Light is a form of energy produced by a light source. Light is made of photons that travel very fast. Photons of light behave like both waves and particles.
Light sources
Something that produces light is called a light source. There are two main kinds of light sources:
Incandescent sources use heat to produce light. Nearly all solids, liquids and gases will start to glow with a dull red colour once they reach a temperature of about 525 °C. At about 2300 °C, the filament in a light bulb will start to produce all of the colours of the visible spectrum, so it will look white. The Sun, stars, a flame and molten metal are all incandescent.
Luminescent sources are normally cooler and can be produced by chemical reactions, such as in a glowstick or a glow-worm. Other luminescent sources include a computer screen, fluorescent lights and LEDs.
Light travels much faster than sound
Light travels at a speed of 299,792,458 m/s (that’s nearly 300,000 km/s!). The distance around the Earth is 40,000 km, so in 1 second, light could travel seven and a half times around the world.
Sound only travels at about 330 m/s through the air, so light is nearly a million times faster than sound.
If lightning flashes 1 kilometre away from you, the light reaches you in 3 millionths of a second, which is almost instantly. The sound of the thunder takes 3 seconds to travel 1 kilometre – to work out many kilometres away lightning is, count the seconds for the thunder to arrive and divide by 3.
Light takes about 8 minutes and 20 seconds to reach the Earth from the Sun. When we see the Sun, we are seeing what it looked like over 8 minutes ago.
Light can travel through empty space
Unlike sound, which needs a medium (like air or water) to travel through, light can travel in the vacuum of space.
Light travels in straight lines
Once light has been produced, it will keep travelling in a straight line until it hits something else.
Shadows are evidence of light travelling in straight lines. An object blocks light so that it can’t reach the surface where we see the shadow. Light fills up all of the space before it hits the object, but the whole region between the object and the surface is in shadow. Shadows don’t appear totally dark because there is still some light reaching the surface that has been reflected off other objects.
Once light has hit another surface or particles, it is then absorbed, reflected (bounces off), scattered (bounces off in all directions), refracted (direction and speed changes) or transmitted (passes straight through).
Models for light
Light as waves
Rainbows and prisms can split white light up into different colours. Experiments can be used to show that each of these colours has a different wavelength.
At the beach, the wavelength of water waves might be measured in metres, but the wavelength of light is measured in nanometres – 10-9 (0.000,000,001) of a metre. Red light has a wavelength of nearly 700 nm (that’s 7 ten-thousandths of a millimetre) while violet light is only 400 nm (4 ten-thousandths of a millimetre).
Visible light is only a very small part of the electromagnetic spectrum – it’s just that this is the range of wavelengths our eyes can detect.
Light as particles
In 1905, Albert Einstein proposed that light is made of billions of small packets of energy that we now call photons. These photons have no mass, but each photon has a specific amount of energy that depends on its frequency (number of vibrations per second). Each photon still has a wavelength. Shorter wavelength photons have more energy.
The photoelectric effect is when light can cause electrons to jump out of a metal. These experiments confirm that light is made of these massless particles called photons.
Simple explanations of some of these concepts can be found in the article Building Science Concepts: Shadows.
Nature of science
In order to understand the world we live in, scientists often use models. Sometimes, several models are needed to explain the properties and behaviours of a phenomenon. For example, to understand the behaviour of light, two models are needed. Light needs to be thought of as both waves and particles.
Useful links
Even though light doesn’t have mass, learn how it still has a tiny amount of momentum. Find out about NASA’s solar sails to power spacecraft.
Read about the LightSail project, a crowdfunded project from The Planetary Society, aiming to demonstrate that solar sailing is a viable means of propulsion for CubeSats (miniature satellites intended for low Earth orbit).
Explore solar sails more in your classroom, with this activity Solar Sails: The Future of Space Travel from the TeachEngineering website.
