Key Facts: Electromagnetic Waves

Having previously covered waves in general (https://physbang.com/2023/04/13/key-facts-waves/) it is now time to look at electromagnetic waves in particular as this is a topic in its own right.

Electromagnetic waves are created by the movement of electrons in different situations. The electrons can be moving in an electric circuit, to produce or receive radio waves, or they can be moving between energy levels in an atom to produce visible colours. Visible light is just a tiny part of the electromagnetic spectrum and is important only because it is the range of wavelengths that our eyes can see. In the modern world, other parts of the electromagnetic spectrum are just as important as visible light.

To learn more about electromagnetic waves, read the following articles;

For an overall summary, including an easy way to remember the order of the different sections of the electromagnetic spectrum (which is a common exam question) read https://physbang.com/2022/04/01/electromagnetic-waves-overview/. Notice that gamma waves are found at the very high energy end of the electromagnetic spectrum so there is an overlap between electromagnetic waves and radioactivity.

For an explanation of electromagnetic waves’ behaviour at a boundary, read https://physbang.com/2022/04/22/properties-of-em-waves/. You are expected to be able to draw ray diagrams that represent reflection from a boundary (such as light bouncing off a mirror) and refraction across a boundary (such as light changing direction as it moves from the air into a glass block or water). Although these two examples both mention light, they also apply to other types of electromagnetic waves but the exact circumstances will differ. For example, radio waves are transmitted through solid walls whereas light waves are reflected (and absorbed) by the same materials. But radio waves can be reflected from different layers in the atmosphere, allowing signals to be heard the other side of the world.

Different types of electromagnetic waves have different uses – and also different types of dangers associated with them. For a description of the various uses and dangers, read https://physbang.com/2022/04/08/em-waves-uses-and-dangers/. Remember that UV waves can cause skin to age prematurely and increase the risk of skin cancer but the risks of cancers are even greater with higher-energy waves, such as x-rays and gamma rays. You might ask why x-rays are used in hospitals if they risk causing cancers but the risks are quite low for patients although they are higher for staff who carry out many x-rays (on different people) each day. Therefore, radiographers usually stand behind a screened wall to reduce their accumulated dose of radiation when x-ray pictures are taken.

Although you are not expected to remember the units used for radiation dosage you are expected to be able to do an appropriate conversion. It is therefore important that you recognise the radiation dose name, which is sieverts, and its symbol, Sv.

In an examination paper, you could be asked to express a dose of 430 mSv in sieverts. This is the same conversion as applies for any unit when going from a “milli-” to a full unit. The correct method is to divide by 1000, giving an answer of 0.430 Sv.

Finally, there is a required practical in the AQA Trilogy course that investigates the behaviour of electromagnetic waves by measuring temperature changes when infrared radiation is absorbed or radiated by different surfaces. The key facts you should know are;

• dull black surfaces absorb and radiate infrared rays (heat) at the fastest rate
• shiny white or polished metal surfaces absorb and radiate infrared rays (heat) slowly

These two facts mean that black surfaces tend to get hot quickly AND they tend to cool down quickly. Surfaces that are intended to cool objects, such as heat sinks on computer chips and radiators for car engines, often have a black surface to help them cool more effectively.

On the other hand, polished metal surfaces tend to heat-up and cool-down much more slowly. The heat shields that keep the James Webb Space Telescope cool, when it is in sunlight, are made of an aluminium-coated multi-layer foil that reflects almost all of the sun’s heat. But it is also true that long-distance runners are sometimes wrapped in metal-foil blankets after a race and this is to keep them warm.

The equilibrium temperature for any object is an energy balance between the heat it produces (or receives) and the heat that it loses. To decide how hot an object will be you must look at both its surface and where the heat source is located. Here is a summary of the important facts;

• If the heat source is outside the object then shiny surfaces will keep the object cool whereas dark surfaces will cause the object to get hotter.
• If the heat source is inside the object then shiny surfaces will keep the object hotter whereas dark surfaces will allow it to cool down quicker.
• In both cases, shiny surfaces mean that the object’s temperature will remain more stable whereas dark surfaces cause the object’s temperature to change faster and by a greater amount.

Note that if a single object has both dark surfaces and shiny surfaces, the dark surface will emit more infrared radiation and will therefore appear hotter, when measured using a non-contact infrared thermometer, than the shiny surface. This is due to the different radiation behaviours of the surfaces and is demonstrated using a piece of equipment known as a Leslie Cube, which reveals variations in thermal radiation emitted from different surfaces at the same temperature. The metal cube is filled with hot water and has an even temperature inside but radiates heat at different rates from its outside surfaces. There is a good summary of the Leslie Cube experiment on the BBC Bitesize website at https://www.bbc.co.uk/bitesize/guides/z32f4qt/revision/5.