A ray diagram shows how light from one point on an object travels through a lens to form an image. You only need to draw two or three standard rays, then see where they meet.
This lesson is part of light and optics. Once the diagram is right, explaining image formation shows how to describe the image in words.
What are the three standard rays for a convex lens?
Each ray follows a fixed rule. Learn the rules and every diagram becomes a routine.
- A ray parallel to the principal axis bends and passes through the focal point F on the other side.
- A ray through the optical centre of the lens carries straight on without bending.
- A ray through the focal point on the object side bends and leaves parallel to the axis.
The image of the top of the object is where these rays cross. Draw a line down from that point to the axis to complete the image.
Worked example: object beyond 2F
A candle 4 cm tall stands 30 cm from a convex lens with a focal length of 10 cm. Use a scale of 1 cm for each 5 cm.
Step 1. Draw the principal axis, the lens as a vertical line, and mark F on both sides at 2 cm from the lens on your drawing (10 cm in real life).
Step 2. Draw the candle as an upright arrow 6 cm to the left of the lens. The height on the page is 0.8 cm, using the same scale.
Step 3. Draw ray 1 from the top of the candle, parallel to the axis, to the lens. Then draw it through the far focal point.
Step 4. Draw ray 2 from the top of the candle straight through the centre of the lens. The two rays cross on the other side.
Where they cross gives the top of the image. The image is between F and 2F on the far side, smaller than the object and upside down.
The mistakes that cost marks
The marks go to accurate construction, not to neat shading. Three slips cost the most marks.
| Slip | What it looks like | Fix |
|---|---|---|
| No arrowheads | Lines without direction | Add an arrow on every ray |
| Bent ray at the wrong place | Ray bends halfway between the lens and the axis | Bend the ray once, at the lens line |
| Wrong focal point | F marked on only one side | Mark F on both sides, same distance |
A fourth slip is to draw a real image with a dashed arrow. Use a solid arrow for a real image and a dashed arrow for a virtual one.
Check yourself
A pin stands on the axis, 15 cm from a convex lens of focal length 10 cm. Describe where you would draw ray 1 and ray 3, and state what the two rays do after the lens.
Answer
Ray 1 leaves the top of the pin parallel to the axis, reaches the lens, and bends through F on the far side. Ray 3 leaves the top of the pin through the near focal point, hits the lens, and emerges parallel to the axis.
After the lens the two rays cross on the far side of the lens, beyond 2F. The crossing gives the top of the image, which is real, inverted and larger than the pin.
What to study next
When you can draw the rays, describe the image with its properties. Continue with explaining image formation, then check your skills with the light and optics practice set.
To go through your own diagrams with a teacher, see online one-to-one Science tuition.