Ray diagrams are used to show the formation of images by convex and…

The idea

A lens ray diagram is built from standard construction rays drawn from the top of the object. For a convex lens: a ray parallel to the principal axis is refracted through the principal focus on the far side; a ray through the centre of the lens carries straight on without being deviated; a ray passing through the principal focus on the near side emerges parallel to the axis. Any two of these are enough, and the point where they cross is the top of the image. For a concave lens the parallel ray is refracted so that it appears to come from the principal focus on the same side, and the central ray again passes straight through; the rays are traced backwards as dotted lines to find the virtual image. Use a ruler and mark object and image with arrows.

What earns the marks

• A ray through the centre of the lens continuing without deviation.
• A ray parallel to the principal axis refracted correctly at the lens.
• The image drawn in the correct position.
• The image drawn with the correct orientation and size, shown with an arrow.

Common mistakes and how to put them right

• Bending the ray that passes through the centre of the lens.

• Drawing the bends at the two curved glass faces instead of at the single lens line, which is the construction the mark scheme expects and makes the angles far harder to draw accurately.

• Drawing solid lines for the virtual rays behind a concave lens instead of dotted construction lines.

• Drawing the image the same way up as the object when the construction gives an inverted image.

Exam tips

• Use a ruler and draw the rays long enough to cross clearly, since short rays cost position marks.
• Two construction rays are enough, but a third is a useful check if the crossing point looks wrong.
• Represent the image with an arrow so that its orientation is unmistakable.