Reflection and Refraction of Waves

(Separate Physics only) This whole topic, including Required practical 9, is assessed in AQA GCSE Physics but not in Combined Science.

When any wave reaches a boundary between two different materials, three things can happen to its energy. Usually all three occur at the same time, in different proportions.

• Reflection — the wave bounces back off the boundary and stays in the first material.
• Absorption — the boundary material takes in the wave's energy, often heating up as a result.
• Transmission — the wave passes through the boundary into the second material and carries on (often with a change of direction, which is refraction).

Which effect dominates depends on the wave and the two materials. A mirror mostly reflects light; a black surface mostly absorbs it; a clear glass window mostly transmits it. Sound striking a soft curtain is largely absorbed, which is why soft furnishings deaden echoes.

At almost every real boundary, reflection, absorption and transmission all happen together. Questions often ask you to say which one dominates for a given surface, and why.

To describe reflection precisely, physicists draw the normal: an imaginary line at 90° to the surface at the exact point where the ray strikes. Every angle is measured from this normal, never from the surface itself.

Two angles matter:

• Angle of incidence (i) — between the incoming (incident) ray and the normal.
• Angle of reflection (r) — between the reflected ray and the normal.

The law of reflection states:

$\text{angle of incidence}=\text{angle of reflection}$

The incident ray, the reflected ray and the normal all lie in the same plane. A ray hitting a mirror at 30° to the normal reflects at 30° to the normal on the other side.

Always measure angles from the normal, not from the mirror surface. A ray at 30° to the surface is at 60° to the normal, so it reflects at 60° to the normal.

The specification requires you to construct ray diagrams for reflection. Follow these steps precisely, because marks are awarded for the construction, not just the final picture.

1. Draw the reflecting surface (for example a plane mirror) as a straight line.
2. At the point where the ray meets the surface, draw the normal as a dashed line at exactly 90° to the surface.
3. Draw the incident ray arriving at the surface, with an arrow showing its direction towards the surface.
4. Measure the angle of incidence from the normal to the incident ray.
5. Draw the reflected ray leaving on the other side of the normal, at an equal angle to the normal, with an arrow pointing away from the surface.
6. Label the angle of incidence, the angle of reflection, and the normal.

Points that examiners look for: the normal is dashed and at 90°, both rays have arrows, the two angles are clearly equal, and every angle is measured from the normal.

The law of reflection holds at every point on a surface, but the overall effect depends on how smooth the surface is.

Type	Surface	What happens	Result
Specular reflection	Smooth (e.g. a mirror, still water)	Parallel rays reflect and stay parallel	A clear, sharp image
Diffuse reflection	Rough (e.g. paper, a wall)	Parallel rays reflect in many directions (scattering)	No clear image; the surface looks matt

In diffuse reflection the law of reflection still applies at each tiny point, but because the surface is bumpy, each part faces a slightly different way and the reflected rays scatter. This is why you can see a matt wall from any angle but see your reflection only in a smooth mirror.

Refraction is the change in direction of a wave when it crosses a boundary and changes speed. It is the transmission part of the story: the wave passes into the new material but bends because its speed changes.

The direction of the bend follows a simple rule:

• Entering a slower (optically denser) material, such as light going from air into glass, the ray bends towards the normal.
• Entering a faster (less dense) material, such as light going from glass into air, the ray bends away from the normal.

A ray hitting the boundary along the normal (angle of incidence 0°) does not bend, though it still changes speed. During refraction the wave's frequency stays the same; its speed and wavelength both change, which is what forces the change of direction.

A helpful analogy: a set of car wheels rolling from tarmac onto mud. The wheel that hits the mud first slows down while the other is still on tarmac, so the axle swings round. A wavefront meeting a boundary at an angle turns the same way.

Required practical 9 (physics only) has two parts: investigating reflection by different surfaces and refraction by different substances. A ray box (light source) and a sheet of paper are used throughout so you can trace the rays.

Part A — reflection.

• Apparatus: ray box with a single slit, plane mirror (and other surfaces to compare), protractor, ruler, sharp pencil, plain paper.
• Method: place the mirror on the paper and draw round its edge. Shine a ray at the mirror. Mark the incident and reflected rays with crosses, remove the mirror, and join the crosses. Draw the normal, then measure the angle of incidence and angle of reflection with a protractor. Repeat for several angles.
• Expected result: the angle of reflection equals the angle of incidence every time, confirming the law of reflection. Comparing surfaces shows smooth surfaces give a clear reflected ray (specular) while rough surfaces scatter it (diffuse).

Part B — refraction.

• Apparatus: ray box, rectangular glass (or Perspex) block, protractor, ruler, pencil, paper.
• Method: draw round the block. Shine a ray into one face at an angle and mark the ray going in, then the ray coming out the far side. Remove the block, join the marks to reveal the path through the glass, draw the normal at the entry point, and measure the angles. Repeat with blocks of different substances.
• Expected result: the ray bends towards the normal on entering the glass (it slows down) and away from the normal on leaving (it speeds up). Denser substances bend the ray more.

Variables: the independent variable is the angle of incidence (or the substance); the dependent variable is the measured angle of reflection or refraction; controls include using the same ray box and the same block face so the test is fair.

1. Measuring angles from the surface instead of the normal

Every angle in reflection and refraction is measured from the normal, the line at 90° to the surface. Measuring from the surface itself gives an angle that is 90° minus the correct value.

2. Forgetting arrows on rays

A ray diagram without arrows loses marks. Arrows show the incident ray heading towards the surface and the reflected or refracted ray heading away from it.

3. Saying refraction always bends light towards the normal

The direction depends on the speed change. Slowing down (into a denser material) bends towards the normal; speeding up (into a less dense material) bends away from it. A ray along the normal does not bend at all.

4. Thinking frequency changes during refraction

When light refracts, its speed and wavelength change but its frequency stays constant. The colour (which depends on frequency) is unchanged; only the direction and wavelength alter.

5. Confusing diffuse reflection with "no reflection"

A rough surface still reflects; the rays just scatter in many directions because the surface faces different ways at each point. The law of reflection still holds at every individual point.