waves&properties

KS4: Electromagnetic

Waves

Electromagnetic Spectrum When particles [usually electrons] accelerate or decelerate, they make electromagnetic waves.

Electromagnetic waves are transverse waves made up of electric and magnetic fields which travel together. All electromagnetic waves can travel through space.

All electromagnetic waves travel at the same speed [300,000,000 m/s in a vacuum].

Electromagnetic Spectrum Visible light is made when negative electrons slow down as they move around inside an atom.

Electromagnetic Spectrum Although all e-m waves travel at the same speed, their wavelength [] and frequency [ƒ] can change.

The properties, dangers and uses of e-m waves depends on the wavelength [].

Waves that cook food.

Waves that cause sun-tans.

Waves that cause cancer.

Electromagnetic Spectrum The whole family of electromagnetic waves is called the electromagnetic spectrum.

Ra

dio

Micro

Infra

-Re

d

Visib

le

Ultra

-Vi

ole

t

X ra

ys

Ga

mm

a

increases

ƒ increases

Electromagnetic SpectrumName Gamma Rays

0.000 000 001 mm

Properties

Very high energy Pass through

body unchanged VERY dangerous

Uses

Kill cancer cells[radiotherapy]

Sterilise medicalequipment

As tracers to lookat lung structure

Electromagnetic SpectrumName X rays

0.000 001 mm

Properties

Energetic Short wave X rays

pass throughflesh but not bone

Dangerous

Uses

Look throughbody i.e. brokenbones and teeth

Scan luggage fordangerous items

Electromagnetic Spectrum

Electromagnetic SpectrumName Ultra Violet

0.000 01 mm

Properties is too short for

eyes to see causes sun-tans

Uses

‘sun’ beds checking

counterfeitbanknotes

in nightclubs

The £5 note on the left is genuine.

The note on the right glows in UV and is counterfeit

Electromagnetic SpectrumName Visible Light

0.005 mm

Properties Our eyes respond

to these Made of ROYGBIV

Uses

To see! Expose

photographic film To generate

electricity in photo-electric cells [solarpanels]

Electromagnetic SpectrumName Infra Red

0.01 mm

Properties

Emitted by warmobjects

Hotter the object,shorter emitted

Uses

Remote controls Thermal imaging

cameras Electric grill

Electromagnetic SpectrumName Microwaves

1 mm – 1 cm

Properties

Can carryinformation

Some absorbedby food

Uses

Microwave ovens Mobile

communications RADAR

Microwaves reflect around the oven and carry energy to about 1 cm into the food. This cooks the food quickly

Electromagnetic SpectrumName Radio

10 cm – 1 km

Properties

Can carryinformation

Travel largedistances

Uses

Broadcasting[carry radio andTV signals]

96.7 FMRadio waves are

emitted when electrons move up and down an aerial very quickly.

Electromagnetic Spectrum1) Match up the following parts of the

electromagnetic spectrum with their uses :

Gamma rays Allow us to see

Radio waves Remote Controls

Ultra Violet ‘See’ broken bones

Visible Carry TV signals

Microwaves RADAR

X rays Sterilise equipment

Infra Red Causes sun-tans

Electromagnetic Spectrum1) A radio station uses waves of frequency 96.7 MHz

If the speed of e-m waves in air is 300,000,000 m/s,

a) calculate the wavelength of the radio waves used.

b) calculate the time taken for the transmission to travel 50 km.

2) Why can we see the Sun but can’t hear it?

3) Write down 3 things all e-m waves have in common.

Reflection : A reminder

From KS3 you should remember :

Pale and shiny surfaces are good reflectors, dark and rough surfaces are not.

The image in a plane mirror is laterally inverted.

The image is the same distance behind the mirror as the object is in front.

The image in a plane mirror is the same size as the object.

angle of incidence = angle of reflection

¡ = r

Reflection : A reminder

Angle i

Angle r¡ =r

Incident ray

reflected ray

Reflection : Curved Mirrors

In KS3 you just dealt with plane mirrors.

By curving a mirror, we can make mirrors more useful:

Concave mirrors curve inwards

Convex mirrors bulge outwards

Reflection : Curved Mirrors

ƒ

Chose a distant object [to get parallel rays of light].

Finding of a concave mirror.

Hold the mirror in the other hand and move it closer to the screen until a clear image appears.

Hold a plain white screen in one hand.

Use a ruler to measure the distance between the lens and the screen - this is the focal length [ƒ].

Reflection : How does

curvature affect ƒ ? Concave mirrors reflect rays of light to a focal point.

The distance between the mirror and the focal point is called the focal length [ƒ].

How can ƒ be changed?

Concave mirrors produce real images because the rays of light meet [unless the object is close].

Reflection : How does

curvature affect ƒ ?ƒ

Take a piece of Al or stainless steel sheet and curve it slightly.

Shine parallel rays of light at the reflector and plot their positions.

Draw around the reflector.

Measure ƒ and record your results.

Carefully bend the reflector and repeat the process to see how ƒ changes with curvature.

Reflection : Convex mirrors

ƒ

Convex mirrors reflect rays of light away from a focal point.

The distance between the mirror and the focal point is called the focal length [ƒ]

Convex mirrors produce virtual images - the rays of light do not meet.

Reflection : Curved mirrors

Concave reflectors are used to focus signals from distant satellites.

Convex reflectors are used to widen the field of view.

Total Internal Reflection

Incident ray

Reflected ray

Refracted ray

Angle i Angle rRefraction orReflection?

15

30

45

60

75

Angle iAngle r

Angle r

At what angle of incidence did the ray change from refraction to reflection?

