The Electro-Magnetic Spectirum

You should remember from your Key Stage 3 studies that white light can be split up into a spectrum of many different colours. You should know that visible light is just part of a continuous spectrum of electromagnetic radiation. Different types of electromagnetic radiation have different hazards and uses.

What is a spectrum?

The visible spectrum

Refraction from a prism

Using a prism, you can split up white light to form a spectrum. A prism is a block of glass with a triangular cross-section. The light waves are refracted as they enter and leave the prism. The shorter the wavelength of the light, the more it is refracted. As a result, red light is refracted the least and violet light is refracted the most, causing the coloured light to spread out to form a spectrum.

Visible light is just one type of electromagnetic radiation. There are various types of electromagnetic radiation with longer wavelengths of light than red light and with shorter wavelengths than violet light. All the different types of electromagnetic waves travel at the same speed through space.

THE ELECTROMAGNETIC SPECTRUM

X-rays, visible light and radio waves are all types of electromagnetic radiation.

The main types of electromagnetic radiation

there are...
-RADIO WAVES
-MICRO WAVES
-INFARED
-VISABLE LIGHT
-ULTRA VIOLET

-X RAYS
 -GAMMA RAYS

All types of electromagnetic radiation:

• are transverse waves
• travel at the same speed in a vacuum - empty space

The speed of electromagnetic radiation in a vacuum is 299,792,458 m/s. This is approximately three hundred million metres per second - nearly nine hundred thousand times faster than sound, which is why you see a flash of lightning before you hear the thunder.

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.

Uses of electromagnetic radiation

Scanning by reflection

The coloured part of your eye, the iris, has a unique pattern. Visible light reflected off the iris can be analysed by a computer and compared to stored patterns. In this way, people can be identified by their iris pattern.

Scanning by emission

All objects give off or emit infrared radiation. The hotter the object, the more infrared radiation it emits. Sensors that can detect this radiation allow the temperature of objects to be measured. This is useful for medical scans to detect abnormally hot parts of the body, which may indicate damage or illness. The detection of infrared emissions is also useful for tracking criminals at night and for military 'night vision' goggles.

Scanning by absorption

Materials absorb different types of electromagnetic radiation to different degrees.

For example X-rays are weakly absorbed by skin and muscle, but strongly absorbed by dense material such as bone and metal. This means that X-rays can be used to produce images of bones inside your body, to check for damage such as fractures.
X-rays are also used in industry to check metal components and welds for cracks or other damage.
Microwave radiation is absorbed by water molecules, so it can be used for cooking. Water in the food absorbs the microwave radiation, which causes the water to heat up and cook the food.
Microwaves are also used to monitor rain. Satellites have been set up to measure how much microwave radiation, sent from ground-based transmitters, is absorbed by water in the atmosphere. Computers analyse the information and build up a picture of the rain and cloud.
Ultraviolet light is used to detect forged bank notes. Certain chemicals in paper absorb ultraviolet light and emit the energy as blue light. Real bank notes have invisible markings that only show up using ultraviolet light.