4.4.2 - Electromagnetic Waves

Question 1

What is an electromagnetic wave?

Answer 1

A self-propagating transverse wave that does not require a medium to travel through.

Question 2

Which seven properties do all electromagnetic waves share?

Answer 2

• They are all transverse waves.
• They can all be reflected, refracted and diffracted.
• They can all demonstrate interference.
• They can all be polarised.
• They can all travel through a vacuum.
• All possess both a magnetic wave and an electrical wave interlocked and at right angles to each other.
• In free space, they all travel at the speed of light, c, or 2.98 * 10^8 ms^-1.

Question 3

What are the orders of magnitude of wavelengths of the principal radiations?

Answer 3

LONGEST WAVELENGTH

Radio: 10^-1 to 10^4 m
Microwave: 10^-4 to 10^-1 m
Infra-Red: 7.4 * 10^-7 to 10^-3 m
Visible Light: 3.7 * 10^-7 to 7.4 * 10^-7 m
Ultra Violet: 10^-9 to 3.7 * 10^-7 m
X-Rays: 10^-12 to 10^-7 m
Gamma Rays: 10^-16 to 10^-9 m

SHORTEST WAVELENGTH

Question 4

Which electromagnetic radiation is non-ionising and why?

Answer 4

• Radio Waves, Microwaves, InfraRed and Visible Light
• As they do not have enough photon energy to remove electrons from the shells of atoms. They cannot produce ions and so are called non-ionising radiations.

Question 5

Which electromagnetic radiation is ionising and why?

Answer 5

• Ultraviolet Rays, X-Rays and Gamma Radiation
• As they all have high photon energies. This means they can cause ionisation by knocking electrons from the shells of atoms.

Question 6

How are X-rays produced?

Answer 6

By firing high-energy electrons at a copper anode.

[INSERT DIAGRAM]

Question 7

How is gamma radiation released?

Answer 7

When the nuclei of unstable atoms give out high-energy photons.

Question 8

What is polarisation?

Answer 8

The process of turning an unpolarised wave into a plane-polarised wave.

Question 9

What is polarisation good evidence for?

Answer 9

The wave nature of light.

Question 10

What type of waves can be polarised?

Answer 10

All transverse waves (including all electromagnetic waves).

Question 11

What are all electromagnetic waves made of?

Answer 11

A magnetic field and an electric field oscillating at right angles to one another and the direction of wave travel.

Question 12

What defines a plane?

Answer 12

The displacements of the oscillating field and the direction of travel.

Question 13

What defines the plane of electromagnetic waves?

Answer 13

The plane of oscillation of the electric field.

Question 14

What is an unpolarised wave?

Answer 14

A transverse wave with a mixture of waves with many different planes, with all the waves oscillating perpendicular to the direction of energy travel.

Question 15

What is a plane-polarised wave?

Answer 15

A transverse wave oscillating in only one plane.

Question 16

What are polarising filters?

Answer 16

Crystalline materials which can cause the oscillating fields to happen in one plane only.

Question 17

How can the polarisation of microwaves be observed?

Answer 17

1) The microwave transmitter emits naturally polarised waves with a specific wavelength.
2) A microwave receiver is placed in front of the transmitter, and then either the receiver/transmitter is rotated around the line between them (to show that the microwaves are polarised when they are emitted).
3) The signal reception (detected either using an ammeter or an audio amplifier and loudspeaker so that the microwave signal can be ‘heard’) rises and falls in intensity as the rotation occurs, and drops to zero when they are ‘crossed’ (at right angles).
4) When the receiver is at the maximum signal position, a metal grille is then placed between the transmitter and receiver.
5) The microwave transmitter is producing vertically plane-polarised radiation.

Question 18

What will happen if the bars of the metal grille are horizontally orientated?

Answer 18

Very few of the microwaves will be absorbed and the ammeter will show a high output.

Question 19

What will happen if the bars of the metal grille are vertically orientated?

Answer 19

All of the microwaves will be absorbed, because they are vertically plane-polarised and the ammeter will show no output.

Question 20

What is Malus’ law?

Answer 20

The law stating that the intensity of a beam of plane-polarised light (after passing through a rotatable polariser) varies as the square of the cosine of the angle through which the polariser, is rotated from the position that gives maximum intensity.

Intensity after the analyser ∝ cos^2 θ

When a perfect polariser is put in a beam of polarised light, the intensity, I, of the light that passes through it is given by I = I max cos^2 θ where:
I - intensity transmitted at angle θ
I max - the maximum intensity transmitted (at θ = 0°)

The law also states that if the analyser is at right angles to the polariser, then θ = 90° and no light will pass through (‘crossed Polaroids’).

