Light, Sound and Waves

Reflection of light

Physics Narrative for 5-11 Supporting Physics Teaching

Reflection from surfaces

Light beams are reflected from surfaces according to the law of reflection, which states that the angle of incidence is equal to the angle of reflection.

Here an explanation of the law builds up, step by step. (Remember that rays are figments of your imagination.)

The angles of incidence (i) and reflection (r) are measured between the incident (incoming) and reflected (outgoing) rays and the normal line. The normal line is a construction line drawn perpendicular to the reflecting surface at the point where the incident ray strikes.

Predicting what will happen using ray diagrams

This ray diagram shows the formation of a shadow by an opaque thing. This might represent what happens when a narrow-beam torch forms the shadow of a book on a wall. This model represents the actual event in a number of ways:

  • The light source is represented as a single point.
  • Just two rays are shown, as straight lines leaving the source.
  • The direction of travel of each ray is shown by an arrow.
  • The book is represented as an opaque barrier.
  • The position of the shadow on the wall is located between the points where the two rays meet the screen.

When thinking about ray diagrams, it is important that you remember that they are a model: they predict what will happen, but do not show a photo-realistic imitation of the phenomenon.

In the case of this shadow, you might be able to make the following predictions:

  • What will happen to the size of the shadow if the torch is moved closer to the opaque barrier? (it gets bigger on the screen).
  • What will happen to the size of the shadow if the screen is moved away from the opaque barrier? (it gets bigger on the screen).

is formalised by Law of Reflection
can be exhibited by Progressive Wave
has the special case Total Internal Reflection
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