Unit 1 - Chapter 5

AQA GCSE PHYSICS

Waves

Keywords of this chapter

● Transverse wave - Wave in which the vibrations are perpendicular to the

direction of energy transfer.

● Oscillation - Moving to and fro about a certain position along a line.

● Perpendicular - At right angles.

● Longitudinal wave - Wave in which the vibrations are parallel to the direction of

energy transfer.

● Compression - Squeezed together.

● Rarefaction - Stretched apart.

● Electromagnetic wave - Electric and magnetic waves disturbance that transfers

energy from one place to another. The spectrum of electromagnetic waves, in

order of increasing wavelength, is as follows: gamma and X-rays, ultraviolet

radiation, microwaves, radio waves.

● Mechanical wave - Vibration that travels through a substance.

More keywords...

● Amplitude - The height of a wave crest or a wave trough of a transverse wave

from the rest position.

● Wavelength - The distance from one wave crest to the next wave crest (along

the waves).

● Frequency - The number of wave crests passing a fixed point every second.

● Speed - Distance moved / time taken

● Plane mirror - A flat mirror.

● Normal - Straight line through the surface or boundary perpendicular to the

surface or boundary

● Angle of incidence - Angle between the incident ray and the normal.

● Angle of reflection - Angle between the reflected ray and the normal.

● Real image - An image formed where light rays meet.

A few more words...

● Virtual image - An image, seen in a lens or a mirror, from which light rays

appear to come after being refracted by the lens or reflected by the mirror.

● Boundary - Line along which two substances meet.

● Refraction - The change of direction of a light ray when it passes across a

boundary between two transparent substances (including air).

● Diffraction - The spreading of waves when they pass through a gap or around

the edges of an obstacle that has a similar size as the wavelength of the wave.

● Sound - A form of mechanical energy.

● Echo - Reflection of sound that can be heard.

● Pitch - The pitch of a sound increases if the frequency of the sound waves

increases.

The nature of waves● Youtube videos: https://www.youtube.com/watch?v=egcB0NbKPQI

● https://www.youtube.com/watch?v=GNPlcoVXL7M

https://www.youtube.com/watch?v=9LkLj8TS9VI&feature=iv&src_vid=UR2rjO0TkU0&annotation_id=annotation_2659666575

● We use waves to transfer energy and transfer information.

● There are different types of waves including mechanical waves and electromagnetic waves.

● Electromagnetic waves can all travel through a vacuum at the same speed of 300 000 kilometres per second. The waves cover a

continuous range of wavelengths called the electromagnetic spectrum.

● Sound waves, water waves and seismic (produced by earthquakes) waves are all examples of mechanical waves. Meaning these

waves are vibrations which travel through a medium (substance) and cannot travel through a vacuum.

Google definition of waves:

Transverse waves

The vibrations of a transverse wave are perpendicular to

the direction in which the waves transfer energy.

The amplitude is the displacement

from the rest position to the crest.

The wavelength is the length of a

full cycle of the wave (from one

crest to the next).

Frequency is the number of

complete waves passing a certain

point per second or the number of

waves produced by a source each

second. Frequency is measured in

hertz (Hz). 1 Hz is 1 wave per

second.

Examples of transverse waves:

1. Light and all other EM waves

2. Ripple on water.

3. Waves on strings.

4. A slinky spring wiggled up and down.

Longitudinal wave

The vibrations of a longitudinal wave is parallel to the direction

in which the waves are travelling.

Examples of longitudinal waves:

1. Sound waves and ultrasound.

2. Shock waves, e.g. seismic waves.

3. A slinky spring when you push the end.

Vibrations in same direction as

wave is traveling

The wavelength of a

longitudinal wave is the

distance from the middle of

one compression to the

middle of the next

compression. This is the

same as the middle of one

rarefaction to the middle of

the next rarefaction.

The frequency of a

longitudinal wave is the

number of compressions

passing a point in one

second.

Equation for the speed of a wave

The speed of the wave is

usually independent of the

frequency or amplitude of

the wave.

All electromagnetic waves

travel at about the speed of

300 000 Km.

For waves of constant frequency, the speed of the

waves depends on the frequency and the wavelength

as follows:

Key points

● We use waves to transfer energy and transfer information.

● Transverse waves vibrate at right angles to the direction of energy transfer of the

waves. All electromagnetic waves are transverse waves.

● Longitudinal waves vibrate parallel to the direction of energy transfer of the waves.

A sound wave is an example of a longitudinal wave.

● Mechanical waves, which need a medium (substance) to travel through, may be

transverse or longitudinal waves.

