Properties and uses of each EM wave type

One hospital, seven kinds of wave

Walk through a large hospital and you will find almost the whole electromagnetic spectrum at work. A thermometer held near your forehead reads the infrared your skin gives out. A lamp in an empty operating theatre fills it with ultraviolet to kill bacteria. A radiographer takes an X-ray of a broken wrist. The wireless network carries your records on microwaves. The syringes were sterilised with gamma rays.

The last lecture showed that all seven families are the same kind of wave, travelling at the same speed. So why can't one kind of wave do every job?

The answer is not a list of uses to learn. It is three questions. Once you can ask them, you can work out why each wave is used where it is, and why it is dangerous where it is.

Three questions that choose the wave

1. What does it pass through, and what absorbs it? Every material lets some frequencies through and absorbs others. Your body lets most X-rays through but stops visible light. The atmosphere stops most ultraviolet but lets radio waves pass. A wave is useful when it passes through what you want to see through, and is stopped by what you want to see.

2. How long is the wavelength, compared with the things involved? The diffraction lecture showed that waves spread round obstacles of about their own size, and pass straight by obstacles much smaller than a wavelength. A long radio wave bends round a hill. A short wave travels in a narrow beam and reflects sharply from small objects. And to see detail, you need a wavelength about as small as the detail.

3. How much energy does one photon carry? The last lecture showed that $E=hf$, and that what a wave can do to a molecule depends on the energy of one photon, not on the total. A low-energy photon can only make molecules move and warm up. A higher-energy photon can break a chemical bond. An even higher one can knock an electron out of an atom: it ionises.

Now take the families in order of frequency, and ask the three questions each time.

Radio waves: long enough to bend round the world

Radio waves have wavelengths from about a metre to many kilometres. Two things follow from that length.

First, they diffract round hills and buildings, so a long-wave or medium-wave radio station can be heard behind a hill where its mast is out of sight. Some of the wave also follows the curve of the ground, as a ground wave.

Second, the upper atmosphere has a layer of charged particles, the ionosphere, made by sunlight about 50 to 400 km up. Waves between about 3 and 30 MHz are bent back down by it, as a sky wave. They come down far beyond the horizon. Ships, aircraft over oceans and international broadcasters use these frequencies to reach thousands of kilometres without satellites.

Higher frequencies pass through the ionosphere and out into space. That is bad for reaching the far side of the world, and exactly right for reaching a satellite. FM radio and television, at higher frequencies, travel mostly in straight lines and need masts in sight of the receiver.

Microwaves: beams, echoes and dinner

Microwaves have wavelengths from about a metre down to a millimetre. They are short enough to send in a narrow beam from a dish, and they pass through the ionosphere and most clouds. So satellite television, satellite phones and GPS all use microwaves. Mobile phones and wireless networks use the long-wavelength end of the microwave range.

Radar sends out a short pulse of microwaves and listens for the echo. The pulse goes to the aircraft and back at speed $c$, so the distance to the aircraft is

where $t$ is the time between sending the pulse and hearing the echo. The 2 is there because the pulse travels the distance twice. Microwaves suit radar because their short wavelength reflects strongly from an aircraft or a ship and stays in a narrow beam, so the radar knows the direction too.

Then there is cooking. Test yourself before reading on.

The oven needs no special frequency, because heating by twisting molecules works over a wide range of frequencies. The danger from microwaves is the same as the use: heating. A leak from a damaged oven could warm the eyes or skin. It cannot ionise them.

Infrared: everything warm is shining

Every object gives out infrared, and the hotter it is, the more it gives out. Your skin, at about 37 °C, gives out most strongly at wavelengths of about 10 μm. You cannot see this, but a thermal camera can. It turns infrared into a picture, with warmer things brighter. Firefighters use thermal cameras to find people through smoke, and engineers use them to find where a house loses heat.

Infrared is absorbed by skin and turned into thermal energy, which is why you feel the warmth of a fire. Grills and toasters cook mainly by infrared, which heats the surface. TV remote controls send coded flashes of infrared.

Long-distance optical fibres also use infrared, near 1550 nm. Silica glass is most transparent near this wavelength. In good fibre, about 1% of the light is still there after 100 km. The light stays inside the fibre by total internal reflection, from the lecture on the critical angle.

