P2.4 Light · KS3 Science

How light travels

Light travels in straight lines (rays) at about 300 000 km per second. It can travel through a vacuum.
Luminous objects give out light (the Sun, a bulb). We see non-luminous objects because light reflects off them into our eyes.
Transparent materials let light through; translucent materials let some light through but scatter it; opaque materials block light, so they make shadows.
Light travels much faster than sound, which is why you see lightning before you hear thunder.

The law of reflection: the angle of incidence equals the angle of reflection. Both are measured from the normal, a line at 90° to the surface.
Smooth surfaces such as mirrors give a clear reflection (specular reflection). Rough surfaces scatter light in all directions (diffuse reflection).
An image in a mirror is the same size, the right way up, reversed left to right, and as far behind the mirror as the object is in front.

Refraction is light changing direction when it passes from one material into another, because its speed changes.
Going from air into glass or water, light slows down and bends towards the normal. Coming out, it speeds up and bends away from the normal.
Refraction makes swimming pools look shallower than they are, and makes a straw look bent in a glass of water.
A convex (converging) lens brings light rays together at a focal point; it is used in magnifying glasses, cameras and the eye. A concave (diverging) lens spreads rays out.

White light is a mixture of colours: red, orange, yellow, green, blue, indigo and violet (the spectrum).
A prism splits white light into a spectrum (dispersion), because each colour refracts by a different amount; violet bends the most.
Objects look coloured because they reflect that colour and absorb the others. A white object reflects all colours; a black object absorbs them all.
A filter only lets its own colour through. A red object looks black in blue light, because there is no red light for it to reflect.

Luminous: Gives out its own light.
Opaque: Doesn't let light through.
Transparent: Lets light through clearly.
Translucent: Lets some light through, but scatters it.
Ray: A line with an arrow that shows the path of light.
Reflection: Light bouncing off a surface.
Normal: A line at 90° to a surface where a ray hits it.
Angle of incidence: The angle between the incoming ray and the normal.
Angle of reflection: The angle between the reflected ray and the normal.
Refraction: Light changing direction as it passes from one material into another.
Convex lens: A lens that brings light rays together at a focal point.
Spectrum: The band of colours that makes up white light.
Dispersion: White light splitting into its colours.
Filter: A material that only lets one colour of light through.