Radio waves and microwaves carry information because so little of them is absorbed on the way

1. Radio waves: the longest wavelengths of the seven regions, and the least likely to be stopped by whatever sits in the way.
   1. Radio and television transmission uses them because a wave metres or tens of metres long diffracts around hills and buildings, so one transmitter reaches aerials it has no clear line of sight to.
   2. Radio astronomy uses them because the radio waves given out by distant stars and galaxies travel through the dust clouds of space and through the atmosphere without being absorbed, while the visible light from the same objects is often blocked.
   3. An RFID tag is a chip and a tiny aerial that answers a reader's radio waves with an identifying code, and because radio waves pass straight through card, plastic and cloth the tag can be read inside a wallet or a sealed box.
2. Microwaves: short enough to be sent as a narrow beam, yet still able to travel through air, cloud and the whole depth of the atmosphere with very little absorption.
   1. Satellite television uses microwaves because they pass up through the atmosphere to the satellite and back down again to the dish without losing much of their energy on either journey.
   2. A mobile phone swaps microwave signals with a nearby mast for the same reason, since air absorbs microwaves weakly and a beam can be aimed at the mast rather than spilled in every direction.
   3. In a microwave oven the waves are strongly absorbed by the water molecules in the food, which raises the food's internal energy right through its volume instead of only at the surface.
   4. Household ovens work at about $2.45GHz$, a frequency that soaks a few centimetres into food before it has all been absorbed, which is why a thick joint of meat still needs standing time for the inside to even out.

Infrared does the heating and sensing jobs, and visible light does the seeing jobs

1. Infrared: given out by every warm object and absorbed readily by skin, food and most solids, where the absorbed energy shows up as a temperature rise.
   1. An electric grill has a very hot element, and the infrared it radiates is absorbed by the outside of the food, which is why grilled food browns on the surface first.
   2. A remote controller flashes coded pulses of infrared from a small diode, which suits short-range communication because the pulses cross a room in a straight line but are absorbed by the walls, so they never reach the set next door.
   3. An intruder alarm watches for a change in the infrared reaching its sensor, and a person walking past emits enough infrared from their warm skin to set it off in complete darkness.
   4. A thermal imaging camera measures how much infrared arrives from each direction and turns those readings into a picture, so a firefighter can pick out a body through smoke and a surveyor can see where a house is losing heat.
   5. Optical fibres carry infrared rather than visible light because glass absorbs infrared least of all, so the signal is still strong after a long journey.
2. Visible light: the one narrow band your eyes respond to, and also the band the Sun delivers most strongly through the atmosphere.
   1. Vision works because the cells of the retina absorb these wavelengths and respond to them, and absorb no other part of the spectrum usefully.
   2. Photography uses a lens to focus visible light onto a sensor whose pixels record its intensity and colour, so the picture matches what an eye at the same place would have seen.
   3. Illumination means flooding a room with visible light so that surfaces reflect it into your eyes; a lamp that gave out mainly infrared would warm you and leave you in the dark.

Ultraviolet, X-rays and gamma rays are useful because they alter cells or pass through solid matter

1. Ultraviolet: invisible, sitting just beyond the violet end, and energetic enough that a single wave can change the molecule that absorbs it.
   1. Security marking uses an ink that is invisible in ordinary light but absorbs ultraviolet and gives out visible light instead, so a postcode written on a laptop glows under a lamp and proves who owns it.
   2. Detecting fake bank notes works the same way, because a genuine note carries fibres and patterns that glow under an ultraviolet lamp while a forgery printed on ordinary paper does not.
   3. Sterilising water uses ultraviolet because the energy it delivers damages the genetic material of bacteria and other microorganisms so that they cannot reproduce, which kills them without putting any chemical into the water.
2. X-rays: they pass through low-density material but are absorbed by dense material, and that difference is what makes a shadow picture possible.
   1. In medical scanning the X-rays go through soft tissue and are absorbed by bone, so the detector behind the patient receives fewer of them where bone was in the way and the bone shows up pale on the finished image.
   2. A security scanner sends X-rays through a suitcase and builds the same kind of image, so a dense metal object inside is seen without the case being opened.
3. Gamma rays: the most penetrating region, able to pass through metal and thick packaging, and able to kill living cells as they go.
   1. Medical equipment is sealed in its packet first and then exposed, because the gamma rays go through the packaging and kill the bacteria inside, so the contents stay sterile until a surgeon opens it.
   2. Food is treated in the same way to kill the bacteria and moulds that spoil it, so it keeps far longer, and the food does not become radioactive itself.
   3. Cancer is detected by giving the patient a substance that emits gamma rays and gathers in the tumour, because those gamma rays are penetrating enough to leave the body and reach a detector outside it.
   4. Cancer is treated by aiming beams of gamma rays into the body from several directions so that they cross at the tumour, where the energy delivered is enough to kill the cancer cells.

How each region harms you follows from where it is absorbed and how much energy it delivers

1. Microwaves: internal heating of body cells.
   1. The water inside your cells absorbs microwave energy, so tissue is heated from within rather than at the surface, and the temperature can rise where you would feel little warning.
   2. That is why an oven has a fine metal mesh in its door and a switch that cuts the power the instant the door is opened.
2. Infrared: skin burns.
   1. Infrared is absorbed in the outer layers of the skin, so all the energy arrives in the one place where a sharp temperature rise is felt at once.
   2. Standing too close to a grill or a bar heater burns the surface of the skin, and the damage stops roughly where the absorption stops.
3. Ultraviolet: damage to surface cells and to the eyes.
   1. Each ultraviolet wave carries enough energy to break molecules inside a surface skin cell, including the DNA that controls how that cell divides.
   2. Damaged DNA can leave cells dividing out of control, which is skin cancer, and sunscreen and covered arms cut the exposure that leads to it.
   3. The cornea and lens absorb ultraviolet too, so exposure without goggles brings on eye conditions such as cataracts, as well as the painful surface damage a welder gets from an unshielded arc.
4. X-rays and gamma rays: mutation of or damage to cells anywhere in the body.
   1. Both regions pass deep into you, so cells well below the skin absorb the energy and not only the ones on the outside.
   2. The energy each wave delivers is large enough to break a strand of DNA, and a cell whose DNA has been altered may die or may start dividing uncontrollably, which is how a cancer begins.
   3. Anyone working with them keeps the dose small: a radiographer stands behind a lead screen and uses the shortest exposure that still gives a usable image.

The higher the frequency, the more energy each wave delivers, and the more serious the damage

1. The energy a single wave delivers is set by its frequency, so one ultraviolet, X-ray or gamma wave arrives with far more energy than one radio or microwave.
2. At the low-frequency end that energy is too small to break a molecule, so what is absorbed simply raises the internal energy of the tissue, which is why microwaves and infrared harm you by heating.
3. From ultraviolet upwards a single wave can strip electrons from atoms and break the bonds in a large molecule, so the harm is damage to the structure of the cell rather than a change in its temperature, and that is why skin cancer and mutation belong to this end.
4. How far the radiation gets in decides how much of you is at risk: infrared stops at the skin, microwaves reach a few centimetres in, and X-rays and gamma rays go right through, so they can alter cells anywhere.
5. Radio waves and visible light do neither at everyday intensities, since tissue absorbs radio waves too weakly to be warmed by them and a single visible wave is far too feeble to damage a cell.