Lenses (physics only): A lens forms an image by refracting light.

The idea

A lens works because light slows down when it enters glass and speeds up when it leaves, so every ray is refracted twice, once at each curved surface. Because the surfaces are curved, rays striking different parts of the lens are bent through different amounts, and that is what brings a whole beam together or spreads it apart. A convex, or converging, lens is thicker in the middle and brings parallel rays together. A concave, or diverging, lens is thinner in the middle and spreads parallel rays apart. The image is simply the place where the refracted rays, or their backward projections, meet. A lens never creates light; it only changes the direction of the light already arriving from the object.

What earns the marks

• Stating that the light is refracted as it enters and as it leaves the lens.
• Saying that refraction happens because the light changes speed in the glass.
• Identifying a convex lens as converging and a concave lens as diverging.
• Defining the image as the point where the refracted rays, or their extensions, meet.

Common mistakes and how to put them right

• Saying the lens reflects light to form the image.

• Saying the lens magnifies by making the object bigger, rather than by changing the directions of rays.

• Showing refraction at only one surface of the lens.

• Confusing which lens shape converges and which diverges.

Exam tips

• Use the words converging and diverging alongside convex and concave, since both are credited.
• Say where the refraction happens, at the surfaces of the lens, not inside it.
• In a ray diagram, bend the ray at the single lens line, which is the standard GCSE simplification.