Light: How We See the World

🔦 Have you ever tried to shine a torch around a corner? No matter how hard you try, the beam refuses to bend — it travels in a perfectly straight line. Light is one of the fastest things in the universe, zipping through air at about 300,000 kilometres every second. Yet despite that incredible speed, light obeys very clear rules. In this lesson we will discover how light leaves a source, bounces off objects, bends through glass or water, and finally reaches our eyes so we can see the world around us. Understanding light helps explain everything from rainbows to magic tricks!

A luminous source is any object that makes its own light — the Sun, a candle flame, a light bulb. Most things we see, however, are non-luminous: they do not make light themselves. We see a non-luminous object only because light from a source travels to that object and bounces back off it into our eyes. The Moon looks bright in the night sky, but it is non-luminous — it simply reflects sunlight towards Earth. Because light always travels in straight lines, the reflected rays enter your eye in a straight path too. Your brain traces those straight lines backwards to work out exactly where the object is.

When light bounces off a surface, we call it reflection. Scientists measure angles from an imaginary line called the normal — a line drawn at exactly 90° (perpendicular) to the surface at the point where light hits. The angle between the incoming ray (incident ray) and the normal is called the angle of incidence. The angle between the bouncing ray (reflected ray) and the normal is the angle of reflection. The law of reflection states that the angle of incidence always equals the angle of reflection. A flat mirror gives a clear, sharp reflection because all light rays bounce off at the same predictable angle. A rough surface scatters light in many directions — that is why paper looks dull compared to a mirror.

Light bends when it crosses the boundary between two different transparent materials — for example, from air into glass or from air into water. This bending is called refraction. It happens because light travels at different speeds in different materials. Light slows down when it enters glass or water and speeds up again when it exits back into air. If the ray hits the boundary at an angle, the slowing down causes it to change direction — like a wheel that rolls onto sand and veers to one side. Refraction explains why a straw in a glass of water looks broken or bent at the water's surface, and why a swimming pool always looks shallower than it really is.

Because light travels in straight lines, it cannot bend around solid, opaque objects. When an opaque object blocks a light source, the area behind it receives no light — this dark patch is called a shadow. The shape of a shadow matches the outline of the object because the straight rays cannot travel through it. The size of a shadow depends on the position of the light source: move the source closer to the object and the shadow grows larger; move it further away and the shadow shrinks. This is because rays from a nearby source spread around more of the object, creating a bigger unlit area behind it.

Your eye is the remarkable detector that collects all this reflected and refracted light. Light enters through the cornea (the curved, transparent front layer) and then through the pupil (the black opening). The lens inside your eye refracts the light rays to focus them onto the retina at the back of the eye. The retina contains millions of special light-sensitive cells called rods (which detect brightness) and cones (which detect colour). These cells convert the light into electrical signals that travel along the optic nerve to your brain. Your brain processes the signals and creates the image you see — all in a fraction of a second!