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SingaporePhysicsSyllabus dot point

How does light bounce off mirrors and bend when it passes between materials?

State the law of reflection and describe refraction when light passes between media

State the law of reflection, describe the image in a plane mirror, and explain how light refracts (bends) when it passes between air and glass or water at N(A)-Level.

Generated by Claude Opus 4.87 min answer

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  1. What this dot point is asking
  2. The answer
  3. Examples in context
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What this dot point is asking

SEAB wants you to state the law of reflection, to describe the image formed by a plane mirror, and to describe refraction (the bending of light) when it passes between materials such as air and glass. The big idea is that light travels in straight lines but changes direction when it reflects off a surface or crosses into a different material.

The answer

Reflection and the normal

When light hits a surface such as a mirror, it bounces off. To describe reflection we draw the normal, a line at right angles to the surface at the point where the ray hits. All angles are measured from the normal, not from the surface.

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

The law of reflection

The law of reflection states that the angle of incidence equals the angle of reflection. The incident ray, the reflected ray and the normal all lie in the same plane.

The image in a plane mirror

A flat (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;
  • upright;
  • laterally inverted (left and right appear swapped);
  • virtual (it cannot be caught on a screen, because the light only appears to come from behind the mirror).

Refraction

Refraction is the change of direction of light when it passes from one material into another. It happens because light travels at different speeds in different materials.

  • When light passes from air into a denser material such as glass or water, it slows down and bends towards the normal.
  • When light passes from glass or water back into air, it speeds up and bends away from the normal.

If the ray hits the surface straight on, along the normal, it does not bend, though it still changes speed.

Why refraction matters

Refraction is why a straw in a glass of water looks bent at the surface, why a pool looks shallower than it really is, and how lenses in glasses and cameras focus light.

Examples in context

Example 1. A periscope. A simple periscope uses two plane mirrors at 45∘45^\circ to let you see over a wall. Each mirror reflects the light through 90∘90^\circ following the law of reflection, so the final image is upright and the right way round. Submarines use the same idea.

Example 2. The bent straw. A straw in a glass of water looks broken at the water surface. Light from the lower part of the straw refracts as it leaves the water into the air, bending away from the normal, so the straw appears to be in a different position. The same effect makes a swimming pool look shallower than it is.

Try this

  • Cue. A ray hits a mirror with an angle of incidence of 35∘35^\circ. State the angle of reflection. [1 mark] By the law of reflection it is also 35∘35^\circ.

  • Cue. State three properties of the image formed in a plane mirror. [3 marks] Same size as the object, as far behind the mirror as the object is in front, upright, laterally inverted and virtual (any three).

  • Cue. Explain why a ray of light bends towards the normal when it enters water from air. [2 marks] Light slows down in the denser water, and this change of speed at the surface makes it change direction towards the normal.

Exam-style practice questions

Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Original4 marksA ray of light hits a plane mirror, making an angle of 30∘30^\circ with the mirror surface. (a) State the law of reflection. (b) Find the angle of incidence (measured from the normal). (c) State the angle of reflection.
Show worked answer →

(a) The law of reflection: the angle of incidence equals the angle of reflection (both measured from the normal), and the incident ray, reflected ray and normal lie in the same plane.

(b) The normal is at right angles to the mirror, so the angle from the normal is 90∘−30∘=60∘90^\circ - 30^\circ = 60^\circ. The angle of incidence is 60∘60^\circ.

(c) By the law of reflection, the angle of reflection equals the angle of incidence, so it is also 60∘60^\circ.

What markers reward: the law stated with angles from the normal, converting the surface angle to the angle from the normal, and equal angles of incidence and reflection.

Original4 marksA ray of light passes from air into a glass block. (a) State what happens to its direction at the surface. (b) Explain why this bending happens. (c) State what happens to the ray as it leaves the glass back into air.
Show worked answer →

(a) The ray bends towards the normal as it enters the glass.

(b) Light slows down when it enters the denser glass. This change of speed at the surface causes the ray to change direction (refraction).

(c) As it leaves the glass into air it speeds up again and bends away from the normal, emerging parallel to the original ray (if the faces are parallel).

What markers reward: bending towards the normal entering glass, the cause being the change of speed in a denser medium, and bending away from the normal on leaving.

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