Electromagnetic Waves
Electromagnetic waves are transverse waves that transfer energy from the source of the waves to an absorber.
Electromagnetic waves form a continuous spectrum and all types of electromagnetic wave travel at the same velocity through a vacuum (space) or air. That velocity is 300 000 000 m/s.
The waves that form the electromagnetic spectrum are grouped in terms of their wavelength and their frequency. Going from long to short wavelength (or from low to high frequency) the groups are: radio, microwave, infrared, visible light (red to violet), ultraviolet, X-rays and gamma rays.
The highest frequency waves (gamma) have the highest energy and shortest wavelength.
Region of EM Spectrum | Facts you need to know: | Hazards to Humans | Ionising? | Uses |
Radio | Produced by oscillations in electrical circuits. When radio waves are absorbed they may create an alternating current with the same frequency as the radio wave itself, so radio waves can themselves induce oscillations in an electrical circuit. | none | no | television and radio transmission |
microwave | Microwaves are not absorbed by the atmosphere which is why they are used to communicate with satellites | overheats cells | no | Satellite communication, wifi, radar guns |
infrared | All bodies emit infrared radiation. The hotter the body the more IR is emitted | Burns | no | IR cameras, remote controls, fibre optic communication |
visible light | The only region our eyes can detect | Eye Damage | no | fibre optic communication |
ultraviolet | can be absorbed by some objects and re-emitted as visible light. This is why some things can glow in the dark. | Blindness, premature skin ageing, skin cancer | no | fraud detection, energy efficient lamps |
X-ray | The “X” in X-ray stands for the unknown, just as x stands for an unknown quantity in maths. | Cell mutation, cancer | yes | medical imaging |
Gamma ray | Changes in atoms and the nuclei of atoms can result in electromagnetic waves being generated or absorbed over a wide frequency range. Gamma rays originate from changes in the nucleus of an atom. | Cell mutation, cancer | yes | medical treatments |
Question: Explain why it is important that the Earth’s atmosphere absorbs gamma rays emitted by the Sun.
To deduce human exposure to gamma rays
because gamma rays damage human cells
Question: A new technology has been developed to charge small electrical devices from the radio waves in the air around us.
Explain how radio waves could charge a small electrical device.
- the small device absorbs / receives the radio waves
- which creates an alternating current of the same frequency (as the radio waves)
Refraction & Reflection of EM Waves
Different substances may absorb, transmit, refract or reflect electromagnetic waves in ways that vary with wavelength.
Refraction is due to the difference in velocity of the waves in different substances. Visible light is refracted and reflected at a boundary with glass

Wave front diagrams can be used to explain refraction in terms of the change of speed that happens when a wave travels from one medium to a different medium. Not all of the wave enters the new medium at the same time, so the first part that enter will slow down more than the part not in the medium. His causes the wave to change direction.

Question: Wave front diagrams are used to explain why light refracts when it passes from air into glass.
Figure 1
(a) Explain why the light refracts as it passes from air into glass. (3 marks)
- speed / velocity of the wave in the glass is lower
- so the edge of the wave(front) entering the glass slows down
- but the part of the wave(front) in the air continues at the higher speed / velocity (causing a change in direction)
Black Body Radiation (Triple Physics Only)
All bodies (objects), no matter what temperature, emit and absorb infrared radiation. The hotter the body, the more infrared radiation it radiates in a given time.
A perfect black body is an object that absorbs all of the radiation that is directed towards it.
Learn this definition
A black body does not reflect or transmit any radiation. Since a good absorber is also a good emitter, a perfect black body would be the best possible emitter.
All bodies (objects) emit radiation
A body at constant temperature is absorbing radiation at the same rate as it is emitting radiation. The temperature of a body increases when the body absorbs radiation faster than it emits radiation.
The temperature of a body is related to the balance between incoming radiation absorbed and radiation emitted.
The temperature of the Earth depends on many factors including: the rates of absorption and emission of radiation, reflection of radiation into space. If the Earth receives more radiation than is reflected/emitted back into space, the Earth will heat up.
Question: Albedo is a measure of the amount of solar radiation reflected by an object compared to the total solar radiation incident on the object.
A perfect reflector has an Albedo value of 1.0
A perfect absorber has an Albedo value of 0.0
What is the Albedo value of a perfect black body?
0.0 (because A perfect black body is an object that absorbs all of the radiation incident on it. )