When we talk about light we usually mean the light we can see with our eyes. This is also called visible or optical light.
Light is a type of energy. Most of the light we have on Earth comes from the Sun. The light energy from the Sun means that life can survive on Earth. There are other sources of light on Earth, like fire and lightbulbs. Lightbulbs use electricity or solar energy to create light energy. Other objects do not create their own light but reflect the light landing on them, like a book or clothing.
For us to see, light must travel from a light source to our eye. The light we see on Earth travels in straight lines. Anything in the path of light has an effect on what we see. For example, a window is transparent – this means that the light can pass straight through it. An opaque object, like a tree, will block the light travelling towards it. This is what creates a shadow. Some of the blocked light is absorbed by the object, making it hotter, and the rest of the light is reflected. This is how we see things. Light hits the surface of an object and bounces off the surface, sending the light to our eyes. Some objects reflect more light than others. We can change the direction of light by place a very reflective object, like a mirror, in its path. This is how most telescopes work.
The light we see looks white, but it is made up of every colour of the rainbow. You can split white light into all of its colours by placing a glass prism in its path. We see this effect when the Sun shines through rain in the air to make a rainbow.
An object's colour is the result of the light which is reflected. White objects reflect all colours of light landing on them, perfectly black objects reflect no light. If an object looks green it’s because it is reflecting the green light landing on it, but absorbing the other colours.
Light is actually much more complicated this. As well as the light we can see with our eyes there are lots of other types of light. The full range of light is called the electromagnetic spectrum.