# P1b - Electromagnetic spectrum

GCSE Physics unit 2

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• Created by: Elizabeth
• Created on: 25-05-11 11:17

## The Electromagnetic spectrum - Introduction

Electromagnetic radiation travels as waves and transfers energy from one place to another. All electromagnetic waves can travel through a vacuum, and they all travel at the same speed in a vacuum.

The electromagnetic spectrum is a continuous range of wavelengths. The types of radiation that occur in different parts of the spectrum have different uses and dangers, which depend on their wavelength and frequency.

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## The visible spectrum

White light can be split up using a prism 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. Therefore, red light is refracted the least and violet light is refracted the most, causing the coloured light to spread out to form a spectrum.

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## The electromagnetic spectrum

There various electromagnetic radiations. Here are the main types of radiation and their uses: (highest frequency = shortest wavelength)                                    (lowest frequency = highest wavelength)

Gamma radiation - Killing cancer cells.

X-rays - Medical images of bones.

Ultraviolet - Detecting forged bank notes (by florescence)

Visible Light - Reflecting light and seeing

Infrared - Optical fibre communication

Microwaves - Cooking

Radio waves - Television signals

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## Electromagnetic spectrum

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## Gamma rays

Gamma waves have a very high frequency.

Gamma radiation cannot be seen or felt.

It mostly passes through skin and soft tissue, but some of it is absorbed by cells.

Gamma radiation is used for the following purposes.

• To sterilise surgical instruments
• To kill harmful bacteria in food
• To kill cancer cells (lower doses of gamma radiation could lead to cells becoming cancerous) :(
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## X-Rays

X-rays have a lower frequency than gamma radiation. They cannot be seen or felt. X-rays mostly pass through skin and soft tissue, but they do not easily pass through bone or metal.

X-rays are used to produce photographs of bones to check for damage such as fractures. They are also used in industry to check metal components and welds for cracks or other damage.

Lower doses of X-rays can cause cells to become cancerous, so precautions are taken in hospitals to limit the dose received by patients and staff when X-ray photographs are taken.

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## Ultraviolet radiation

Ultraviolet radiation is found naturally in sunlight. We cannot see or feel ultraviolet radiation, but our skin responds to it by turning darker. This happens in an attempt to reduce the amount of ultraviolet radiation that reaches 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.

Ultraviolet radiation is used in:

• Sun beds
• Security pens
• Fluorescent lights (coatings inside the tube or bulb absorb the ultraviolet light and re-emit it as visible light)
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## Infrared radiation

Infrared radiation is absorbed by the skin and we feel it as heat. It is used in heaters, toasters and grills. It is also used for television remote controls and in optical fibre communications.

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## Radio waves

Radio waves have lower frequencies and longer wavelengths than microwaves. They are used to transmit television and radio programmes. Television uses higher frequencies than radio.

A radio programme receiver does not need to be directly in view of the transmitter to receive programme signals. For low frequency radio waves diffraction can allow them to be received behind hills, although repeater stations are often used to improve the quality of the signals.

The lowest frequency radio waves are also reflected from an electrically charged layer of the upper atmosphere, called the Ionosphere. This means that they can reach receivers that are not in the line of sight because of the curvature of the Earth's surface.

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## Microwaves

Cooking - Microwave radiation has lower frequencies and longer wavelengths than visible light. Microwaves with certain wavelengths are absorbed by water molecules and can be used for cooking. Water in the food absorbs the microwave radiation, which causes the water to heat up and cook the food. The water in living cells can also absorb microwave radiation. As a result, they can be killed or damaged by the heat released.

Communications - Microwave radiation can also be used to transmit signals such as mobile phone calls. Microwave transmitters and receivers on buildings and masts communicate with the mobile telephones in their range. Certain microwave radiation wavelengths pass through the Earth's atmosphere and can be used to transmit information to and from satellites in orbit.

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## Questions

1. Which of the following types of electromagnetic radiation has the highest frequency?

• Gamma radiation
• Visible light
• Microwaves

2. Ultraviolet light can be used:

• For mobile phone communications
• For fluorescent lights

3. Which type of electromagnetic radiation travels at the greatest speed in a vacuum?

• They all travel at the same speed
• Microwaves
• Radiowaves
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