● For any wave, its amplitude is the height of the wave crest or the depth of the wave

trough from the position at rest.

● For any wave, its frequency is the number of wave crests passing a point in one

second.

● For any wave, its wavelength is the distance from one wave crest to the next wave

crest. This is the same as the distance from one wave trough to the next wave

trough.

● V = f x λ

The electromagnetic spectrumThe electromagnetic spectrum extends from high frequency or high energy

(short wavelength 10^-15 m) waves, e.g. gamma rays, to low frequency or

low energy (long wavelengths 10^4m) waves, e.g. radio waves.

Visible light is one type of electromagnetic radiation and is the only part of the

electromagnetic spectrum that can be seen with the eye. It consists of seven

primary band of colour from red to violet.

Long wavelengths, low

frequencyShort wavelengths, high

frequency

Communication

● Different parts of the electromagnetic spectrum can be used for communication.

● Radio waves - used for television and radio signals allowing communication across the Earth.

● Microwaves - Used for mobile phone networks and satellite communication (although there are

potential risks of using mobile phones, e.g. possible links with brain tumourS) and can be used for

cooking - water molecules absorb microwaves and heat up.

● Infrared - Used for remote controls for televisions. Also used in grills, toasters and radiant heaters

(e.g. electric fires). Infrared is also used for optical fibre communication.

● Visible light is used for morse code with torches, photography and fibre optics.

Reflection 1

Youtube video: https://www.youtube.com/watch?v=iUguHQMf0i8

● All waves can be reflected.

● Reflection of light is what allows us to see objects. Light bounces off them into

our eyes.

● When light travelling in the same direction reflects from an uneven surface

such as a piece of paper, the light reflects off at different angles. However,

when a light travelling in the same direction reflects from an even surface

(smooth and shiny like a mirror) then it’s all reflected at the same angle and

you get a clear reflection.

● A plane (flat) mirror enables you to see an exact mirror image of yourself.

Google definition of

reflection.

Reflection 2

Angle of incidence = Angle of reflection

The normal is a line perpendicular to

the mirror at the point where the

incident ray hits the mirror.

The angle of incidence is the angle

between the incident ray and the

normal.

The angle of reflection is the angle

between the reflected ray and the

normal.

Image formation by a plane mirror

This ray diagram shows the path of two

light rays from an object that reflect off

the mirror. The image and the object

are at equal distances from the mirror.

The image formed by a plane mirror is virtual,

upright (the same way up as the object) and

laterally inverted (back to front but not upside

down). A virtual image can’t be projected on the

screen like the movie images that are seen in the

cinema. An image on a screen is described as a

real image because it is formed by focusing light

rays on the screen.

A real image is one that can be formed on a screen,

because the rays of light that produce the image

actually pass through it.

A virtual image cannot be formed on a screen,

because the rays of light that produce the image only

appear to pass through it.

Refraction ● Youtube:

https://www.youtube.com/watch?v=7aU8sX8cFNs,

https://www.youtube.com/watch?v=dwu3Bv9KARw,

https://www.youtube.com/watch?v=QoZA9EwUt-M

● Refraction is a property of all forms of waves

including light and sound.

● When a wave crosses a boundary between two

substances (e.g. from air to glass) it changes

direction.

● A light ray changes directions towards the normal

when it travels from air into glass. The angle of

refraction (r) is smaller than the angle of incidence (i).

This is because the light slows down.

● The light ray changes direction away from the normal

when it travels from glass to air. This is because the

light ray speeds up. The angle of refraction (r) is

greater than the angle of incidence.

● Wave change speed when they cross a boundary

between different substances. The wavelength of the

wave also changes, but the frequency stays the

same.

● The change in speed of the waves causes a change

in direction.

● If the wave is travelling along a normal, then it will not

change direction.

Glass block

Ray

box

Normali

r

Refraction by a prism

A narrow beam of white light passes through a

triangular glass prism where the beams comes out of

the prism in a different direction to the incident ray and

is split into the colours of the spectrum. This is called

dispersion.

White light contains all the colours of the spectrum.

Each colour of light is refracted slightly differently. So

the prism splits the light into colours.

Violet light is refracted the most and red light is

refracted the least.

Diffraction

● Youtube: https://www.youtube.com/watch?v=UR2rjO0TkU0

https://www.youtube.com/watch?v=-QYQDJ7CSVQ,

https://www.youtube.com/watch?v=POMsF0Fd_SE

● Diffraction is a property of all waves, including light and sound.