Question 21

How can the polarisation of light be observed?

Answer 21

1) Look through a polarising filter at different light sources (i.e. Sun, light bulb, laptop/calculator screen, reflection in glass).
2) Try one filter, then with two filters aligned at different angles to each other.

3) As filter B is rotated through different angles with respect to filter A, the intensity of transmitted light rises and falls.
____________________________________________
1) Look through a single polarising filter at light reflected at a shallow angle from water, such as a pond, or from shiny surfaces in the lab.

2) Rotate the polarising filter until you notice that the reflected image gets dimmer.

Question 22

State and describe three uses of polarisation.

Answer 22

• Although sunlight/light from most sources are not polarised, some naturally occurring light can be partially polarised. This means that there is more light with the direction of oscillation in one direction than there is in any other direction. This mainly occurs in light that has been reflected.
  e. g. light reflected from the surface of a lake
• Photographers often use Polaroid filters to enhance the colour of the sky. The filters remove some of the polarised light from a blue sky, so the colour of the sky seems more intense.
• Strain analysis allows coloured images to be produced that change as the plastics are stretched or squashed.

Question 23

What two things will happen to a wave when refraction occurs?

Answer 23

• The wave will change speed.

- The wave will change direction (unless it is travelling along the normal, in which case it will not).

Question 24

What will happen to an electromagnetic wave if it moves from a material of lower refractive index to a material of higher refractive index?

Answer 24

The speed of the wave will decrease.

Question 25

How is refractive index calculated?

Answer 25

n = c / v

refractive index = speed of light in a vacuum (ms^-1) / speed of light in the material (ms^-1)

Question 26

What is the relationship between the refractive index and the speed of light in the material?

Answer 26

The higher the refractive index, the slower the electromagnetic wave travels through this material compared with its speed in a vacuum.

Question 27

What is Snell’s law?

Answer 27

An electromagnetic wave travelling at an angle, θ1, to the normal in ‘material 1’ will travel at an angle, θ2, to the normal in ‘material 2’.

n1 * sin θ1 = n2 * sin θ2

n sin θ = constant (at a boundary where θ is the angle to the normal)

Question 28

How can the refractive index of a transparent semi-circular block be determined?

Answer 28

1) The equipment is set up as shown below.

[INSERT DIAGRAM]

2) Measure the angle of the incidence, θ1, and the angle of refraction, θ2, using a protractor.
3) Use readings for θ1 over a range of values between 0° and 80° (since light rays exit the block at 90°, you do not have to deal with a refraction at this second interference). This will give sufficient data to plot a graph of sin θ1 against sin θ2.
4) To measure values of θ2, you will need to remove the glass block each time and join the points from where the emergent refracted ray left the block to the point at which it entered the block, and draw a normal line.

5) Plot a graph of sin θ1 against sin θ2.
The gradient of the graph = sin θ1 / sin θ2 = n2 / n1

Since the refractive index of air is very close to 1, the equation simplifies to n = sin θ1 / sin θ2.

Question 29

What is total internal reflection (TIR)?

Answer 29

The result of both reflection and refraction. It occurs when light, or other electromagnetic radiation, travels from a material of a higher refractive index to one of a lower refractive index.When the angle of incidence is increased above the critical angle, no refraction will take place and so all the light will be reflected.

Question 30

What is the critical angle?

Answer 30

The angle of incidence beyond which rays of light passing through a denser medium to the surface of a less dense medium are no longer refracted, but totally reflected.

Question 31

How is the critical angle calculated for materials in contact that involve air?

Answer 31

sin C = 1 / n

sine of critical angle = 1 / refractive index of material

Question 32

How is the critical angle calculated for materials in contact that do not involve air?

Answer 32

sin C = n2 / n1

sine of critical angle = refractive index of material 1 / refractive index of material 2

NOTE:

• Ratio n2 / n1 must always be less than 1, so n2 < n1.
• Smaller refractive index goes on the top of the fraction.

Question 33

How can the critical angle of a transport material be determined?

Answer 33

1) Use a semicircular shaped block of the material and draw around it on a piece of paper.
2) Make a mark at the centre of the straight edge and draw a normal at that point.
3) Shine a ray of light from a ray box towards the curved edge of the block in an arc until the ray emerges along the surface of the boundary between the material and air.
4) The angle the ray makes with the normal is the critical angle.
5) The critical angle can then be used to find the refractive index of the material in question using sin C = 1 / n.