Lenses
Converging lenses focus light; diverging lenses spread it.
About this topic
A lens refracts light to form images. A converging (convex) lens brings parallel rays to a focus at the focal length f, while a diverging (concave) lens spreads them out. The thin lens equation is 1/f = 1/u + 1/v, and magnification is m = v/u.
Depending on the object's distance, a convex lens can form a real inverted image, as in a camera or projector, or a virtual magnified one, as in a magnifying glass when the object is inside the focal length. Concave lenses always give virtual, upright, smaller images and correct short-sightedness. Lens power is 1/f in dioptres, so a lens of f = 0.5 m has power 2 D.
Worked example: an object 30 cm from a convex lens of f = 10 cm gives 1/v = 1/10 − 1/30 = 2/30, so v = 15 cm, a real image on the other side with magnification 15/30 = 0.5, so it is half size and inverted. Moving the object closer than 10 cm makes the image virtual and larger.
Lenses are essential in spectacles, cameras, telescopes and microscopes and in your own eye. Ray diagrams use three rays: parallel to the axis through the focus, through the centre undeviated, and through the near focus to emerge parallel. A common error is mixing sign conventions, so follow one consistently.
Key ideas
- Convex lenses converge
- Concave lenses diverge
- 1/f = 1/u + 1/v
Quick check
Use cases: where Lenses helps
- Homework: review the key idea behind Lenses before you start a problem set.
- Exam revision: use the quick check to test whether you can recall Lenses without notes.
- Visual learning: play with the interactive lab to see how Lenses changes when you adjust its inputs.
- Teaching a friend: explain Lenses in your own words, then compare your version with the key ideas.
The same laws describe car crashes, sports, satellites and everyday machines.
Frequently asked questions
What is Lenses?
Converging lenses focus light; diverging lenses spread it.
What will I learn in Lenses?
Convex lenses converge. Concave lenses diverge. 1/f = 1/u + 1/v.
Is Lenses part of Physics?
Yes. It is one of 150 Physics topics on LearnViz HighSchoolEdu, with 3 key ideas, a live animation and a quick check.
Does the Lenses lab work on a phone or tablet?
Yes. The animation resizes to fit your screen and supports touch, so you can drag, tap and slide with your finger.
How should I study Lenses?
Read the summary and key ideas first, play with the interactive lab to see the idea in motion, then answer the quick check. If you get a question wrong, go back to the key idea it tests.
Do I need an account, and is my progress saved?
No account is needed. Marking a topic as learned is saved in your own browser, so it stays on that device and is not sent to a server.