Electromagnetic Waves Flashcards

All 50 cards in this deck

What are the three properties that all electromagnetic waves share?

They are transverse, they transfer energy from a source to an absorber, and they all travel at the same speed in a vacuum (or air).

List the seven groups of the electromagnetic spectrum in order from longest to shortest wavelength.

Radio, microwave, infrared, visible light, ultraviolet, X-rays, gamma rays.

What is the speed of all electromagnetic waves in a vacuum?

$3 \times 10^8$ m/s (the speed of light).

True or false? Gamma rays travel faster than radio waves in a vacuum.

False. All electromagnetic waves travel at the same speed, $3 \times 10^8$ m/s, in a vacuum.

Which colour of visible light has the longest wavelength, and which has the shortest?

Red has the longest wavelength; violet has the shortest.

What are the steps to convert a wavelength given in nanometres into metres in standard form?

e.g. 300 nm

1. Use $1$ nm $= 1 \times 10^{-9}$ m: $300 \times 10^{-9}$ m
2. Write in standard form: $3 \times 10^{-7}$ m

What are the steps to find the wavelength of an electromagnetic wave in air from its frequency?

e.g. a radio wave of frequency 100 MHz ($1 \times 10^8$ Hz)

1. Rearrange $v = f\lambda$ to $\lambda = v \div f$
2. Use $v = 3 \times 10^8$ m/s: $\lambda = 3 \times 10^8 \div 1 \times 10^8$
3. Wavelength $= 3$ m

What do electromagnetic waves transfer, and between what?

They transfer energy from the source of the waves to an absorber.

Give an example that illustrates the transfer of energy by electromagnetic waves.

Food in a microwave oven absorbs microwaves and its temperature rises (also: infrared from a fire warming your skin).

Why do human eyes detect only a limited range of electromagnetic waves?

Our eyes only detect visible light, which is one small group of the continuous EM spectrum.

True or false? Human eyes can detect infrared radiation.

False. Eyes detect visible light only; infrared must be detected with an instrument such as an IR camera or thermometer.

True or false? Electromagnetic waves need a medium to travel through.

False. They can travel through a vacuum, unlike sound waves.

What four things may a substance do to electromagnetic waves reaching it?

Absorb, transmit, refract or reflect them.

Does a substance treat all electromagnetic waves in the same way?

No. How much it absorbs, transmits, refracts or reflects varies with the wavelength of the wave.

What causes an electromagnetic wave to refract at a boundary?

The wave travels at a different velocity in the two substances.

Which property of a light wave changes when it is refracted at a boundary?

Its velocity (and therefore its wavelength).

True or false? The frequency of a light wave changes when it is refracted.

False. Frequency stays the same; the velocity and wavelength change.

What are the steps to draw a ray diagram for refraction at a boundary?

e.g. a light ray travelling from air into a glass block

1. Draw the normal at 90° to the boundary at the point where the ray hits it.
2. Draw the incident ray to that point and mark the angle of incidence from the normal.
3. The light slows in glass, so draw the refracted ray bent towards the normal (smaller angle).

In a refraction ray diagram, which way does a ray bend when it enters a medium in which it travels more slowly?

e.g. light going from air into glass

Towards the normal, so the angle of refraction is smaller than the angle of incidence.

True or false? A light ray passing from glass into air bends towards the normal.

False. It speeds up, so it bends away from the normal.

In a refraction ray diagram, what is the normal?

A construction line drawn at 90° to the boundary at the point where the ray meets it.

What happens to a ray that meets a boundary along the normal (angle of incidence 0°)?

It carries straight on without changing direction, although its speed still changes.

On a wave front diagram, what happens to the spacing of the wave fronts when a wave crosses into a medium where it travels more slowly?

They get closer together (wavelength decreases), because the frequency stays the same.

What does Required practical 10 investigate?

How the amount of infrared radiation absorbed or radiated by a surface depends on the nature of that surface.

