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:
IncandescentLight 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. 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 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).