Mr Palmer's AQA GCSE Physics Notes
Electromagnetic Spectrum
What is a Wave?
A wave diagram.
A wave transfers energy from one place to another without transferring matter. Each part of the wave oscillates (vibrates) in place as the wave as a whole moves.
There are two parts of the wave that you must be able to identify:
Waves also have a property called frequency. The frequency is the number of cycles that pass a point in a second. Frequency is measured in hertz, abbreviated Hz.
The Wave Formula
All parts of the electromagnetic spectrum travel at the same speed. This is the speed of light, about 300,000,000 meters per second. (You need to memorise this number).
An Example Question:
A tsunami wave has a speed of approximately 200m/s and their frequency is about 0.1Hz. What is their wavelength?
EM Spectrum Overview
The colours we see with our eye are just a small part of something called the electromagnetic spectrum. All the parts of the spectrum are waves and travel at the same speed (300,000,000m/s) in a vacuum.
Different parts of the spectrum affect materials differently. Sometimes they are reflected and sometimes absorbed.
Confusingly, the parts of the spectrum may also be referred to as `radiation' but this is not (with the exception of Gamma Rays) the same as atomic radiation. If a material absorbs radiation from the electromagnetic spectrum, it will get hotter. Also, if the material is a metal, it will generate a current of the same frequency as the wave itself.
You must memorise the order of the spectrum, and which parts have the highest/lowest frequency and the highest/lowest wavelength.
Gamma Rays
Gamma rays have the highest frequency and the shortest wavelength of the electromagnetic spectrum. They are highly penetrative, able to pass through bone and metal. High doses of gamma rays kill living material and low doses can cause cancer.
Gamma rays are used to kill harmful bacteria in food, to sterilise medical equipment and in a medical procedure known as gamma-knife surgery.
In gamma-knife surgery, several gamma sources are placed around a difficult-to-operate tumour. Each source emits gamma radiation which damages all the cells in its path, but the tumour, which is hit by all the sources simultaneously, is damaged the most and hopefully killed.
X-Rays
An x-ray of human feet
X-rays are used to make pictures of dense materials, such as bone and metal. A photographic negative is placed behind the subject and x-rays are shined through the subject, onto the negative. Less dense areas, such as skin and tissue allow the x-rays to pass through and expose the negative. Denser areas, such as bone, absorb the x-rays, not allowing the negative beneath them to be exposed. When the negative is developed, the shadow of the more dense materials can be seen.
Prolonged exposure to x-rays increases the chance of developing cancer.
X-ray that come from outer space are absorbed by the atmosphere before reaching Earth's surface.
Ultraviolet Radiation
Ultraviolet radiation is the cause of both suntans and skin cancer. Ultraviolet light has difficulty penetrating further into the human body than the skin. A suntan is the body's way of protecting skin from UV light. The darker the skin, the more UV light it absorbs, preventing it from getting deeper into the body.
Some kinds of materials, such as security pens, fluoresce under UV light, meaning that they give off light our eyes can see.
Scorpions will fluoresce under UV lights.
Visible Light
Visible light is between ultraviolet and infra red light on the electromagnetic spectrum. The order of the colours is: Red, Orange, Yellow, Green, Blue, Indigo and Violet.
Red light has the lowest frequency and longest wavelength of the colours.
Violet light has the highest frequency and the shortest wavelength of the colours.
Visible light undergoing total internal reflection in an optical fibre
Fibre optic wires carry light long distances by using total internal reflection to keep the light inside. The incident ray always strikes the edge of the wire at an angle greater than the critical angle.
Fibre optic wires are used in endoscopes. A camera is attached to one end of the wire and the other is inserted into the patient. Light is shown down the wire, reflected off the inside of the patient and back up the wire, allowing the doctor to see inside without invasive surgery.
Fibre optics are also used in telecommunications for either telephone cables or internet connections. Fibre optics are used because they are able to carry vastly more information than a same-sized copper wire which uses electricity to send signals.
Most often, visible light is used in fibre optics, but infra red light can be used as well.
Infra Red Radiation
We can't see infra red radiation, but we can perceive it as heat. Infra red radiation is used in electrical heaters, and remote controls. It can also travel through optical fibres.
Various objects as viewed through an infra red camera
Microwaves
Microwaves have two useful properties. Firstly, microwaves are strongly absorbed by water molecules, which is how we use them for cooking. A microwave oven cooks the food by heating the water within it.
Secondly microwaves can travel through the atmosphere. We use microwaves to transmit messages to satellites which then relay them back to Earth. This is how mobile phones send messages to each other.
Radio Waves
Radio waves have the longest wavelength and the lowest frequency of any wave in the electromagnetic spectrum. Because of this, they are able to very easily diffract around obstacles, such as mountains.
The longest radio waves also reflect off of the Earth's atmosphere. This allows people to send messages around the curvature of the Earth, without the use of satellites.
When a radio wave is absorbed by an radio antenna (aerial) it causes an alternating current in the antenna with the same frequency as the radio wave. It will also cause the antenna to heat up slightly.
Physics P1 Topics
Physics P2 Topics
- Motion
- Forces
- Energy
- Static Electricity
- Current Electricity
- Mains Electricity
- Momentum
- Nuclear Physics
Other