Reflection and refraction

When a wave reaches the boundary between two materials, it can be:

• reflected: it bounces back
• absorbed: its energy goes into the material
• transmitted: it passes through, often being refracted (changing direction) as it does

What happens depends on the wavelength and the materials. Glass transmits visible light but absorbs much of the ultraviolet.

In a ray diagram, light is drawn as straight lines with arrows, called rays.

Where a ray hits a surface, draw the normal: a dashed line at right angles to the surface.

• The angle of incidence is between the incoming ray and the normal.
• The angle of reflection is between the reflected ray and the normal.

For a mirror, the angle of incidence = angle of reflection.

Refraction happens because a wave changes speed when it passes from one material into another.

• Light going from air into glass slows down and bends towards the normal. The angle of refraction is smaller than the angle of incidence.
• Light going from glass into air speeds up and bends away from the normal.
• A ray that hits the boundary along the normal (at 90° to the surface) changes speed but doesn't change direction.

In a ray diagram, draw the ray in, the normal, and the ray out. Mark the angle of incidence and the angle of refraction, both measured from the normal.

A wavefront is a line joining the same point on a wave, such as all the crests. Picture wavefronts reaching a glass block at an angle:

1. One end of each wavefront reaches the glass first and slows down.
2. The rest of the wavefront is still in the air, moving faster.
3. So the wavefront turns, and the wave changes direction towards the normal.

Inside the glass, the wavefronts are closer together: the wavelength is shorter. The frequency stays the same, because the source sets it.

If the wave hits along the normal, the whole wavefront slows down at once, so it doesn't turn.

Refraction:

1. Put a glass block on paper and draw round it.
2. Shine a thin ray of light from a ray box into one side of the block.
3. Mark the path of the ray going in and coming out with small crosses.
4. Remove the block, join the crosses, and draw the normal where the ray entered.
5. Measure the angles of incidence and refraction with a protractor.
6. Repeat for different angles and different materials, such as a Perspex block.

Reflection: use the same method with a plane mirror instead, measuring the angles of incidence and reflection. Repeat with different surfaces to compare how clearly they reflect.

• Independent variable: the angle of incidence (or the material).
• Dependent variable: the angle of refraction (or reflection).
• Control variables: the same ray box and the same block for each set of readings.

Hazard: the ray box gets hot, so don't touch the bulb. The room is dim, so keep bags off the floor.

Improving accuracy: use a thin ray, mark crosses far apart along each ray, and use a sharp pencil.