The light we see is just a tiny fraction of the waves constantly moving around us. The is a continuous range of all possible frequencies of electromagnetic radiation.
The spectrum consists of seven main groupings. In order from longest wavelength to shortest wavelength, they are:
As you move from to , the wavelength decreases while the frequency increases. Because higher frequency waves carry more energy per photon, the photon energy also increases across this sequence.
The highest frequency, shortest wavelength waves (, , and ) carry enough energy to be . This means they can knock electrons out of atoms, making them potentially harmful to living cells and DNA.
Despite the vast range of the EM spectrum, our bodies are only equipped to detect a sliver of it. The human eye detects a very limited range of frequencies known as .
accounts for an exceptionally tiny fraction (approximately 0.0035%) of the entire EM spectrum. It sits squarely between (which has a lower frequency) and (which has a higher frequency).
The at the back of the eye acts as the biological for these waves. Within the visible spectrum, red light has the longest wavelength (approx. ) and lowest frequency, while violet light has the shortest wavelength (approx. ) and highest frequency.
Whether it is a radio signal to a spacecraft or light from a distant star, all EM waves race through space at an identical, unimaginable speed. Unlike sound waves, EM waves do not require a medium and can freely propagate through the empty space of a .
All EM waves travel through a at the exact same constant, very high speed: . While they travel at this identical speed in space, EM waves slow down when entering different substances like glass or water, which leads to .
Because the wave speed is constant in a , there is a strict inverse relationship between frequency and wavelength. As frequency increases, wavelength must proportionally decrease, as shown by the wave speed equation:
Where:
When you feel the warmth of the Sun on your face, you are experiencing the direct transfer of energy across millions of miles of empty space. EM waves carry energy away from a and deliver it to an .
Once the radiation is taken in by an object, it ceases to exist as radiation. Its energy is typically converted into other forms, most commonly thermal energy, or it can cause specific chemical or electrical effects.
Examples of this energy transfer include:
Why can light pass through a window, but the heat from a fire is partially blocked? It all comes down to the size of the wave. A material's behaviour is entirely .
Depending on the wavelength of the incident EM radiation, a substance will undergo , , or .
Examples of how these interactions change with wavelength include:
Students often confuse the speed of light with the speed of sound. Remember that all EM waves travel at in a vacuum, which is nearly a million times faster than sound.
Exam Technique: When asked to list the EM spectrum, pay close attention to whether the question asks for the order of increasing or decreasing wavelength or frequency. Use the mnemonic 'Raging Martians Invaded Venus Using X-ray Guns' to remember the order from longest to shortest wavelength.
OCR exams heavily emphasise precise wording for energy transfer; you must explicitly state that EM radiation transfers energy from a 'source' to an 'absorber' and that it 'ceases to exist as radiation' once absorbed.
For recall questions on visible light, you must specifically state that the eye detects a 'very limited range' of frequencies, as vague phrases like 'a small part' may not score the mark.
Electromagnetic (EM) spectrum
A continuous range of frequencies of electromagnetic radiation, grouped into seven main regions.
Radio waves
The region of the electromagnetic spectrum with the longest wavelengths and lowest frequencies.
Microwaves
A region of the electromagnetic spectrum commonly used for satellite communication and heating food.
Infrared
A region of the electromagnetic spectrum emitted by all objects based on their temperature, typically felt as thermal radiation.
Visible light
The very limited range of electromagnetic frequencies that can be detected by the human eye.
Ultraviolet
A region of the electromagnetic spectrum with frequencies higher than visible light, capable of causing sunburn and ionising atoms.
X-rays
A high-energy, ionising region of the electromagnetic spectrum commonly used in medical imaging to transmit through soft tissues.
Gamma rays
The region of the electromagnetic spectrum with the shortest wavelengths and highest frequencies, carrying the most photon energy.
Ionising radiation
High-frequency radiation that carries enough energy to knock electrons out of atoms, potentially damaging living cells and DNA.
Retina
The light-sensitive layer at the back of the eye that acts as the biological absorber for visible light.
Vacuum
A space entirely devoid of matter, through which all electromagnetic waves travel at exactly .
Refraction
The change in direction of a wave as it moves from one medium to another due to a change in its speed.
Source
An object or device that emits electromagnetic radiation.
Absorber
A material or object that takes in electromagnetic radiation, converting its energy into other forms.
Wavelength-dependent
A property where a material's interaction with a wave changes based entirely on the physical length of the wave.
Absorption
The process where a substance takes in the energy of an electromagnetic wave, often resulting in heating or a chemical effect.
Transmission
The process where electromagnetic waves pass straight through a material without being absorbed.
Reflection
The process where electromagnetic waves bounce off the surface of a material.
Put your knowledge into practice — try past paper questions for Physics B
Electromagnetic (EM) spectrum
A continuous range of frequencies of electromagnetic radiation, grouped into seven main regions.
Radio waves
The region of the electromagnetic spectrum with the longest wavelengths and lowest frequencies.
Microwaves
A region of the electromagnetic spectrum commonly used for satellite communication and heating food.
Infrared
A region of the electromagnetic spectrum emitted by all objects based on their temperature, typically felt as thermal radiation.
Visible light
The very limited range of electromagnetic frequencies that can be detected by the human eye.
Ultraviolet
A region of the electromagnetic spectrum with frequencies higher than visible light, capable of causing sunburn and ionising atoms.
X-rays
A high-energy, ionising region of the electromagnetic spectrum commonly used in medical imaging to transmit through soft tissues.
Gamma rays
The region of the electromagnetic spectrum with the shortest wavelengths and highest frequencies, carrying the most photon energy.
Ionising radiation
High-frequency radiation that carries enough energy to knock electrons out of atoms, potentially damaging living cells and DNA.
Retina
The light-sensitive layer at the back of the eye that acts as the biological absorber for visible light.
Vacuum
A space entirely devoid of matter, through which all electromagnetic waves travel at exactly .
Refraction
The change in direction of a wave as it moves from one medium to another due to a change in its speed.
Source
An object or device that emits electromagnetic radiation.
Absorber
A material or object that takes in electromagnetic radiation, converting its energy into other forms.
Wavelength-dependent
A property where a material's interaction with a wave changes based entirely on the physical length of the wave.
Absorption
The process where a substance takes in the energy of an electromagnetic wave, often resulting in heating or a chemical effect.
Transmission
The process where electromagnetic waves pass straight through a material without being absorbed.
Reflection
The process where electromagnetic waves bounce off the surface of a material.