Visible light: the narrow window

Visible light, from about 700 nm (red) to 400 nm (violet), is a tiny part of the spectrum. It matters because the atmosphere and water let it through, and because the Sun gives out most strongly near it. Eyes evolved to use it, and plants use red and blue light for photosynthesis.

Visible photons have between about 2.8 × 10⁻¹⁹ J (red) and 5 × 10⁻¹⁹ J (violet). That is enough to change the shape of a molecule in your eye and so start a nerve signal. It is not enough to break most strong bonds.

Ultraviolet: enough energy to break bonds

Here the third question takes over. The figure compares the energy of one photon with two thresholds. The first is the energy needed to break one carbon–carbon bond, about 5.75 × 10⁻¹⁹ J (from the average bond enthalpy, 346 kJ/mol, divided by the number of bonds in a mole). The second is the energy to ionise a hydrogen atom, 2.18 × 10⁻¹⁸ J.

Look at where ultraviolet sits. Its photons are close to, and often above, the bond line, but still below the ionising line. So most ultraviolet does not ionise. It does something different but still harmful: it breaks or rearranges bonds directly.

The most important example is DNA. DNA absorbs ultraviolet most strongly near 260 nm. When it does, two neighbouring thymine bases can bond to each other, forming a "thymine dimer". A cell with this damage cannot copy its DNA properly. Germicidal lamps use exactly this: a low-pressure mercury lamp gives out almost all its ultraviolet at 254 nm, very close to that peak, and is used to disinfect water, air and surfaces.

Sunlight contains ultraviolet too, but the ozone layer absorbs almost all of the shortest kind (UV-C) before it reaches the ground. The UV-B that gets through causes sunburn and can damage skin cell DNA, which increases the risk of skin cancer. It also starts the making of vitamin D in the skin. Some materials absorb ultraviolet and give out visible light. This is fluorescence, and it makes security marks on banknotes glow under an ultraviolet lamp.

X-rays: through flesh, stopped by bone

X-ray photons are far above the ionising line. They pass through soft tissue, which is mostly light atoms such as hydrogen, carbon and oxygen. Bone contains calcium, a heavier atom, and absorbs much more of them. So in a radiograph, the X-rays that pass through the body reach a detector, and the bones cast a light shadow. A CT scan takes many X-ray images from different angles and lets a computer build a three-dimensional picture. Airport scanners do the same with luggage.

X-rays have a second, very different use, which depends on the second question. Their wavelengths, around 0.1 nm, are similar to the spacing between atoms in a crystal. So a crystal diffracts X-rays, like a grating for light, and the pattern of spots shows how the atoms are arranged. This is X-ray crystallography. In May 1952, Raymond Gosling, working with Rosalind Franklin at King's College London, took the X-ray diffraction image of DNA now called Photo 51. Its pattern pointed to a helix, and it was key evidence that James Watson and Francis Crick used in building their double-helix model, published in 1953.

Because X-rays ionise, they can damage DNA. Medical X-rays use the smallest dose that gives a useful image, and radiographers stand behind shielding.

Gamma rays: from the nucleus

Gamma rays come from changes inside the nucleus of an atom. As the last lecture explained, a gamma ray and an X-ray of the same energy are the same kind of photon; the names describe where they came from. The most energetic gamma rays carry the most energy per photon in the spectrum, and they pass through most materials. Stopping them needs thick lead or concrete.

Their uses all come from that penetration and that energy. Gamma rays from cobalt-60 sterilise syringes, dressings and surgical instruments after they are sealed in their packaging, with no heat needed. In a PET scan, a patient receives a substance that gives out positrons. Each positron meets an electron, and the pair becomes two gamma rays travelling in opposite directions, which detectors around the patient record. In cancer treatment, beams of ionising photons are aimed at a tumour from many directions, so that the tumour receives a large dose and the healthy tissue around it a smaller one. Most treatment machines today make these photons as high-energy X-rays from an accelerator, but some, such as the Gamma Knife, still use gamma rays from cobalt-60.

Now test the three questions on some real uses.

Some cards had a second reason too: gamma sterilisation needs both energy and penetration. A good exam answer names every property that matters.

Worked examples

Use $c=3.00\times 10^8$ m/s and $h=6.63\times 10^{-34}$ J s.

Three traps