Light is an electromagnetic wave
Definition
Electromagnetic wave
A transverse wave, such as light or a radio wave, that transfers energy and can travel through a vacuum.
Definition
Visible light
The narrow band of electromagnetic waves, with wavelengths from about 400 nm400\ \text{nm}400 nm to 700 nm700\ \text{nm}700 nm, that the human eye can detect.
- Light is an electromagnetic wave, and it is the only part of the electromagnetic spectrum that your eyes can detect.
- This means light belongs to the same family as radio waves, microwaves, infrared, ultraviolet, X-rays and gamma rays.
- In the spectrum, visible light sits between infrared and ultraviolet.
- Light is not passed along by vibrating particles, which makes it a completely different kind of wave from sound.
Properties of light
- It is transverse.
- The oscillations of a light wave are at right angles to the direction in which the light travels.
- It can travel through a vacuum.
- Light from the Sun and from distant stars crosses empty space to reach your eyes.
- It travels at 3.0×108 m/s3.0\times10^{8}\ \text{m/s}3.0×108 m/s in a vacuum.
- This is the same speed as every other electromagnetic wave, which is why this value is called the speed of light.
- It transfers energy from a source to an absorber.
- Sunlight absorbed by a solar cell or a leaf transfers energy to it.
- It obeys the wave equation.
- The relationship v=fλv=f\lambdav=fλ links the speed, frequency and wavelength of light, just as it does for any other wave.
Evidence from space
- Light from the Sun reaches the Earth across about 1.5×1011 m1.5\times10^{11}\ \text{m}1.5×1011 m of almost empty space, so light cannot need a medium to travel.
- Sound does need particles, which is why astronauts on a spacewalk can see each other clearly but have to talk by radio.
- Light and radio signals from a spacecraft arrive together, which shows that they travel at the same speed.
Example
Light and radio signals from the Moon
- The Moon is about 3.8×108 m3.8\times10^{8}\ \text{m}3.8×108 m from the Earth.
- Moonlight and a radio signal from a lunar lander both travel 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:
- Both arrive after the same time, about 1.31.31.3 seconds, because light and radio waves are both electromagnetic waves.
Common Mistake
- Do not describe light as a sound wave or say that it needs air to travel, because light crosses the vacuum of space.
- Do not give only "transverse" when a question asks what type of wave light is, because many waves are transverse and the key word is electromagnetic.
Exam technique
Recall questions on light
- If a one-mark question asks what type of wave light is, write electromagnetic wave in full rather than "electric" or "magnetic".
- When asked how we know light can cross empty space, use light from the Sun or the stars as the evidence.
- Any property you learn for electromagnetic waves in general, such as their speed or their transverse nature, can be used in an answer about light.
Self review
- What type of wave is light?
- Between which two groups of the electromagnetic spectrum does visible light lie?
- What is the speed of light in a vacuum?
- What evidence shows that light does not need a medium to travel?
- Why do light and radio signals from the Moon arrive at the same time?
