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:
Incandescent1 sources use heat2 to produce light. Nearly all solids, liquids and gases3 will start to glow with a dull red colour once they reach a temperature4 of about 525 °C. At about 2300 °C, the filament5 in a light bulb6 will start to produce all of the colours of the visible spectrum, so it will look white. The Sun, stars, a flame and molten metal7 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 lightning8 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 vacuum9 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 evidence10 of light travelling in straight lines. An object blocks light so that it can’t reach the surface where we see the shadow11. 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. Shadows12 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 wavelength13.
At the beach, the wavelength of water waves might be measured in metres14, but the wavelength of light is measured in nanometres – 10-9 (0.000,000,001) of a metre15. 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 mass16, but each photon17 has a specific amount of energy that depends on its frequency18 (number of vibrations per second). Each photon still has a wavelength. Shorter wavelength photons have more energy.
The photoelectric effect19 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 momentum20. 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 viable21 means of propulsion for CubeSats (miniature satellites intended for low Earth orbit22).
Explore solar sails more in your classroom, with this activity Solar Sails: The Future of Space Travel from the TeachEngineering website.
- incandescent: Light produced as a result of heating or combustion. For example, the filament in an electric light bulb glows white hot when a suitable electric current passes through it.
- heat energy (heat): Heat energy: the transfer of energy in materials from the random movement of the particles in that material. The greater the random movement of particles the more heat energy the material has. Temperature is a measure of the heat energy of a material.
Heat: the flow of energy from a warm object to a cooler object. - gases: The state of matter distinguished from the solid and liquid states. Gases have the ability to diffuse readily and to become distributed uniformly throughout any container.
- temperature: A measure of the degree of hotness or coldness of an object or substance. Temperature is measured with a thermometer calibrated in one or more temperature scales. Kelvin scale temperature is a measure of the average energy of the molecules of a body.
- filament: A long thin strand of a polymeric substance.
- bulb: A circuit component used to transform electrical energy into light energy. Heat energy is also formed in this process.
- metal: Any of a category of elements that usually have a shiny surface, are generally good conductors of heat and electricity and can be melted or fused, hammered into thin sheets or drawn into wires (for example, copper).
- lightning: A large-scale natural spark discharge, visible as a flash of blue-white light, that occurs within the atmosphere or between the atmosphere and the Earth’s surface. The lightning channel consists of extremely high-temperature plasma.
- vacuum: An absence of matter. In practice, a space that contains a very low density of matter (very low pressure) is often referred to as a vacuum.
- evidence: Data, or information, used to prove or disprove something.
- shadow: A dark image produced when an object blocks rays of light from reaching a surface.
- shadow: A dark image produced when an object blocks rays of light from reaching a surface.
- wavelength: The distance between two successive points of a wave (from one peak or crest of a wave and the next peak or crest). Usually refers to an electromagnetic wave, measured in nanometres (nm).
- metre: The base unit of length in the International System of Units (SI).
- metre: The base unit of length in the International System of Units (SI).
- mass: The amount of matter an object has, measured in kilograms.
- photon: A discrete bundle (or quantum) of electromagnetic (or light) energy. Photons are always in motion and, in a vacuum, have a constant speed of light to all observers, at the vacuum speed of light (more commonly just called the speed of light) of c = 2.998 x 108 m/s.
- frequency: 1. How often something occurs within a specified time. 2. The number of waves per second that pass a given point or the number of waves produced per second by a source.
- photoelectric effect: Given the right conditions, when light with sufficient energy is shone onto a metal surface, electrons can be emitted or ejected from the surface of the metal. The energies of the emitted electrons are independent of the intensity of the incident light.
- momentum: The property of a moving object that keeps it moving until a force changes that motion. Momentum is equal to mass (kg) x velocity (m/s).
- viable: 1. Feasible, capable of working successfully. 2. Capable of surviving.
- orbit: The path of an object as it revolves around another object. For example, the path the Moon takes as it moves around the Earth is its orbit.
