Electromagnetic waves

Electromagnetic waves are transverse waves that transfer energy from the source of the waves to an absorber.

Electromagnetic waves form a continuous spectrum, and all types of electromagnetic wave travel at the same velocity through a vacuum — space — or air.

The waves that form the electromagnetic spectrum are grouped in terms of their wavelength and their frequency. Going from long to short wavelength, or from low to high frequency, the groups are: radio, microwave, infrared, visible light (red to violet), ultraviolet, X-rays and gamma rays.

Our eyes only detect visible light, and so detect a limited range of electromagnetic waves.

The order must be learnt, and the direction matters: as you go from radio to gamma the wavelength decreases and the frequency increases. Since v = f λ and the speed is the same for all of them in a vacuum, the two must change in opposite directions.

You should be able to give examples that illustrate the transfer of energy by electromagnetic waves — sunlight warming a surface, a microwave oven heating food, an infrared heater warming a room, or an X-ray depositing energy in body tissue.

You should be able to construct ray diagrams to illustrate the refraction of a wave at the boundary between two different media.

The convention is the same as for reflection: draw the normal as a dashed line at right angles to the boundary where the ray meets it, and measure all angles from the normal. Entering a denser medium the ray bends towards the normal; leaving it, the ray bends away.

Higher tier: different substances may absorb, transmit, refract or reflect electromagnetic waves in ways that vary with wavelength. Some effects, for example refraction, are due to the difference in velocity of the waves in different substances.

You should be able to use wave front diagrams to explain refraction in terms of the change of speed that happens when a wave travels from one medium to a different medium.

The explanation a wave front diagram makes visible: the wave fronts are drawn as parallel lines at right angles to the direction of travel. When they meet the boundary at an angle, one end of each front enters the new medium — and slows down — before the other end does. That end travels a shorter distance in the same time, so the whole front pivots, and the direction of travel changes. If the wave meets the boundary head-on, both ends slow at the same moment, so the front does not pivot and no bending occurs.

The frequency does not change on crossing the boundary, so from v = f λ a lower speed means a shorter wavelength — which is why the fronts are drawn closer together in the slower medium.

Changes in atoms and the nuclei of atoms can result in electromagnetic waves being generated or absorbed over a wide frequency range. Gamma rays originate from changes in the nucleus of an atom.

Ultraviolet waves, X-rays and gamma rays can have hazardous effects on human body tissue. The effects depend on the type of radiation and the size of the dose.

Radiation dose, in sieverts, is a measure of the risk of harm resulting from an exposure of the body to the radiation. 1000 millisieverts (mSv) = 1 sievert (Sv). You will not be required to recall the unit of radiation dose.

Ultraviolet waves can cause skin to age prematurely and increase the risk of skin cancer. X-rays and gamma rays are ionising radiation that can cause the mutation of genes and cancer.

You should be able to draw conclusions from given data about the risks and consequences of exposure to radiation. That means using the figures — comparing a dose with background levels, or comparing two occupations — rather than stating in general terms that radiation is dangerous.

Higher tier: radio waves can be produced by oscillations in electrical circuits.

When radio waves are absorbed they may create an alternating current with the same frequency as the radio wave itself, so radio waves can themselves induce oscillations in an electrical circuit.

That is transmission and reception described in two sentences, and the symmetry is the point. At the transmitter, an alternating current in a circuit produces radio waves of the same frequency. At the receiver, those radio waves are absorbed by an aerial and produce an alternating current of that same frequency, which the receiver then decodes. The frequency carries through unchanged from one circuit to the other, which is why a radio can be tuned to a station.

Electromagnetic waves have many practical applications. These are the examples named in the specification, and they are worth learning as a list.

Higher tier students should be able to give brief explanations why each type of electromagnetic wave is suitable for the practical application.

The explanation always comes from a property, so pair each use with the reason:

• Radio waves are not strongly absorbed by the atmosphere and can be reflected or diffracted around obstacles, so they travel long distances to aerials. • Microwaves pass through the atmosphere, so they reach satellites; a different wavelength is absorbed by water molecules in food, which heats it. • Infrared is strongly absorbed by surfaces and transfers energy to their thermal store, which is why it heats and cooks; and because all warm objects emit it, a camera can detect it. • Visible light is transmitted along glass fibres with very little absorption, so it carries data with little loss. • Ultraviolet makes some materials fluoresce, giving visible light efficiently, and it darkens skin. • X-rays are transmitted by flesh but absorbed by bone and metal, which is what makes the image; gamma rays penetrate deeply enough to reach and destroy tumours inside the body.

A lens forms an image by refracting light. In a convex lens, parallel rays of light are brought to a focus at the principal focus. The distance from the lens to the principal focus is called the focal length.

Ray diagrams are used to show the formation of images by convex and concave lenses. The image produced by a convex lens can be either real or virtual. The image produced by a concave lens is always virtual.

You should be able to construct ray diagrams to illustrate the similarities and differences between convex and concave lenses.

For a ray diagram, draw two rays from the top of the object:

• one travelling parallel to the axis, which is refracted through the principal focus • one passing straight through the centre of the lens without bending.

Where they meet is the top of the image. If they actually cross, the image is real and is drawn inverted; if they only appear to come from a point when traced backwards, the image is virtual and is drawn upright with dashed rays.

In ray diagrams a convex lens is represented by a vertical line with outward-pointing arrowheads at each end, and a concave lens by one with inward-pointing arrowheads.

The magnification produced by a lens can be calculated using the equation:

magnification = image height / object height

This equation is given on the Physics equation sheet. Magnification is a ratio and so has no units. Image height and object height should both be measured in either mm or cm — the same unit for both, so that the units cancel.

Each colour within the visible light spectrum has its own narrow band of wavelength and frequency.

Reflection from a smooth surface in a single direction is called specular reflection. Reflection from a rough surface causes scattering: this is called diffuse reflection.

Colour filters work by absorbing certain wavelengths, and colour, and transmitting other wavelengths, and colour.

The colour of an opaque object is determined by which wavelengths of light are more strongly reflected. Wavelengths that are not reflected are absorbed. If all wavelengths are reflected equally the object appears white. If all wavelengths are absorbed the object appears black.

Objects that transmit light are either transparent or translucent.

You should be able to explain how the colour of an object is related to the differential absorption, transmission and reflection of different wavelengths of light by the object; the effect of viewing objects through filters, or the effect on light of passing through filters; and why an opaque object has a particular colour.

The filter questions follow directly. A red object viewed through a red filter still looks red, because the red light it reflects is transmitted by the filter. The same red object viewed through a blue filter looks black — it reflects only red light, and the blue filter absorbs red, so no light reaches the eye.