O-Level Physics Lenses: Construct the Image Before Describing It

Build the diagram from two rays

A student asking for help with Physics and prelim practice identified lens diagrams as a specific difficulty. A useful response is to practise constructing one image before attempting another complete paper. The construction tells you where an image forms and supports the description that follows.

The 2026 O-Level Physics 6091 syllabus, under Light, includes converging lenses, focal length, real and virtual image ray diagrams, and image characteristics. This guide concentrates on those ideas. Its examples use an ideal thin converging lens with the object on the left and light travelling towards the right.

Set up the page before drawing the image

Draw a horizontal principal axis and place the lens vertically across it. Mark the optical centre where lens and axis meet. Mark a principal focus on each side, with equal distances from the centre, and add the positions twice as far away. Label these F and 2F clearly. The focal length is the distance from the optical centre to a principal focus.

Draw the object as an upright arrow with its base on the axis. Start both construction rays from its tip. Using rays from different points of the object can produce an intersection that does not represent the image of either point. Keeping one starting point makes the geometry much easier to follow.

Draw two paths and locate their meeting point

Send the first ray parallel to the principal axis until it reaches the lens. After passing through the lens, draw it through the principal focus on the far side. Send the second ray through the optical centre; in the ideal thin-lens construction it continues straight. Add arrows to show the direction of light.

If the emerging rays meet on the right, their intersection locates the tip of a real image. Draw the image arrow from the axis to that point. If the intersection lies below the axis, the image is inverted. Compare the arrow heights to decide whether it is magnified, diminished or the same size. Do not decide the height from how far the arrow happens to be from a page margin.

Compare two positions using the same focal length

For an original scale-drawing example, use a focal length of 10 cm and a 2 cm tall object 30 cm from the lens. A convenient drawing scale is one centimetre on paper for five centimetres in the setup. The two construction rays meet 15 cm to the right of the lens and 1 cm below the axis. The image is real, inverted and diminished.

Now move the same object to 15 cm from the lens. The rays meet 30 cm to the right and 4 cm below the axis. The image is real, inverted and magnified. Both objects are outside the focal point, so both give real images. The change in size comes from the changed object position. These numbers are checked ideal-model examples, not a required lens-formula method for an exam answer.

Use backward extensions for a virtual image

Place the object inside the focal length, for example 5 cm from that same lens. Draw the actual emerging rays towards the right. They diverge, so they do not meet there. Extend their lines backwards with dashed lines. Those extensions meet on the left, locating the virtual image.

For this example, the virtual image is 10 cm to the left and 4 cm tall: upright and magnified. The dashed extensions show where light appears to come from; they are not extra paths along which the light actually travels. A screen at that apparent image position would not capture this virtual image. Keep that distinction clear in both drawing and explanation.

Check the limiting positions

An object at 2F gives a real inverted image at 2F on the other side with the same size in this model. At F, the emerging construction rays are parallel and there is no finite image position to mark. If a drawing gives a nearby intersection for an object exactly at F, check the focal marks and ray slopes.

Practise one object beyond 2F, one between F and 2F, and one inside F. For each, record position, orientation, relative size and real or virtual. Then choose a lens question from the Secondary Pure Physics collection. Redraw any incorrect construction before checking another answer. Accurate rays give the image description a reason.