EM Spectrum Tour
Seven regions, seven uses and seven dangers is fourteen things to memorise. There is one question underneath all of them: does this material absorb, reflect or transmit this wave? Answer that and the uses, the hazards and the practical all fall out of it.
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Fourteen facts, or one question
The usual way to learn this topic is seven regions, a use for each and a danger for each, which is fourteen things to hold in your head and no way of working out any of them if you forget. There is a better way in, and it also happens to be what the required practical is about. When an electromagnetic wave meets a material, exactly three things can happen to it. It can be absorbed, it can be reflected, or it can be transmitted straight through. That is the whole menu. Nearly every use in this topic is somebody choosing a material for one of those three. An aerial absorbs radio waves; a mirror reflects light; a window transmits it; the metal mesh in a microwave oven door reflects microwaves and keeps them inside. And harm only ever happens where the wave is absorbed, because a wave that goes straight through leaves nothing behind and a wave that bounces off never got in. So carry one question through: does this material absorb, reflect, or transmit this wave? What a wave IS, and how to measure one, is the other module on this topic.
Five words, three of them a menu
The first three are the only things that can happen to a wave when it meets a material. The last two matter later, and the fifth is the one people use loosely and examiners do not.
Which one is in the remote control
The seven regions are shown here in order, each labelled. Tap the region a television remote control uses to send its signal.
Why a window is not a shield
Ordinary window glass transmits visible light almost completely, and it also transmits a good deal of the ultraviolet that causes sunburn less well, absorbing much of it. What does that tell you about why absorption matters?
- Harm can only be done where the wave energy is actually taken in. A wave that passes straight through has left nothing behind, and a wave that is absorbed has deposited everything it was carrying
- Because waves travel more slowly through glass, which reduces the harm they can do
- Because glass reflects all ultraviolet, so none of it reaches anybody
- Because only frequency decides harm, and the material makes no difference at all
Three fates, three kinds of use
Look at any application of electromagnetic waves and you will find somebody choosing a material because of which of these three it does. That is what an application IS.
Five everyday things to explain
- The door of a microwave oven has a metal mesh in it, and you can still see the food through it
- A remote control works when pointed at the television but not from the next room with the door shut
- A matt black car left in the sun gets far hotter inside than a white one parked beside it
- A satellite dish is a curved metal bowl rather than a flat sheet of anything
- Ultraviolet, X-rays and gamma are hazardous in a way that infrared and microwaves are not, even at the same intensity
- The holes are far too small for microwaves to get through, so those are reflected back inside, while visible light has a much shorter wavelength and passes between them easily
- The wave it uses is transmitted by air but absorbed by an ordinary door, so it needs a clear path and cannot go round a corner
- A matt dark surface absorbs far more of the radiation falling on it than a light shiny one, which reflects most of it straight back
- The shape reflects waves arriving over a wide area to a single point, where the detector that absorbs them can be put
- They carry enough energy in each wave to knock electrons off atoms, which can damage the DNA inside a cell. The other two heat things instead, which is a matter of how much rather than of what kind
Compare two cooling cans
In a required practical, two identical cans of hot water are left to cool, one painted matt black and one polished shiny. Assume each cools at a steady rate: the matt black one by 3 degrees Celsius each minute and the shiny one by 1 degree Celsius each minute. After 12 minutes, how many MORE degrees Celsius has the matt black can cooled by?
Name the region, five in a row
Each description points to one region of the spectrum. Three lives.
The practical that is the same question
One of the required practicals for this topic asks how much infrared a surface absorbs or gives out, and it is usually learned as a separate fact. It is not separate. It is the absorb-reflect-transmit question, asked of one region and answered by measurement. The apparatus is four surfaces treated differently, or two cans of hot water, and a way of measuring how fast each warms up or cools down. The result is always the same. A matt black surface absorbs infrared best, and a shiny light surface absorbs it worst, reflecting most of it straight back. ⭐ And now the part worth a mark of its own. A good absorber turns out to be a good EMITTER too. The matt black can does not just warm up fastest in the sun, it also cools down fastest when it is the hot one. The surface that takes energy in readily gives it out readily, and stating both halves is what separates a full answer from half of one. ⚠️ Fair test: the cans must start at the same temperature, hold the same volume, sit the same distance from the source, and differ ONLY in the surface.
Two claims to strike out
Five statements from a set of revision notes. Tap the TWO that are wrong.
- All electromagnetic waves travel at the same speed through a vacuum
- Going from radio to gamma, the wavelength gets shorter and the frequency gets higher
- A shiny silver surface is the best emitter of infrared
- A wave that passes straight through a material still deposits its energy in it
- Ultraviolet, X-rays and gamma rays are ionising
Rain on three roofs
Rain falls on a row of three buildings, and the same weather has three completely different consequences, entirely because of what each roof is made of. The first has a thatched roof. The water soaks into it and stays there, and the thatch gets heavier and colder all afternoon. The rain stopped when it arrived, and everything it was carrying was handed over to the roof. The second has a steep slate roof. The water hits it and runs straight off into the gutter. It never got in at all, and the slate is no wetter at the end of the day than it was at the start. The third has an open courtyard where the roof should be, and the rain goes all the way through and lands on the floor below. Nothing happened to the building on the way past. Three roofs, one shower, three outcomes. And notice which one is the problem: the only building that gained anything from the rain is the one that took it in. If you wanted to keep something dry, the thatch is the one to worry about, and it is no use asking how heavy the rain was without asking what it landed on.
Put the proper words back
When a wave meets a material it can be taken in, in which case it is _____; it can bounce off, in which case it is _____; or it can pass straight through, in which case it is _____. Harm can only be done where the energy is taken in. Going from radio towards gamma the wavelength gets shorter while the frequency and the energy get _____, and the three highest-energy regions are described as _____ because each wave carries enough energy to knock electrons off atoms. A matt black surface is the best absorber of infrared and also the best _____ of it.
How a satellite dish earns its shape
A satellite dish is a curved metal bowl with a small detector mounted in front of it. Select every statement that correctly explains a feature of it.
- It is made of metal because metal reflects the waves rather than letting them through
- It is curved so that waves arriving across the whole bowl are brought together at one point
- The detector is placed at that point because its job is to absorb the waves
- It is large so that it can transmit the waves through to the receiver behind it
- It is shiny so that it absorbs as much of the signal as possible
Four design decisions
Four people are choosing a material, and every one of them is really answering the same question. Each stage sets out where things stand and hands you the decision.
- An engineer is choosing what to put in the window of a microwave oven door, so that the food can be seen but the microwaves stay inside. Decide what is needed.
- A radiographer needs an image of a bone. X-rays are directed through the limb onto a detector behind it. Decide what makes the image appear.
- A householder is choosing the colour and finish for the radiators in a cold room, and wants them to warm the room as effectively as possible. Decide what to tell them.
- A class is setting up the infrared practical with two cans of hot water, one matt black and one shiny. One student fills the black can fuller than the shiny one, because it is the one being tested. Decide what to say.
Build the two-part answer
Assemble the sentence that answers a use-and-danger question properly.
Explain uses and dangers from one idea
Explain how the use made of an electromagnetic wave depends on what materials do to it, and explain why the regions at the high-frequency end are dangerous in a different way from the ones at the low-frequency end.
- Name the three things that can happen when a wave meets a material, and say what each one means
- Give one use that depends on a material absorbing a wave, and one that depends on a material reflecting it
- Explain why harm can only be done where a wave is absorbed
- Explain what ionising means, and say which regions are ionising and why
- Finish with the practical: say which surface absorbs infrared best, and say what else that same surface does best