Electromagnetic waves
Definition
Electromagnetic wave
A transverse wave, such as light or a radio wave, that transfers energy and can travel through a vacuum.
Definition
Transverse wave
A wave in which the oscillations are perpendicular to the direction in which the wave transfers energy.
- Electromagnetic waves are a family of waves that includes radio waves, microwaves, infrared, visible light, ultraviolet, X-rays and gamma rays.
- Every electromagnetic wave is a transverse wave, so its oscillations are at right angles to the direction in which the wave travels.
- The direction of travel is also the direction in which the wave transfers energy.
- In an electromagnetic wave, nothing material moves up and down, because the oscillations are changing electric and magnetic fields rather than vibrating particles.
- All the members of the family share the properties in this article, and they differ from one another only in their wavelength and frequency.
- A diagram of a transverse wave shows the oscillations as a line of crests and troughs drawn along the direction of travel.
- If an electromagnetic wave travels across a page from left to right, its oscillations are drawn as up-and-down displacements on that page.
Travelling through a vacuum
Definition
Vacuum
A space that contains no particles of matter.
- Almost all of space between the Sun, the planets and the stars is a vacuum.
- Electromagnetic waves do not need a medium, so they do not need particles of a solid, liquid or gas to pass their oscillations along.
- This is why light and infrared from the Sun can cross about 1.5×1011 m1.5\times10^{11}\ \text{m}1.5×1011 m of empty space to reach the Earth.
- It is also why mission control can send radio signals to space probes far beyond the Moon and receive their replies.
- Sound is different, because it is passed on by vibrating particles and so it cannot travel through a vacuum at all.
- Electromagnetic waves can also travel through many materials, such as air, glass and water, but a material is never needed for them to travel.
The same speed in a vacuum
- All electromagnetic waves travel through a vacuum at exactly the same speed, whatever their wavelength or frequency.
- This speed is called the speed of light, and its value is 3.0×108 m/s3.0\times10^{8}\ \text{m/s}3.0×108 m/s, which is 300 000 km300\,000\ \text{km}300000 km every second.
- A radio wave and a gamma ray that leave a distant star at the same moment therefore arrive at the Earth at the same moment.
- In air, electromagnetic waves travel at almost exactly the same speed, so 3.0×108 m/s3.0\times10^{8}\ \text{m/s}3.0×108 m/s is also used for calculations in air.
- The speed has a fixed direction as well as a size, which is why it is also described as the velocity of electromagnetic waves in space.
Example
Time for sunlight to reach the Earth
- The Sun is 1.5×1011 m1.5\times10^{11}\ \text{m}1.5×1011 m from the Earth, and light travels through space at 3.0×108 m/s3.0\times10^{8}\ \text{m/s}3.0×108 m/s.
- Use the equation that links distance, speed and time:
- Substitute the values:
- This is just over 888 minutes, so the sunlight reaching you now left the Sun about 888 minutes ago.
- Infrared and ultraviolet from the Sun arrive at exactly the same time, because they travel at the same speed.
Common Mistake
- Do not say that electromagnetic waves need air, particles or any other medium to travel, because they cross the vacuum of space.
- Do not say that high-frequency waves such as gamma rays travel faster in a vacuum, because every electromagnetic wave has the same speed there.
- Do not describe a transverse wave as one that "moves up and down", because the oscillations must be compared with the direction of travel.
Exam technique
Describing electromagnetic waves
- A "state" question on this topic usually wants separate properties, so give transverse, can travel through a vacuum and same speed in a vacuum as three clear points.
- When you describe a transverse wave, name both directions: the oscillations are perpendicular to the direction of energy transfer.
- Use "at right angles" or "perpendicular", because vaguer words such as "sideways" do not make the comparison clear.
- Learn the value 3.0×108 m/s3.0\times10^{8}\ \text{m/s}3.0×108 m/s, because many calculations on electromagnetic waves depend on it.
Self review
- What type of wave is every electromagnetic wave?
- In a transverse wave, how does the direction of the oscillations compare with the direction of travel?
- Why can electromagnetic waves travel through space when sound waves cannot?
- What is the speed of all electromagnetic waves in a vacuum?
- Does a gamma ray travel faster through a vacuum than a radio wave?
