Course: Separate Physics (lenses) | Tier: Foundation + Higher | See also: Lenses and Visible Light | Magnification
📌 KEY DEFINITIONS – learn them word for word
Real image: formed where rays of light actually meet. It can be shown on a screen.
Virtual image: rays of light only appear to come from it. It cannot be shown on a screen.
Principal focus (F): the point where rays parallel to the principal axis meet after a convex lens (or appear to come from after a concave lens).
Focal length: the distance from the centre of the lens to the principal focus.
Before you start
You need a sharp pencil, a ruler and ideally 1 cm squared paper. Pause each video after every step and copy it. Then try the questions at the bottom without the video.
✏️ Golden rules for every ray diagram
- Draw every ray with a ruler. Light travels in straight lines.
- Put an arrow on each ray to show which way the light is going.
- Use solid lines for real rays and dashed lines for rays that are only traced back (virtual).
- For a lens, bend the ray at the centre line of the lens, not at its curved surfaces.
- Always finish by describing the image: real or virtual, upright or inverted, magnified or diminished.
1. Plane mirror
- Draw the image the same distance behind the mirror as the object is in front, and the same size.
- Draw a dashed line from the image to the eye. Stop it at the mirror.
- Carry on with a solid line from the mirror to the eye, with an arrow pointing to the eye.
- Draw a solid line from the object to the point where the first line touched the mirror, with an arrow pointing to the mirror.
The image is: virtual, upright, the same size, and the same distance behind the mirror as the object is in front.
2. Convex (converging) lens – real image
- Draw the principal axis and the lens as a vertical line with arrowheads pointing outwards.
- Mark F on both sides of the lens, one focal length from the centre.
- Draw the object as an upright arrow standing on the axis.
- Ray 1: from the top of the object, parallel to the axis to the lens, then bend it so it passes through F on the other side.
- Ray 2: from the top of the object straight through the centre of the lens. It does not bend.
- Where the two rays cross is the top of the image. Draw the image as an arrow from the axis to that point.
If the object is further away than F: the image is real and inverted. If the object is closer than F: the rays never cross, so you trace them back with dashed lines. This gives a virtual, upright, magnified image – a magnifying glass.
3. Concave (diverging) lens
- Draw the principal axis and the lens as a vertical line with arrowheads pointing inwards. Mark F on both sides.
- Ray 1: from the top of the object, parallel to the axis to the lens. After the lens it spreads out as if it came from F on the object’s side. Draw it going up and away, then draw a dashed line back from the lens to that F.
- Ray 2: from the top of the object straight through the centre of the lens.
- The top of the image is where the dashed line meets Ray 2. Draw the image arrow there.
The image from a concave lens is always: virtual, upright and diminished, on the same side of the lens as the object.
4. Exam recap: six easy steps
Watch this once you can draw both lens diagrams. It pulls everything together into a routine you can use in the exam.
All videos on this page are by physics teacher Simon Poliakoff. Find more of his free draw-along videos and resources at teachphysics.org.
Questions
Q1 (F) Why must you use a ruler to draw rays of light?
Show answer
Light travels in straight lines.
Q2 (F) An object is 15 cm in front of a plane mirror. Where is its image?
Show answer
15 cm behind the mirror.
Q3 (F) What is the difference between a real image and a virtual image?
Show answer
A real image is formed where light rays actually meet and can be shown on a screen. A virtual image is where rays only appear to come from, so it cannot be shown on a screen.
Q4 (F) In a lens ray diagram, what happens to a ray that passes through the centre of the lens?
Show answer
It carries straight on without changing direction.
Q5 (F/H) A ray travels parallel to the principal axis into a convex lens. Describe its path after the lens.
Show answer
It is refracted so that it passes through the principal focus (F) on the other side of the lens.
Q6 (F/H) Draw a ray diagram for an object 2 cm tall placed 6 cm from a convex lens of focal length 3 cm. Describe the image.
Show answer
The object is at twice the focal length, so the image forms 6 cm on the other side of the lens. It is real, inverted and the same size (2 cm tall).
Q7 (H) Explain why a concave lens can never form a real image.
Show answer
A concave lens makes rays spread out (diverge), so the rays never actually meet. They only appear to come from a point on the object’s side of the lens, which gives a virtual image.
Q8 (H) A convex lens is used as a magnifying glass. A 4 mm tall insect produces an image 12 mm tall. Where must the insect be, and what is the magnification?
Show answer
The insect must be closer to the lens than the principal focus (F). Magnification = image height ÷ object height = 12 ÷ 4 = 3. The image is virtual, upright and magnified.