What piece of apparatus is used to compare infrared radiation emitted by different surfaces, and how is it set up?

A Leslie cube (faces with different surfaces, e.g. matt black and shiny silver) filled with hot water, with an infrared detector aimed at each face.

What are the steps to investigate how infrared emission depends on the surface?

e.g. a Leslie cube with a matt black face and a shiny silver face

1. Fill the Leslie cube with hot water.
2. Hold the infrared detector the same distance from each face and record the reading.
3. Compare readings: the matt black face gives the highest reading.

In the infrared radiation practical, which type of surface is the best emitter and absorber of infrared?

A matt black surface; a shiny silver surface is the worst emitter and absorber.

Name two control variables in the infrared radiation practical.

The distance from the surface to the detector, and the temperature (and volume) of the hot water.

True or false? In the Leslie cube experiment the detector may be moved closer to some faces than others.

False. The distance must be kept the same, or the comparison between surfaces is not valid.

Where in the atom do gamma rays originate?

From changes in the nucleus of the atom.

What can changes in atoms and in the nuclei of atoms do to electromagnetic waves?

They can generate or absorb electromagnetic waves over a wide frequency range.

True or false? X-rays and gamma rays are both emitted from the nucleus of an atom.

False. Only gamma rays come from the nucleus; X-rays are produced by changes in the atom outside the nucleus (fast electrons hitting a metal target).

Can atoms absorb electromagnetic waves as well as emit them?

Yes — changes in atoms and their nuclei can result in EM waves being absorbed as well as generated.

How are radio waves produced?

By oscillations in electrical circuits (an alternating current makes electrons oscillate in an aerial).

How does the frequency of a radio wave compare with the frequency of the oscillations in the circuit that produced it?

It is the same as the frequency of the oscillations.

What happens in an electrical circuit when its aerial absorbs radio waves?

An alternating current is induced in the circuit with the same frequency as the radio wave.

True or false? Radio waves absorbed by an aerial induce a direct current in the circuit.

False. They induce an alternating current, of the same frequency as the radio wave.

Give two ways in which radio waves are different from sound waves.

Radio waves are transverse electromagnetic waves (sound is longitudinal and mechanical), and they travel through a vacuum and at a much higher speed.

Which three groups of electromagnetic waves can have hazardous effects on human body tissue?

Ultraviolet, X-rays and gamma rays.

What are the hazards of ultraviolet waves to the body?

They can make skin age prematurely and increase the risk of skin cancer.

What are the hazards of X-rays and gamma rays to the body?

They are ionising radiation, so they can cause mutation of genes and cancer.

What is radiation dose a measure of, and in what unit is it given?

A measure of the risk of harm from exposure of the body to radiation, measured in sieverts (Sv).

What two things do the harmful effects of radiation on the body depend on?

The type of radiation and the size of the dose.

Give a use of radio waves and why they are suitable for it.

Television and radio broadcasting — they can be transmitted over long distances and are not strongly absorbed by the atmosphere.

Give two uses of microwaves and why they are suitable.

Satellite communications (they pass through the atmosphere) and cooking food (they are absorbed by water molecules in the food, heating it).

Give two uses of infrared radiation and why it is suitable.

Electric heaters, cooking food and infrared cameras — it is readily absorbed by surfaces, transferring energy and raising temperature, and is emitted by warm objects.

Give a use of visible light and why it is suitable.

Fibre optic communications (and photography) — visible light can be transmitted along glass fibres with little absorption.

Give two uses of ultraviolet waves.

Energy efficient lamps, sun tanning, detecting forged banknotes (security marking) and sterilising water or equipment by killing microbes.

Give a use of X-rays and a use of gamma rays in medicine.

X-rays: medical imaging of bones. Gamma rays: medical imaging (e.g. tracers/PET scans) and radiotherapy to treat cancer.

True or false? The electromagnetic waves form a continuous spectrum.

True. The seven groups merge into one another, so there are no sharp gaps between them.