Physics With Kate
Empowering Girls in Physics
Spec-aligned revision resources, group courses, and 1:1 tutoring for GCSE and A Level Physics — built by an experienced teacher and examiner.
AQA GCSE Physics · Topic 4.6 Waves · Physics only
Refraction and Lenses: Ray Diagrams for Converging and Diverging Lenses
Physics only Combined Science students don't need this page.
In this lesson you'll learn to: draw a ray refracting into a glass block and explain why it bends towards the normal, use wavefronts to show the wavelength getting shorter, do the reflection and refraction required practical, draw ray diagrams for converging and diverging lenses, describe each image (real or virtual, upright or inverted, magnified or diminished), and calculate magnification.
▶
Prefer to watch? Scroll to the bottom for the video on this topic
▶
A video walkthrough is coming soon — it will appear at the bottom of this page
In this lesson you'll learn to: draw a ray refracting into a glass block and explain why it bends towards the normal, use wavefronts to show the wavelength getting shorter, do the reflection and refraction required practical, draw ray diagrams for converging and diverging lenses, describe each image (real or virtual, upright or inverted, magnified or diminished), and calculate magnification.
▶
Start here — have a play
Don't read anything yet. On tab 1, slide the angle of incidence and watch the ray bend towards the normal, then switch on the wavefronts. On tab 2, put the object beyond 2F, between 2F and F, then closer than F, and watch the rays being drawn. What happens to the image when you cross F? Tab 3 does the same with a diverging lens.
Angle of incidence
40°
What the ray does
Object
or slide it
Move the object
Move the object anywhere. A diverging lens always gives an image that is virtual, upright and diminished.
Physics with Kate · physicswithkate.com
▲ Three tabs: refraction in a glass block, a converging lens, a diverging lens
Seen the image flip? Now learn to draw it yourself.
The notes below have the refraction rules, the required practical, the two rays for every lens diagram and a table of every image you need to know.
✎
Revision notes
Refraction
⭐ What is refraction?
Refraction is when a wave moves into an optically denser medium, slows down and changes direction: it bends towards the normal.
Coming back out into air it speeds up and bends away from the normal. Out of a parallel-sided block, the ray leaves at the same angle it went in: parallel to the incoming ray.
- The normal is a dashed line drawn at 90° to the surface where the ray hits it.
- The angle of incidence (i) and the angle of refraction (r) are measured from the normal, not from the surface.
- A ray along the normal (i = 0°) doesn't change direction, but it still slows down.
Why it bends: wavefronts Higher
The frequency stays the same when a wave crosses a boundary. Light is slower in glass, so from v = f λ the wavelength is shorter and the wavefronts are closer together. One end of each wavefront reaches the glass first and slows first, so the wavefront swings round and the wave changes direction.
| Wave | Speed | Frequency | Wavelength |
|---|---|---|---|
| light: air → glass | decreases | same | decreases |
Required practical: reflection and refraction of light
- Put a glass block on paper and draw round it. Shine a thin ray from a ray box at the side of the block.
- Mark the ray going in, the reflected ray and the ray coming out with crosses, then join them with a ruler.
- Draw the normal and measure i, the angle of reflection and r with a protractor. Repeat for different values of i.
- Conclusion: the angle of incidence equals the angle of reflection; the angle of refraction in the glass is smaller than the angle of incidence.
- More accurate: work in a darkened room, use a thin ray, and mark crosses far apart.
Lenses
( )
Converging (convex)
Thicker in the middle. Rays parallel to the axis are brought together through the principal focus F.
) (
Diverging (concave)
Thinner in the middle. Parallel rays spread out as if they came from the principal focus.
The focal length is the distance from the centre of the lens to F. A lens has a focus on each side.
⭐ The two rays for every lens diagram
1. From the top of the object, parallel to the principal axis to the lens, then through F (converging) or as if from F (diverging). One arrow on each part.
2. From the top of the object straight through the centre of the lens, without bending. One arrow each side of the lens.
The top of the image is where they cross. If they spread apart, trace them back with dashed lines: that image is virtual.
| Lens and object | Image | Use |
|---|---|---|
| Converging, object further than 2F | real, inverted, diminished | camera (including a phone camera), the eye |
| Converging, object between 2F and F | real, inverted, magnified | projector, cinema screen |
| Converging, object closer than F | virtual, upright, magnified | magnifying glass |
| Diverging, object anywhere | virtual, upright, diminished | glasses for short sight |
⭐ Real or virtual?
