BMAT Physics Section 2

Amplitude

displacement from rest position to crest

Wavelength

length of a full cycle of the wave (from crest to crest)

Frequency

number of complete waves passing a certain point per second or number of waves produced by a source each second

Unit of frequency

herts (Hz)
1 wave per second

Do waves transfer matter?

- no but they can make matter oscillate (move up and down/side to side) but there's no net travel

Transverse

vibrations that are perpendicular to the direction of energy transfer of the wave.

Examples of transverse waves

- light waves (EM waves)
- slinky spring wiggled up and down
- waves on strings
- S waves (seismic)

Longitudinal waves

vibrations are parallel to the direction of energy transfer of the wave

Examples of longitudinal waves

- sound waves and ultrasound
- a slinky spring when you push the end
- P waves (seismic)

Period

time taken for one whole cycle of a wave

frequency of wave =

the reciprocal of its period
1/T

speed of a wave =

frequency x wavelength

Speed of wave is (usually) independent of

frequency and amplitude of wave

Speed and wavelength can vary depending on

medium that the wave is travelling through

Why does frequency not vary according to medium?

as it is set at its source

When any wave arrives at an obstacle/meet a new material

direction of travel can be changed by reflection/refraction

Reflection of light

- allows us to see objects
- light travelling in same direction reflects from an even surface at the same angle (clear reflection)
- light travelling in same direction reflects from an uneven surface at different angles

Law of reflection

angle of incidence = angle of reflection

What does reflection change

direction of light but doesn't change speed, wavelength or frequency

Refraction

by different boundaries cause objects to look bent

Refraction in a prism

- different wavelengths of light refract by different amounts so white light disperses into different colours when it enters a prism
- boundaries aren't parallel, different wavelengths emerge not parallel
- rainbow

Refraction in rectangular prism

- rectangular block has parallel boundaries so ray bend by same amount when they leave block as when they enters
- rays emerge as parallel

Water waves showing reflection

Sound waves

- longitudinal wave
- caused by vibrating objects
- mechanical vibrations are passed through surrounding medium as a series of compression

Sound travels faster in

solids than liquids and faster in liquids than in gases

Sounds cannot travel through

space as it is a vacuum so there are no particles

Sounds are reflected by

hard flat surfaces

Echoes =

reflected sound waves

Why is there a delay in the echo?

as echoed sound waves have to travel further so take longer to reach your ears

Sound waves refract when

- they enter a different media

When sound waves enter denser material...

they speed up

The higher the frequency

the higher the pitch
(shorter wavelength)

The lower the frequency

the lower the pitch
(longer wavelength)

Frequency is the

number of complete vibrations each second

The bigger the amplitude

the louder the sound

Ultrasound

sound that has a frequency above the range of human hearing (about 20kHz)

Uses of ultrasound

pre-natal scanning and sonar

Infrasound

sound that is too low for humans to hear (below 20Hz)

Electromagnetic waves all

- travel at the same speed (3x10? m/s) in a vacuum
- are transverse waves

EM waves with higher frequencies have

shorter wavelengths

Order of EM waves in increasing frequency and energy and decreasing wavelength

Radio Waves, Microwaves, Infra Red, Visible Light, Ultra Violet, X rays, Gamma rays

Radio waves

- wavelengths longer than 10cm

Long-wave radio (1-10km)

- can diffract around hills into tunnels
- diffraction effect makes it possible for radio signals to be received even if receiver isn't in line of sight of transmitter

Medium-wave radio

- can reflect from ionosphere depending on atmospheric conditions and time of day

Short-wave radio (10m-100m)

- can be received at long distances from transmitter
- reflected from ionosphere an electrically charged layer in earth's upper atmosphere

Microwaves

- used to communicate to and from satellites that can pass easily through earth's watery atmosphere
- signal from transmitter is transmitted into space where it is picked up by satellite receiver dish orbiting above earth
- satellite transmits signal back

Microwaves and Microwave ovens

1. Microwave ovens, microwaves need to be absorbed by water molecules in food to be able to heat it up so use a different wavelength to those used in satellite communications
2. Microwave penetrate up to a few cm into food before being absorbed by water m

Health risks with microwaves

- no conclusive proof
- phone emits microwave radiation
- if some of this radiation were to be absorbed by your body causing heating of your body tissue cells would be burned or killed

Infrared : Temperature

- heat radiation
- given out by hot objects
- can monitor tempertures
- detected by night-vision equipment
- the hotter the object the brighter it appears

Uses of IR

- cooking
- short distance with mobile phones and computers
- remote controls: emitting different patters of infrared waves to send different comands

IR: Optical Fibres

- carry data over long distances quickly
- use IR and visible
- signal is carried as pulses of light/IR and is reflected off sides of a very narrow core from one end of fibre to other

IR: Greenhouse Effect (Step 1)

EM radiation from sun can pass through earth's atmosphere to heat up surface

IR: Greenhouse Effect (Step 2)

When radiation is emitted back from surface it is at lower frequency - IR radiation

IR: Greenhouse Effect (Step 3)

Lots of this IR radiation is absorbed by atmospheric gases called green house gases.
(CO?, CH? and H?O)

IR: Greenhouse Effect (Step 4)

Gases then re-radiate heat in all direction, including back towards the Earth. So atmosphere acts as an insulating layer, stopping Earth losing all its heat at night.

IR: Greenhouse Effect (Step 5)

Without greenhouse effect in our atmosphere earth would be a lot colder but too much leads to global warming.

IR : Skin burns

- IR warms up skin
- too much causes burns

Visible light seeing with your eye

When light enters your eye, it gets refracted through lens and focused onto the retina at back of eye.
Retina then sends messages to brain via optic nerve and brain interprets them.

Visible light photography

Camera uses lens to focus visible light onto a light-sensitive film or electronic sensor that records the image

Lens aperture controls

how much light enters the camera (like the pupil in an eye)

Shutter speed determines

how long film or sensor is exposed to the light

UV uses:

- fluorescence
- bank notes
- security pens
- fluorescent lamps
- can cause skin cancer

Ionising radiation

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EM radiation

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Ionisation is dangerous

- damages DNA molecules and cause mutations
- cells might start dividing over and over again = cancer
- high doses of radiation can kill your cells altogether

X-rays

- look inside objects
- cannot pass easily through denser material
- can cause cancer
- to scan for luggage

Gamma rays

- to treat/diagnose cancer
- radioactive isotope is injected into patient - a gamma camera is then used to detect where radioactive isotope travels in body
- creates image

EM waves towards end of spectrum

can pass through material

EM waves towards middle

are absorbed

The higher the frequency of EM radiation

the more ionising and so the more harmful it is

Microwaves: HARM

- similar frequency to vibrations of many molecules
- increase these vibrations
- heats human body cells

Infrared: HARM

- make surface molecules of any substance vibrate causing a heating effect
- skin burns

Visible: HARM

- blindness

UV: HARM

- sun burn
- skin cancer
- eye damage

X-rays: HARM

- cell mutation
- cancer