Students should be able to construct ray diagrams to illustrate the…

The idea

Refraction occurs when a wave crosses a boundary between two media and changes speed, so its direction changes unless it travels along the normal. To construct a ray diagram, draw the boundary as a straight line and a dashed normal at 90° to it at the point where the ray meets it. Draw the incident ray meeting that point, then draw the refracted ray on the other side. If the wave slows down, for example light entering glass from air, the ray bends towards the normal, so the angle of refraction is smaller than the angle of incidence. If it speeds up, for example light leaving glass into air, the ray bends away from the normal. Label the normal, incident ray, refracted ray and the two media, and mark the angles from the normal, not from the boundary.

What earns the marks

• Draw the boundary between the two media as a straight line and mark the point where the incident ray meets it.
• Draw a dashed normal at 90° to the boundary at the point of incidence, and measure or mark angles between each ray and the normal.
• Show the correct direction of bending: towards the normal when the wave slows down, away from the normal when it speeds up.
• Keep the incident and refracted rays on opposite sides of the normal, with the refracted ray continuing into the second medium.
• Label the normal, incident ray, refracted ray and the two media clearly so the diagram communicates the refraction event.
• For a ray travelling along the normal, show it continuing straight through without bending.

Common mistakes and how to put them right

• Measuring the angle of incidence from the boundary rather than from the normal.

  Put it right: Draw and use the normal, because angles in refraction are defined relative to it.

• Drawing the refracted ray bending the wrong way, for example bending away from the normal when light enters glass.

  Put it right: Decide whether the wave slows down or speeds up, then bend towards the normal for slowing and away for speeding up.

• Drawing the refracted ray on the same side of the normal as the incident ray.

  Put it right: The refracted ray passes into the second medium on the opposite side of the normal.

• Forgetting the normal or drawing it along the boundary.

  Put it right: The normal is a construction line at 90° to the boundary at the point of incidence.

Exam tips

• Use a sharp pencil and a ruler for the boundary, rays and normal so the geometry is clear and angles can be checked.
• Annotate the diagram with 'towards the normal' or 'away from the normal' to make your reasoning explicit.
• If the question gives refractive indices or wave speeds, use them to decide the direction of bending before drawing the ray.
• Check that every ray is straight except at the boundary, where the change of direction occurs.