A real image can be projected onto a screen: the light really goes there.
A virtual image cannot be projected onto a screen: the rays only appear to come from it.
Magnification
magnification = image height ÷ object height. It has no units: it's a ratio, so measure both heights in the same unit. A 2.0 cm object with a 6.0 cm image: magnification = 6.0 ÷ 2.0 = 3.
Lens ray diagrams can be drawn with a convex symbol (a line with arrowheads pointing out) or a concave symbol (arrowheads pointing in).
✍
How to write it in the exam
Describing an image, the same three words every time
real or virtual →
upright or inverted →
magnified or diminished, then give the use.
can it go on a screen?which way up?bigger or smaller?
Workbook page 15 · refraction and wavefronts
The ray bends towards the normal.
Same frequency, shorter wavelength.
Why do the wavefronts get closer? The frequency stays the same. In glass the speed is slower, so (v = f λ) the wavelength is shorter: the wavefronts are closer together.
Workbook page 15 · required practical
Ray box, glass block, reflected and refracted rays.
| angle of incidence (°) | angle of reflection (°) | angle of refraction in glass (°) |
|---|---|---|
| 20 | 20 | 13 |
| 30 | 30 | 19 |
| 40 | 40 | 25 |
| 50 | 50 | 31 |
| 60 | 60 | 35 |
- The angle of incidence is equal to the angle of reflection.
- The angle of refraction in the glass is smaller than the angle of incidence in the air.
- More accurate: a darkened room so the ray is easier to see; a thin ray, marked with crosses far apart.
Exam-style question · page 15 · 6 marks
(a) A ray enters the block at an angle of incidence of 50°. Use the table to give the angle of refraction.
- 31° (1)
(b) Explain why light changes direction when it goes from air into glass at an angle.
- Light slows down in glass, which is optically denser (1).
- One side of the wavefront slows first, so the wave changes direction, bending towards the normal (1).
(c) Higher A 500 Hz sound wave goes from air into water, where it travels at 1500 m/s. Calculate the wavelength in water and say what happens to the frequency.
- λ = v ÷ f = 1500 ÷ 500 (1) = 3.0 m (1)
- The frequency stays the same: only the speed and wavelength change (1).
Workbook page 16 · converging lens · three positions
Further than 2F: real, inverted, diminished. Camera, the eye.
Between 2F and F: real, inverted, magnified. Projector.
Closer than F: virtual, upright, magnified. Magnifying glass.
Workbook page 16 · diverging lens
Object at 2F.
Object closer than F.
Wherever the object is, the image is always virtual, upright and diminished. Use: glasses for short sight.
Exam-style question · a magnifying glass · page 16 · 5 marks
Drawn full size. Each square is 5 mm.
(a) What type of lens is shown?
- Converging (1)
(b) Calculate the magnification. The object and image are drawn full size.
- Image height = 20 mm, object height = 10 mm (measured) (1)
- Magnification = 20 ÷ 10 = 2, no unit (1)
(c) Is the image real or virtual? Give a reason.
- Virtual (1): it cannot be projected onto a screen; the rays only appear to come from it (the dashed lines) (1).
📘 Now do it in your workbook
Page 15: draw the refracted ray and the wavefronts, then the reflection and refraction required practical with its results table and an exam question. Page 16: complete five lens ray diagrams, ring the words for each image, and the magnifying-glass exam question.
AQA GCSE Physics Workbook · WAVES · pages 15 and 16The full workbook covers the whole of Topic 4.6 Waves, with exam-style questions and worked answers.
✅ Can you do it? Tick as you go
0% complete🎉 Nice work! You've ticked off every objective for this spec point.
Don't forget to hit “Mark complete” at the bottom of the lesson.
▶
Prefer to watch? Here's the whole thing
Video walkthrough — coming soon
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it.
Everything above, explained out loud — useful for a last-minute recap, or if you'd rather hear it than read it. The video is on its way.
🎬
Video coming soon
The walkthrough for this lesson is being filmed and will appear right here. Until then, the animation and the notes above cover every mark.
Now put it into practice
Pages 15 and 16 of the workbook have refraction, the required practical, five lens ray diagrams and two exam questions, with answers at the back.
Proud member · Accredited · Trusted partners
© 2026 Physics With Kate · All rights reserved