EM Spectrum Tour

EM Spectrum Tour

Visible light is only the sliver your eyes happen to detect. It is one small part of the electromagnetic (EM) spectrum: a continuous family of transverse waves that all travel at the same speed through a vacuum, 300,000,000 m/s. Grouped by wavelength, from the longest to the shortest, there are seven regions: radio → microwave → infrared → visible → ultraviolet → X-ray → gamma They are all transverse and they all travel at that same speed. What changes across the spectrum is the wavelength, the frequency and the energy, and almost every exam question on this topic turns on one of those three.

Rank the sources

Six everyday sources, each emitting mainly one region of the spectrum. Put them in order from the LOWEST energy to the HIGHEST.

• A transmitter mast broadcasting a television channel
• A microwave oven heating a bowl of soup
• The warmth you feel standing near a radiator
• The glow of a desk lamp
• A sunbed that tans skin
• A dental X-ray machine

The trade-off

Move from the radio end towards the gamma end and three things happen together: • the wavelength gets shorter • the frequency gets higher • the energy gets higher They are locked together, which is why you only ever need to know one of them to work out the other two. Gamma rays therefore have the shortest wavelength, the highest frequency and the most energy. One thing does not change: the speed. Every EM wave travels at the same speed in a vacuum, whatever its wavelength. Candidates lose marks every year by writing that gamma rays travel faster.

Two waves

Wave A has a longer wavelength than wave B. Both are electromagnetic waves travelling through a vacuum. Which statement is true?

• Wave A has the lower frequency and carries less energy, but both travel at the same speed
• Wave A has the lower frequency and travels more slowly
• Wave A has the higher frequency and carries more energy
• Wave A carries more energy, because a longer wave carries more

Every region has a job

Each region earns its place. These are the uses named in the specification, and they are worth knowing precisely rather than approximately:

Which region is at work?

Name the seven

Reading the spectrum from the longest wavelength to the shortest, the seven regions run: radio, _____, infrared, _____, _____, X-rays and _____.

Two ways to be harmed

EM waves can damage you in two quite different ways, and exam answers that mix them up lose the mark even when the verdict is right. The dividing line is whether a single wave carries enough energy to knock an electron off an atom:

Find it on the band

Here is the spectrum as a labelled band. Tap the LOWEST-ENERGY region that is still ionising.

Which are true?

Select ALL THREE statements that are TRUE.

• A sunbed can cause skin cancer because ultraviolet is ionising
• Standing too close to a very hot radiator can burn skin, and that is a heating effect rather than an ionising one
• A radio mast broadcasting all day does not ionise the atoms in a nearby house
• Microwaves are dangerous because they ionise the water in your cells
• Visible light becomes ionising if the lamp is bright enough
• Gamma rays are dangerous mainly because they heat tissue

Two different harms

A microwave oven and a sunbed can both damage human tissue. How do the two mechanisms differ?

• The oven heats tissue that absorbs the microwaves; the sunbed ionises atoms, which can damage DNA
• Both ionise atoms, but the sunbed does it faster
• Both simply heat tissue, but the sunbed gets it hotter
• The oven ionises the water in cells; the sunbed only warms the skin

Reflected, absorbed or transmitted

When an EM wave meets a surface, one of three things happens, and which one depends on both the surface and the wavelength. Take one pane of ordinary window glass. It transmits visible light, which is why you can see through it. It largely absorbs infrared, which is why a car left in the sun gets hot and stays hot: sunlight goes in as visible light, is absorbed by the seats, is re-radiated as infrared, and then cannot get back out. The same glass, the same afternoon, doing opposite things to two neighbouring regions. That selectivity is also how coloured filters work, and how a coloured object gets its colour at all.

Why is it red?

A red jumper looks red under white light. What is the jumper doing?

• Reflecting the red wavelengths and absorbing the others
• Transmitting the red wavelengths and reflecting all the others
• Emitting red light of its own
• Absorbing the red wavelengths and reflecting the others

Tour summary

The EM spectrum runs from radio to _____. As you move towards that end the wavelength gets shorter and the frequency and _____ get higher, while the _____ stays the same for every region in a vacuum. The ionising regions, ultraviolet, X-rays and gamma, can damage living _____, whereas the lower-energy regions can only heat the tissue that absorbs them.