Total Internal Reflection This angle is called the critical angle [c]

i < c

Refraction

i = c

Critical case

i > c


[TIR] Different materials have different critical angles - diamond has the lowest at 24º which is why it reflects so much light.


i = r

Optical fibre

Total Internal Reflection Why do communications systems now use

optical fibres instead of copper wires?

ADVANTAGES

Can carry much more information as digital signals.

Carry information at the speed of light [300, 000 km/s].

Clear signals unaffected by electrical interference.

DISADVANTAGES

Expensive to make as very high quality glass is needed.

Need careful handling - signal loss if cracked.

Refraction : A reminder

When light bends this is called refraction.

Refraction happens because the light changes speed [or velocity].

If the incident ray hits a surface at 0º, no refraction occurs.

air

glass

Refraction : Lenses

At KS4, you need to be able to explain how to change the size and nature of an image

formed by a convex lens.

Refraction : Lenses

1. Find the focal length [ƒ] of your lens.

2. Fix the lens to the centre of a metre rule and mark the distances F and 2F either side of the lens.

2F F F 2F

3. Place the candle >2F away from the lens and move the screen until an image appears.

4. Measure the distances between the candle, image and lens and describe the image in the results table.

Refraction : Lenses

Objectposition

[as F]

Distancefrom O tolens [cm]

Imageposition

[as F]

Distancefrom I tolens [cm]

ImageDescrip

tionGraph

>2Faway

2F away

betweenF & 2F

at F

betweenF andlens

Magnification

Refraction : Lenses

Object >2F away

O

2F F F 2F

I

The image [ l ] is formed between F and 2F away from the lens, is inverted and diminished.

Refraction : Lenses

Object at 2F

O

2F F F 2F

I

The image [ l ] is formed at 2F away from the lens, is inverted and the same size.

Refraction : Lenses

Object between 2Fand F away

O

2F F F 2F

I The image [ l ] is formed further than 2F away from the lens, is inverted and magnified.

Refraction : Lenses

Object at F away

O

2F F F 2F

The image [ l ] is formed at infinity - the rays never meet [we use this set-up for searchlights].

Refraction : Lenses

Object between F and lens

O

I

The VIRTUAL image [ l ] is formed on the same side of the lens as the object, is the right way up and magnified.

2F F F 2F

Refraction : Lenses

Magnification = Distance from lens to image

Distance from object to lens

2F F F 2F

Using Refraction : Sound

Sound waves can be refracted as well as light waves

Move the microphone across the balloon and watch the CRO trace of the sound wave. What does the CO2 in the balloon do to the sound waves? Why?

CO2

Diffraction & Interference Waves travel in straight lines but when they go past an edge they spread out in a new direction.

This is called diffraction

Diffraction & Interference

When 2 waves meet, they interfere with each other. If they meet each other exactly in phase, the amplitudes ‘add up’ to produce large crests and troughs.

+ =

This is called constructive interference.


If they meet each other exactly out of phase, the amplitudes ‘subtract’ to produce no peaks or crests.

+ =

This is called destructive interference.


To get 2 waves of light to interfere, the waves must be very similar.

We use a single source of

monochromatic light and split

it into 2 waves using a diffraction

grating like this:

In 1801, a physicist called Young first performed this classic investigation which showed the interference of light waves.


The light source emits rays of light which diffract towards the double slit

S1

S2

S1 and S2 act as 2 coherent light sources

The waves interfere - constructively [bright fringes].

destructively [dark fringes].

Fringes


What would the fringes look like if white light was used as the source instead?


The coloured fringes on these CDs are the result of interference.

Light reflecting from the Aluminium diffracts and interferes.

Some colours are diffracted more than others.

CommunicationCommunicate v. Make known; transmit; pass

information to and fro; have means of access

To pass information quickly over large distances, we use waves.

These dishes collect and focus microwaves from a communications satellite

100’s of km above the Earth.

The effects of reflection, refraction and diffraction are important to consider when designing communications systems.

CommunicationThe meters at the top are analogue meters - they use a needle to represent the reading.

The digital meters below give the reading as a number.

Computers handle digital readings much faster and easier.

3.4 2.6

Communication

0

1

2

3

4

5

6

0 1 2 3 4 5

Time [1/10,000 s]

Sam

plin

g L

evel

Use the chart on the next page to turn the analogue signal into binary code and then a voltage sequence.

CommunicationSampling

LevelBinaryCode

Voltage Sequence

0 000 LOW LOW LOW

1 001 LOW LOW HIGH

2 010 LOW HIGH LOW

3 011 LOW HIGH HIGH

4 100 HIGH LOW LOW

5 101 HIGH LOW HIGH

6 110 HIGH HIGH LOW

Communication

ADVANTAGES

Signals are clearer.

Can be used quickly by computers.

Carry digital signals using electromagnetic waves which travel at the speed of light.

Carry much more information.

Digital hardware is much smaller.

DISADVANTAGES

Digital hardware is expensive at the moment.

Although digital signals are unaffected by electrical interference, they don’t give a complete signal [just lots of samples] - some people feel that analogue vinyl records sound better than digital CDs for this reason.

Communication

ionosphere

Transmitter dish

Receiver dish

Gugliemo Marconi first reflected radio waves off the ionosphere in 1901 [from England to Canada].

Communication

Transmitter dish

Receiver dish

The UHF radio waves we use for TV carry a lot of information but don’t reflect off the ionosphere.

We use communications satellites which amplify and transmit the signal.

Communication : Diffraction UHF radio waves carry high quality TV signals but can’t diffract round hills very well - you get a poor signal in valleys.

LW and MW signals diffract round hills so you get a good signal in valleys.

Communication : Diffraction Waves from the transmitter dish spread out due to diffraction.

The receiver dish can’t collect all the waves and so some energy is wasted - the signal must be amplified.

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