Flashcards in 4.4.2 - Electromagnetic Waves Deck (33):
What is an electromagnetic wave?
A self-propagating transverse wave that does not require a medium to travel through.
Which seven properties do all electromagnetic waves share?
- 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.
What are the orders of magnitude of wavelengths of the principal radiations?
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
Which electromagnetic radiation is non-ionising and why?
- 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.
Which electromagnetic radiation is ionising and why?
- 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.
How are X-rays produced?
By firing high-energy electrons at a copper anode.
How is gamma radiation released?
When the nuclei of unstable atoms give out high-energy photons.
What is polarisation?
The process of turning an unpolarised wave into a plane-polarised wave.
What is polarisation good evidence for?
The wave nature of light.
What type of waves can be polarised?
All transverse waves (including all electromagnetic waves).
What are all electromagnetic waves made of?
A magnetic field and an electric field oscillating at right angles to one another and the direction of wave travel.
What defines a plane?
The displacements of the oscillating field and the direction of travel.
What defines the plane of electromagnetic waves?
The plane of oscillation of the electric field.
What is an unpolarised wave?
A transverse wave with a mixture of waves with many different planes, with all the waves oscillating perpendicular to the direction of energy travel.
What is a plane-polarised wave?
A transverse wave oscillating in only one plane.
What are polarising filters?
Crystalline materials which can cause the oscillating fields to happen in one plane only.
How can the polarisation of microwaves be observed?
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.
What will happen if the bars of the metal grille are horizontally orientated?
Very few of the microwaves will be absorbed and the ammeter will show a high output.
What will happen if the bars of the metal grille are vertically orientated?
All of the microwaves will be absorbed, because they are vertically plane-polarised and the ammeter will show no output.
What is Malus' law?
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').
How can the polarisation of light be observed?
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.
State and describe three uses of polarisation.
- 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.
What two things will happen to a wave when refraction occurs?
- The wave will change speed.
- The wave will change direction (unless it is travelling along the normal, in which case it will not).
What will happen to an electromagnetic wave if it moves from a material of lower refractive index to a material of higher refractive index?
The speed of the wave will decrease.
How is refractive index calculated?
n = c / v
refractive index = speed of light in a vacuum (ms^-1) / speed of light in the material (ms^-1)
What is the relationship between the refractive index and the speed of light in the material?
The higher the refractive index, the slower the electromagnetic wave travels through this material compared with its speed in a vacuum.
What is Snell's law?
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)
How can the refractive index of a transparent semi-circular block be determined?
1) The equipment is set up as shown below.
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.
What is total internal reflection (TIR)?
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.
What is the critical angle?
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.
How is the critical angle calculated for materials in contact that involve air?
sin C = 1 / n
sine of critical angle = 1 / refractive index of material
How is the critical angle calculated for materials in contact that do not involve air?
sin C = n2 / n1
sine of critical angle = refractive index of material 1 / refractive index of material 2
- Ratio n2 / n1 must always be less than 1, so n2 < n1.
- Smaller refractive index goes on the top of the fraction.