Unit P1b     Radiation and the Universe

13.5 What are the uses and hazards of the waves that form the electromagnetic spectrum?
Electromagnetic radiations are disturbances in an electric field. They travel as waves and move energy from one place to another. They can all travel through a vacuum and do so at the same speed. The waves cover a continuous range of wavelengths called the electromagnetic spectrum. The uses and hazards of the radiations in different parts of the electromagnetic spectrum depend on their wavelength and frequency.

You need to be able:
• to evaluate the possible hazards associated with the use of different types of electromagnetic radiation
• to evaluate methods to reduce exposure to different types of electromagnetic radiation.

The Electromagnetic Spectrum
Electromagnetic radiation travels as waves and moves energy from one place to another.
All types of electromagnetic waves travel at the same speed through a vacuum (space).

The electromagnetic spectrum is continuous but the wavelengths within it can be grouped into types of increasing wavelength and
decreasing frequency:
gamma rays, X-rays, ultraviolet rays, visible light, infra red rays, microwaves and radio waves.

Properties of EM waves
Different wavelengths of electromagnetic radiation are reflected, absorbed or transmitted differently by different substances and
types of surface.
When radiation is absorbed the energy it carries makes the substance which absorbs it hotter and may create an alternating
current with the same frequency as the radiation itself.

Dangers of radiation
Different wavelengths of electromagnetic radiation have different effects on living cells. Some radiations mostly pass through soft
tissue without being absorbed, some produce heat, some may cause cancerous changes and some may kill cells.
These effects depend on the type of radiation and the size of the dose.

Radio waves, microwaves, infra red and visible light can be used for communication.
Microwaves can pass through the Earths atmosphere and are used to send information to and from satellites and within mobile
phone networks.
Infra red and visible light can be used to send signals along optical fibres and so travel in curved paths.

Signals
Communication signals may be analogue (continuously varying) or digital (discrete values only, generally on and off).
Digital signals are less prone to interference than analogue and can be easily processed by computers.

The Wave Equation
Electromagnetic waves obey the wave formula:
wave speed = frequency × wavelength
(metre/second, m/s)        (hertz, Hz)                 (metre, m)
 

13.6 What are the uses and dangers of emissions from radioactive substances?
Radioactive substances emit radiation from the nuclei of their atoms all the time. These nuclear radiations can be very useful but may also be very dangerous. It is important to understand the properties of different types of nuclear radiation.

You need to be able:
• to evaluate the possible hazards associated with the use of different types of nuclear radiation
• to evaluate measures that can be taken to reduce exposure to nuclear radiations
• to evaluate the appropriateness of radioactive sources for particular uses, including as tracers, in terms of the type/types of radiation emitted and their half-lives.

Atomic Structure
The basic structure of an atom is a small central nucleus composed of protons and neutrons surrounded by electrons.
The atoms of an element always have the same number of protons, but have a different number of neutrons for each isotope.

Radioactivity
Some substances give out radiation from the nuclei of their atoms all the time, whatever is done to them. These substances are said
to be radioactive.
An alpha particle is a helium nucleus, a beta particle is an electron from the nucleus and gamma radiation is electromagnetic radiation.
Properties of the alpha, beta and gamma radiations - their relative ionising power, their penetration through materials and
their range in air.
Alpha and beta radiations are deflected by both electric and magnetic fields but gamma radiation is not.

The uses of and the dangers associated with each type of nuclear radiation.

The half-life of a radioactive isotope is defined as the time it takes for the number of nuclei of the isotope in a sample to halve or the
time it takes for the count rate from a sample containing the isotope to fall to half its initial level.
 

13.7 What do we know about the origins of the Universe and how it continues to change?
Current evidence suggests that the universe is expanding and that matter and space expanded violently and rapidly from a very small initial point i.e. the universe began with a big bang.

You should be able:
• to compare and contrast the particular advantages and disadvantages of using different types of telescope on Earth and
in space to make observations on and deductions about the universe.

Red Shift and the Big Bang
If a wave source is moving relative to an observer there will be a change in the observed wavelength and frequency.
There is a red-shift in light observed from most distant galaxies.
The further away galaxies are the bigger the red-shift.
How the observed red-shift provides evidence that the universe is expanding and supports the big bang theory (that the universe
began from a very small initial point).

Telescopes
Observations of the solar system and the galaxies in the universe can be carried out on the Earth or from space.
Observations are made with telescopes that may detect visible light or other electromagnetic radiations such as radio waves or X-rays.