# OCR GCSE P2

Some of the things you see are light sources that emit light. Everything else that you see reflects light. Black objects do not reflect much light, they just absorb it whereas shiny objects reflect most of the light that falls on them. Light is one part of the electromagnetic spectrum:

All waves in the electromagnetic spectrum transmit through a vacuum at 300,000km/s. The energy of a wave in the spectrum increases with increasing frequency. Waves with higher frequencies carry more energy.

Electromagnetic waves transfer energy in packets called photons. The energy of a photon depends only on the frequency of the wave. The principal frequency of electromagnetic radiation is the one with the greatest intensity. This increases with increasing temperature. Radiation from the hot sun has a higher frequency than radiation from the cool earth.

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Solar panels absorb electromagnetic radiation from the sun and transfer it into electrical energy. The energy absorbed each second from an electromagnetic wave depends on its intensity. This depends on:

• The number of photons per second (intensity increases with number of photons)
• The energy transferred by each photon (intensity increases with energy)

The energy of a wave is spread over an increasing area as it moves away from its source. This means that, with distance, the wave decreases in intensity.

The intensity of an electromagnetic wave is the energy transferred to each square metre of absorbing surface each second (J/m^2/s). The intensity of a wave in a vacuum is inversely proportional to the square of its distance from its source. If the wave is partially absorbed by the medium it is passing through, then energy is lost even more rapidly than an inverse square law.

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## Ionisation

Atoms and molecules have no overall electric charge

Electrons are negatively charged

Ions have either a positive or negative charge.

Atoms or molecules are ionised when they lose electrons. The photons of ionising radiation have enough energy to ionise atoms or molecules. The only ionising radiations in the electromagnetic spectrum are high energy UV, X-rays and gamma rays. Ionistaion of a molecule can start off a chemical reaction of that molecule.

Radioactive materials emit gamma rays. Gamma rays pass easily through the human body.

X-rays pass through muscle but are absorbed by the bone and other dense materials. This is why they are used in X-rays and to check luggage. People who work in this area are protected by lead or concrete screens.

Cella are ionised and damaged when they absorb gamma rays or X-rays. The damaged cells can either die or develop cancer. Physical barriers absorb some ionising radiation

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## Microwaves

Things that absorb radiation heat up. Cells that absorb radiation are damaged if they get too hot. You can increase the thermal energy transferred by increasing the exposure time or intensity.

Water molecules are good at transferring energy in microwaves to thermal energy. As the water absorbs the microwaves, they vibrate more and share this energy with molecules around them. Cells contain water so heat up when they absorb microwaves.

Microwave ovens can be used to heat up food which contains water in this way. Microwaves reflect off the metal on the walls and door or are absorbed by them instead of escaping, protecting people from the radiation.

Mobile phone networks use low intensity microwaves. The heating effect of these microwaves is very small, but some people are concerned about the health risks from the radiation. The risk of cell damage from mobile phones is measured by comparing cancer rates in large groups of people who do and do not use mobile phones.

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## The Ozone Layer & the Greenhouse Effect

The ozone layer: Sunlight contains UV radiation. UV radiation can cause sunburn and skin cancer. Sun creams and clothing protect people by absorbing UV radiation. A layer of ozone at the top of the atmosphere absorbs UV radiation from the sun. This protects living things on earth from some of the harmful effects of UV radiation. If the ozone layer thins by 1%, the risk of skin cancer increases by about 4%.

The greenhouse effect: Radiation from the sun contains a range of frequencies. Only some of these frequencies pass through the atmosphere of the earth. The earth warms up when it absorbs radiation from the sun. The infared radiation emitted by the earth makes it cool down. Radiation from the earth may pass into space, reflect off clouds or radiate back from gases which absorb it. When the earth's radiation is absorbed or reflected back, this keeps the earth warmer. We call this the greenhouse effect.

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## Carbon dioxide in the atmosphere

Carbon is found in all living things. It is constantly being recycled through the carbon cycle. Carbon dioxide in the atmosphere is found in very small amounts. It is absorbed by plants through photosynthesis and released by organisms as they decompose or respire.

Carbon dioxide is one of the main greenhouse gases found in the earth's atmosphere. Its level in the atmosphere has been steady for thousands of years, because the rates of absorbtion and release have been the same. In the last 200 years, however, the level of carbon dioxide has steadily increased because:

• the burning of fossil fuels as an energy source has increased the rate of release
• deforestation to clear land has decreased the rate of absorbtion.

The recent increase in the temperature of the earth correlates wih the rise in carbon dioxide levels in the earth's atmosphere. Many scientists therefore believe that carbon dioxide is a greenhouse gas.

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## Global warming

The greenhouse effect is slowly increasing global temperatures. This is called global warming. The effects of this will be:

• changes to the crops which can grow in a region
• flooding of low-lying land due to rising sea levels
• more extreme weather

Various gases in the atmosphere are responsible for the greenhouse effect:

• water vapour has the most effect because there is so much of it
• The small amount of carbon dioxide has an effect
• methane is a strong absorber of infrared radiation, but there is very little of it.

Scientists use models to predict the effects of global warming.They suggest that it will result in more extreme weather events because of:

• increased water vapour in the hotter atmosphere
• increased convection in the atmosphere increasing wind speed
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## Electromagnetic waves for communication

Some electromagnetic waves can carry information from one place to another. That information includes text, voice, music and pictures. Radio waves and microwaves are not absorbed by air, meaning that they can carry TV and radio broadcasts through the atmosphere. Infrared and microwaves are not easily absorbed by the glass of optical fibres. This means that they can be used for long distance telephone and internet communication.

Radio waves use a carrier wave to transfer information. The information is used to change the amplitude or frequency of the carrier, in a process called modulation. A radio receiver demodulates the carrier wave to recover the information.

An analogue signal varies continuously with any value. Sound is an example of an analogue signal. Modulation which varies the amplitude of a wave continuously makes analogue signals.

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## Digital signals

A digital signal only has two values called 1 and 0. Analogue signals, such as sound, can be sent by a radio wave in digital code. Radio receivers use the string of 1s and 0s in the digital signal to decode the origianl analogue signal and produce a copy of it.

Radio waves can be affected by unwanted information called noise. Noise changes the amplitude of the carrier continuously. Noise always reduces the quality of the signal recovered by an analogue receiver, because the original signal cannot be separated from the noise.

The wave pulses which carry information can usually be separated from the continuously varying noise in a digital receiver, so the quality of the signal is not affected.

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## Storing digital information

Digital information is stored as bytes. A byte is a string of 8 binary digits. Each binary digit can be a 1 or a 0.

Digital information builds up images from many small dots called pixels. The colour and brightness of a pixel is set by binary digits. Increasing the number of binary digits for a picture increases the sharpness of the image.

Digital information builds up sound from a rapid series of values called samples. The value of each sample is set by binary digits. Increasing the number of binary digits, increases the sound quality.

An advantage of using digital signals is that they are easily stored in electronic memories so can be processed by computers.

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