● Diffraction is the spreading of waves when they pass through an obstacle.

This effect is more noticeable if the wavelength of the waves is about the

same size as the gap or obstacle.

● Light is really hard to diffract due its really small wavelength.

The narrower the gap, the more waves spread out. The

wider the gap, the less waves spread out.

Diffraction of ultrasonic waves is an important factor in

the design of an ultrasonic scanner. Ultrasonic waves

are sound waves at frequencies above the range of the

human ear. An ultrasonic scan can be made of a baby in

the womb.

TV signals are carried by radio waves. People living in

hilly areas may not be able to receive a signal because

it is blocked by a hill. Radio waves passing he hill will be

diffracted round the hill. If they do not diffract enough,

the radio and TV signals will be poor.

The ultrasonic waves spread out

from a hand-held transmitter and

then reflect from the tissue

boundaries inside the womb. If

the transmitter is too narrow, the

waves spread out too much and

image is not very clear.

Sound● Youtube: https://www.youtube.com/watch?v=Fsob8X15S2Y , https://www.youtube.com/watch?v=PCN4RIQIVTo

● Sound waves are longitudinal waves because the air particles vibrate along the direction in which the waves transfer energy

and cause vibrations in a medium, which are detected as sound. Also sound waves cannot travel through a vacuum

● Any objects vibrating in air makes the layers of air near the object vibrate making the further layers of air also vibrate. This

sends out the vibrations of the sir in waves of compressions and rarefactions. When the waves reach your ears, they make

your eardrums vibrate in and out so you hear sound as a result.

● A machine called an oscilloscope converts longitudinal waves into a transverse wave making it easier to analyse.

● Echoes are reflections of sounds - can be heard in places with bare, flat and smooth surfaces. If the walls are covered in

fabric, the fabric will absorb sound instead of reflecting it, meaning no echoes will be heard. Also if the wall surface is uneven

(not smooth), echoes will not be heard because the reflected sound is scattered and broken up. Soft tings absorb sound well.

● Sound waves travel through air at a frequency of about 340 m/s. The warmer the air, the greater the speed of sound.

Refraction takes place at the boundaries between layers of air at different temperature. In the daytime, sound refracts

upwards, not downwards(like at night), because the air near the ground is warmer than the air higher up.

● The pitch of a sound is determined by its frequency (as the frequency increases the pitch becomes higher) and loudness by

its amplitude(higher the amplitude, the louder the sound).

● The human ear can hear frequencies of sound in the range between 20 Hz and 20 000 Hz (20kHz). The ability to hear higher

frequencies declines with age.

● Sound waves generally travel fastest through solids and slowest through gases.

The higher the frequency of the sound waves, the higher the

pitch of the sound. The higher the amplitude, the louder the

sound. Differences of waveform can be show in an

oscilloscope similar to the one to the left.

Turning fork and signal generators produce ‘pure’

waveforms. The quality of a note depends on the

waveform. Musical instruments sound so different

from each other because each instrument make

different waveforms. Vibrations created in an

instrument when it is played produce sound

waves.

In some instruments, a saxophone has a column

of air which vibrates. In a violin the strings

vibrates An xylophone vibrate when they are

struck.

The instrument resonates at these certain

frequencies when a new cycle of vibrations

makes a vibration stronger at certain frequencies.

Key points

● The normal at a point on a mirror is a line drawn perpendicular to the mirror.

● For a light ray reflected by a plane mirror: The angle of incidence is the angle between the incident ray

and the normal and the angle of reflection is the angle between the reflected ray and the normal.

● The law of reflection states that: the angle of incidence = the angle of reflection.

● Refraction is the change of direction of waves when they travel across a boundary.

● When a light ray refracts as it travels from air to glass, the angle of refraction is less than the angle of

incidence. However, when a light ray refracts as it travels from glass into the air, the angle of refraction is

greater than the angle of incidence.

● Diffraction is the spreading out of waves when they pass through a gap or round the edge of an obstacle.

● The narrower a gap is, the greater the diffraction is.

● If radio waves do not diffract enough when they go over hills, radio and TV reception will be poor.

● The frequency range of the normal human ear is from about 20 Hz to about 20 000 Hz.

● Sound waves are vibrations that travel through a medium (substance). They cannot travel through a

vacuum (as in space).

● Echoes are due to sound waves reflected from a smooth, hard surface.

● The pitch of a note increases if the frequency of the sound wave increases. Similarly, the loudness of a

note increases if the amplitude of the sound waves increases.

● Vibrations created in an instrument when it is played produce sound waves.