Describe reflection and refraction of light, recall the regions and uses of the electromagnetic spectrum, and apply the law of reflection

What this dot point is asking

SEAB wants you to describe how light reflects and refracts, to apply the law of reflection, and to recall the order, properties and everyday uses of the regions of the electromagnetic spectrum. The central idea is that light is one part of a family of waves, all of which travel at the same very high speed in a vacuum.

The answer

Reflection of light

When light hits a smooth surface such as a mirror, it bounces off. We measure angles from the normal, a line drawn at $90^\circ$ to the surface at the point where the ray hits. The law of reflection states:

Both angles are measured between the ray and the normal. A flat mirror forms an image that is the same size as the object, upright, and as far behind the mirror as the object is in front.

Refraction of light

When light passes from one material into another, such as from air into glass or water, it changes speed and usually changes direction. This bending is called refraction:

• Going into a denser material (air to glass), light slows down and bends towards the normal.
• Going into a less dense material (glass to air), light speeds up and bends away from the normal.

Refraction is why a straw in a glass of water looks bent at the surface.

The electromagnetic spectrum

Visible light is just one part of a much wider family of waves called the electromagnetic spectrum. They all travel at the same speed in a vacuum (the speed of light). In order of increasing frequency they are:

• radio waves (broadcasting and communications),
• microwaves (cooking and mobile phones),
• infrared (remote controls, thermal imaging, heating),
• visible light (seeing, photography),
• ultraviolet (security marks, causes a suntan),
• X-rays (imaging bones, airport security),
• gamma rays (sterilising equipment, treating cancer).

Higher-frequency waves (ultraviolet, X-rays, gamma rays) carry more energy and can be harmful, which is why exposure is controlled.

Examples in context

Example 1. A periscope. A periscope uses two plane mirrors set at $45^\circ$ so that light reflects twice and lets you see over a wall. Each reflection obeys the law of reflection, turning the light through $90^\circ$ at each mirror.

Example 2. Why a swimming pool looks shallower than it is. Light from the bottom of the pool refracts away from the normal as it leaves the water into air, so the bottom appears raised and the pool looks shallower than its true depth. This is the same refraction effect that makes a straw look bent.

Try this

• Cue. A ray hits a mirror at $25^\circ$ to the normal. State the angle of reflection: $25^\circ$, by the law of reflection.
• Cue. Name one use of microwaves and one use of gamma rays. Microwaves heat food or carry mobile signals; gamma rays sterilise equipment or treat cancer.
• Cue. Explain why a straw in water looks bent. Light from the straw refracts as it leaves the water, so the part underwater appears shifted.