are some uses of radio waves?

Radio transmission

What is the approximate wavelength of microwaves?

10^-3 - 10^-1

What is the penetration of microwaves?

They can mostly pass through matter but cause some heating

What are some uses of microwaves?

Radar, Microwave cooking, TV transmissions

What is the approximate wavelength of infrared waves?

7x10^-7 - 10^-3

What is the penetration of infrared waves?

They are mostly absorbed by matter, causing it to heat up

What are some uses of infrared waves?

Heat detection, Night vision cameras, Remote controls, Optical fibres

What is the approximate wavelength of visible light?

4x10^-7 - 7x10^-7

What is the penetration of visible light?

they are absorbed by matter, causing some heating

What are some uses of visible light?

Human sight, Optical fibres

What is the approximate wavelength of ultraviolet light?

10^-8 - 4x10^-7

What is the penetration of ultraviolet light?

They are absorbed by matter and cause some ionisation

What are some uses of ultraviolet light?

Sunbeds, Security marks that show up under UV light

What is the approximate wavelength of x-rays?

10^-13 - 10^-8

What is the penetration of x-rays?

They mostly pass through matter, but cause ionisation as they pass

What are some uses of x-rays?

To see damage to bones and teeth, Airport security scanners, To kill cancer cells

What is the approximate wavelength of gamma rays?

10^-16 - 10^-10

What is the penetration of gamma rays?

Mostly pass through matter, but cause ionisation as they pass

What are some uses of gamma rays?

Irradiation of food, Sterilisation of medical instruments, To kill cancer cells

What does it mean if a wave has been polarised?

The wave is only moving in one plane(one direction)

What is an unpolarised wave like?

The wave moves in a mixture of directions

What is the plane of polarisation?

The plane in which a plane polarised wave vibrates

Which type of wave can be polarised?

Transverse

How can you show that EM waves are transverse?

They can be polarised

What can be used to polarise light?

A polarising filter

What does a polarising filter do?

It will only transmit vibrations in one direction

If you have two polarising filters at right angles to each other what will happen?

No light will get through

What does the transmission axis of a polarising filter show?

The only direction of vibrations which can pass through the filter

How can you observe polarisation?

By shining unpolarised white lights through two polarising filters

How do you investigate polarisation?

Start by aligning the transmission axes of two filters so they are both vertical. Shine unpolarised light on the first filter, keep the position of the first filter fixed and rotate the second one

What polarisation will the light have when it passes through the first filter

It will always be vertically polarised

What happens when the transmission axes of two filters are aligned?

All of the light that passes through the first filter will also pass through the second filter

What happens as the second filter is rotated?

Less light will get through it as the vertical component of the second filter's transmission axis decreases. This means the intensity of the light getting through the second filter will gradually decrease

What happens when two transmission axis are at 45 degrees to each other?

The intensity will be half that getting to the first filter

When happens when two transmission axes are at right angles to each other?

No light will get through, intensity is 0

What is the relationship between light intensity and the angle of the filters

It looks like a cosine wave but goes from the max to 0 with 0 as 90 and 270

What are some uses of polarising filters?

3D films use polarised light to create depth — the filters in each lens are at right angles to each other so each eye gets a slightly different picture. Polaroid sunglasses also use polarising filters - light reflected off some surfaces is partially polarised so the sunglasses block this out to prevent glare

Why do polarising filters not work on microwaves?

Their wavelength is too long

What are used instead of polarising filters in a microwave?

Metal grilles (squares full of metal wires which are all aligned)

How do you set up the equipment to investigate the polarisation of microwaves?

Use a microwave transmitter and a microwave receiver linked to a voltmeter. Place a metal grille between the microwave transmitter and receiver.

What type of waves do microwave transmitters transmit?

Vertically polarised microwaves

What happens when microwaves meet a metal grille?

Some of their energy is absorbed by the grille

Why does some energy get absorbed by the grille?

The vibrating electric field of the microwave increases the movement and energy in electrons of the metal.