5.10–5.12Types of 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.
Common mistake
"Continuous spectrum" means there are no gaps between the groups — the boundaries are conventions, not physical divisions. And all seven travel at the same speed in a vacuum; only their frequency and wavelength differ.
- Electromagnetic waves
- transverse waves that transfer energy from the source of the waves to an absorber, forming a continuous spectrum and travelling at the same velocity through a vacuum or air
5.7–5.8Properties of electromagnetic waves 1
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.
Required practical 10
Investigate how the infrared radiation absorbed or radiated by a surface depends on the nature of that surface
- Emission: fill a Leslie cube with hot water and place it so that its four differently finished faces — matt black, shiny black, matt white and shiny metal — are exposed.
- Hold an infrared detector the same short distance from each face in turn, and record the reading for each.
- Keep the distance and the water temperature the same for every face, so only the surface differs.
- Absorption: place metal plates with different surfaces the same distance from an infrared heater, each with a thermometer or temperature sensor attached.
- Record the starting temperature of each, switch on the heater, and record the temperature of each at regular intervals.
- Repeat both parts and take means.
- Refraction
- the change in direction of a wave as it crosses a boundary between two media, caused by the difference in the velocity of the wave in each
- Normal
- a line drawn at right angles to a boundary where a ray meets it, from which all angles are measured
5.13–5.14How substances affect electromagnetic wavesHigher tier
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.
Common mistake
Refraction happens because the speed changes, not because the wave "prefers" a shorter path. And a ray meeting the boundary along the normal is not refracted at all, even though it does change speed.
Before you practise
- In order of increasing frequency: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.
- All electromagnetic waves are transverse and all travel at the same speed in a vacuum.
- They all transfer energy from a source to an observer.
- The spectrum is continuous, and the groupings are a convenience rather than sharp boundaries.
- The eye detects only the narrow visible band, from red at the lowest frequency to violet at the highest.
- What a material does to a wave depends on the wavelength — glass transmits light and absorbs ultraviolet.
- A change in velocity at a boundary is what produces refraction, and it depends on the substance and the wavelength.