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1.5.5Electricity, Energy and Waves

Reflection and Refraction of Waves — WJEC GCSE Science (Double Award) - Physics Revision Notes & Topical Questions

5 min read 9 parts Lesson 33 of 36

📚 WJEC GCSE Science (Double Award) - Physics Hub / Topical Revision
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Section 1 of 9Reflection

When a wave strikes a boundary, some or all of it bounces back. This is reflection. For a smooth, flat surface, the reflected rays obey the law of reflection: the angle of incidence i is equal to the angle of reflection r.

i = r

Both angles are measured from the normal, a line drawn at 90° to the surface at the point where the ray strikes. They are not measured from the surface itself. The incident ray, reflected ray and normal all lie in the same plane.

A plane mirror forms an image that is the same size as the object, the same distance behind the mirror as the object is in front, and virtual (it cannot be projected on a screen). The image is laterally inverted.

Section 2 of 9Specular and diffuse reflection

A smooth surface such as a mirror gives specular reflection: parallel rays stay parallel after reflection, so a clear image is formed.

Diffuse
A rough surface such as paper gives diffuse reflection: the normal differs from point to point, so the reflected rays go in many directions.
The law of reflection still holds at each point.
This is why we can see a page from any direction.

Section 3 of 9Refraction

Refraction is the change in direction of a wave when it passes from one medium into another, caused by a change in the wave speed. If the wave meets the boundary at an angle:

Towards
Entering a more optically dense medium (for example air to glass), the light slows down and bends towards the normal.
Away from
Entering a less dense medium (for example glass to air), the light speeds up and bends away from the normal.
If it travels along the normal (i = 0), it does not bend, but its speed still changes.

The frequency does not change at the boundary. Since v = f λ, a change in speed means the wavelength changes in the same proportion: shorter when the wave slows.

Section 4 of 9Why refraction happens

Think of a line of marching soldiers who walk from pavement onto mud at an angle.

Why refraction happens
The soldiers who reach the mud first slow down while the others continue at the original speed, so the line of marchers turns.
In the same way, one side of a wavefront slows down first when it enters a new medium, and the wavefront pivots.
The ray, which is drawn perpendicular to the wavefront, changes direction.

Everyday effects include a pencil looking bent in a glass of water, a swimming pool looking shallower than it is, and the dispersal of white light into colours by a prism, because each colour has a slightly different speed in glass, so is refracted by a different amount.

Section 5 of 9Drawing ray diagrams

To draw a refraction diagram

  • draw the boundary and the normal;
    • draw the incident ray to the boundary;
    • mark the angle i from the normal;
    • draw the refracted ray in the second medium, closer to the normal if it is denser;
    • label the angle of refraction r.

1
Use a ruler and keep rays straight, with arrows.
2
For a rectangular glass block the emergent ray is parallel to the incident ray but shifted sideways.
3
Water waves also refract as they pass from deep to shallow water, where they slow down and the wavelength decreases.
4
The quantitative form of refraction is Snell's Law, and a special case is Total Internal Reflection.

Section 6 of 9Plane mirrors and image formation

To find the image formed in a plane mirror, draw two rays from a point on the object to the mirror, apply the law of reflection to each, and then extend the reflected rays backwards behind the mirror as dotted lines.

Plane mirrors and image formation
They appear to meet at a point behind the mirror, which is the position of the image.
This image is virtual because the light does not really pass through that point; it only appears to come from there.

The distance from the image to the mirror equals the distance from the object to the mirror.

Plane mirrors and image formation
A person standing 2 m in front of a mirror sees their image 2 m behind the mirror, so the apparent distance between them and their image is 4 m.
If the mirror is rotated by an angle, the reflected ray rotates by twice that angle, a fact used in light-beam galvanometers and in laser scanners.
Reflecting surfaces are also used in periscopes,
  • where two mirrors at 45° raise the line of sight,
  • and in car mirrors,
  • where wide-angle views are needed.

Section 7 of 9Worked examples

  1. A ray strikes a plane mirror with an angle between the ray and the mirror surface of 35°. Find the angle of reflection.

  2. Light passes from air into glass at an angle of incidence of 40°. State whether the refracted ray is closer to or further from the normal and what happens to its speed and wavelength.

Section 8 of 9Key terms

TermMeaning
NormalLine at right angles to the surface
Angle of incidenceAngle between the incident ray and the normal
ReflectionWave bouncing back at a boundary
RefractionChange in direction caused by a change in speed
Virtual imageImage that cannot be formed on a screen

Section 9 of 9Exam technique

1
Measuring angles from the surface instead of the normal.
2
Saying light speeds up when it enters glass.
3
Saying the frequency changes on refraction.
4
Drawing the refracted ray bending away from the normal when entering a denser medium.
5
Thinking an image in a plane mirror can be shown on a screen.

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