CCEA GCSE Physics 2.2 Light Exam Questions

About CCEA GCSE Physics 2.2 Light

Light is a Unit 2 topic in CCEA GCSE Physics that relies on accurate ray diagrams. You need the law of reflection and the properties of an image in a plane mirror, how light refracts when it changes speed, and the prescribed practical tracing rays through a glass block. The topic also covers dispersion of white light by a prism, critical angle and total internal reflection with optical fibres, converging and diverging lenses, the correction of short and long sight, and the camera, projector and magnifying glass. It builds on 2.1 Waves. Examiners regularly ask candidates to complete ray diagrams, state the conditions for total internal reflection and describe images.

What you need to know

• Reflection The angle of incidence equals the angle of reflection, both measured from the normal. The image in a plane mirror is virtual, upright, the same size as the object, the same distance behind the mirror as the object is in front, and laterally inverted.
• Refraction Light changes speed when it passes between materials of different density. Entering a denser material such as glass, it slows down and bends towards the normal; leaving, it speeds up and bends away from the normal. A ray along the normal does not change direction.
• Prescribed practical: glass block Shine a ray box beam into a rectangular glass block at different angles of incidence, mark the rays with crosses, draw the normal and measure the angles of incidence and refraction with a protractor. The angle of refraction is smaller than the angle of incidence.
• Dispersion A prism splits white light into the colours of the visible spectrum because each colour is refracted by a different amount. Violet is refracted most and red least.
• Total internal reflection When light travels from a denser to a less dense material and the angle of incidence is greater than the critical angle, all the light is reflected. Optical fibres use this to carry light for communications and in endoscopes.
• Lenses A converging (convex) lens brings parallel rays to a focus at the principal focus; a diverging (concave) lens spreads them out. Short sight is corrected by a diverging lens and long sight by a converging lens. A camera and a projector form real images; a magnifying glass forms a virtual, upright, magnified image when the object is closer than the focal length.

Common mistakes in 2.2 Light

• Measuring angles from the surface instead of from the normal.
• Stating the conditions for total internal reflection incompletely. Light must go from a denser to a less dense material and the angle of incidence must be greater than the critical angle.
• Drawing rays without arrows or drawing virtual rays as solid lines. Virtual rays should be dashed.
• Saying light bends towards the normal when it speeds up. It bends towards the normal when it slows down.
• Mixing up the lenses for sight defects. Short sight uses a diverging lens; long sight uses a converging lens.

Try three 2.2 Light exam-style questions