Microwaves cannot be seen, but they can cause burns if they are absorbed by body tissues. Microwave ovens rely on the ability of microwaves to penetrate about 1cm into food. They are absorbed by water in the food, causing it to warm up.

Microwaves go through plastics and glass, but are reflected by metals. To protect us from the harmful effects of microwaves, microwave ovens have metal cases, and the glass doors have a metal mesh.

There is a British Standard which sets a safe limit of 50 watts per square metre at 5cm. This means that measurements of microwave power taken at 5cm from the microwave must be less than 50 watts per square metre.

Cooking with waves - higher

Transferring energy

You should be able to explain how infrared radiation and microwaves transfer energy to materials such as food.

In each case, the kinetic energy of particles is increased when the radiation is absorbed:

infrared radiation is absorbed by all particles on the surface

microwave radiation is absorbed by water particles, both on the surface and up to about 1cm deep into the food

The kinetic energy is transferred to the centre of the food by conduction or convection.

Potential dangers

The higher the frequency of an electromagnetic wave, the greater the energy it transfers for a given amplitude. Infrared radiation has a higher frequency than microwaves. It can transfer enough energy to break chemical bonds. Microwaves cannot.

Hazards of electromagnetic radiation

Over-exposure to certain types of electromagnetic radiation can be harmful. The higher the frequency of the radiation, the more damage it is likely to cause to the body:

microwaves cause internal heating of body tissues

infrared radiation is felt as heat and causes skin burns

X-rays damage cells, causing mutations (which may lead to cancer) and cell death

gamma rays also damage cells, causing mutations (which may lead to cancer) and cell death.

Microwaves

Microwave radiation can be used to transmit signals such as those for mobile phone calls. Microwave transmitters and receivers on buildings and masts communicate with the mobile phones in their range

Ultraviolet light

Ultraviolet radiation - UV - is found naturally in sunlight. We cannot see or feel ultraviolet radiation, but our skin responds to it by turning darker. This happens as our bodies attempt to reduce the amount of ultraviolet radiation reaching deeper skin tissues. Darker skins absorb more ultraviolet light, so less ultraviolet radiation reaches the deeper tissues. This is important, because ultraviolet radiation can cause normal cells to become cancerous.

The three main types of ultraviolet radiation, and some of their effects

type frequency hazard

UV C high causes severe damage to cells

type frequency hazard

UV B medium causes severe sunburn and damage to cells

UV A low weaker effects than UV B

Uses of electromagnetic radiation

Some uses of electromagnetic radiation

Electromagnetic radiation Uses

radiowaves broadcasting

communications

satellite transmissions

microwaves cookingcommunications

satellite transmissions

infrared cookingthermal imaging

short range communications

optical fibres

television remote controls

security systems

visible light visionphotography

illumination

ultraviolet security markingfluorescent lamps

detecting forged bank notes

disinfecting water

Electromagnetic radiation Uses

X-rays observing the internal structure of objectsairport security scanners

medical X-rays

gamma rays sterilising food and medical equipmentdetection of cancer and its treatment

Ionising radiation

Radioactive substances give out radiation all of the time. There are three types of nuclear radiation: alpha, beta and gamma. Alpha is the least penetrating, while gamma is the most penetrating. Nonetheless, all three are ionising radiation: they can knock electrons out of atoms and form charged particles.

Radiation can be harmful, but it can also be useful - the uses of radiation include to:

detect smoke gauge the thickness of paper treat cancer sterilise medical equipment.

Types of radiation

Nuclear radiation comes from the nucleus of an atom. Substances that give out radiation are said to be radioactive. All radiation transfers energy. There are three types of nuclear radiation:

alpha beta gamma.

Radiation can be absorbed by substances in its path. For example, alpha radiation travels only a few centimetres in air, beta radiation travels tens of centimetres in air, while gamma radiation travels many metres. All types of radiation become less intense the further the distance from the radioactive material, as the particles or rays become more spread out.

The thicker the substance, the more radiation is absorbed. The three types of radiation penetrate materials in different ways.

Penetrative properties of different types of radiation

Alpha radiation

Alpha radiation is the least penetrating. It can be stopped (or absorbed) by a sheet of paper.

Beta radiation

Beta radiation can penetrate air and paper. It can be stopped by a thin sheet of aluminium.

Gamma radiation

Gamma radiation is the most penetrating. Even small levels can penetrate air, paper or thin metal. Higher levels can only be stopped by many centimetres of lead or many metres of concrete.

Check your understanding by having a go at this animation. Click on each image of the rock to discover the reading on the radiation meter. Use the readings to confirm that the rock gives out beta radiation.

The waves just make the molecules vibrate with a larger amplitude, which heats the food up.