What are the convex lens rules?

One focal length separates real from virtual, two focal lengths separate smaller from bigger. u > 2f gives an inverted, diminished real image (camera). u = 2f gives an inverted, same-size real image. f < u < 2f gives an inverted, magnified real image (projector). u = f gives no image. u < f gives an upright, magnified virtual image (magnifying glass). For real images, a closer object gives a farther, bigger image.

Why can’t I find an image on the screen?

Usually one of three reasons: the candle is at or inside the focal length, so there is no real image; the flame, lens and screen centers aren’t level, so the image misses the screen; or the image distance is longer than the bench. The tool tells you which one applies to your setup.

If I cover half the lens, do I lose half the image?

No. Light from every point of the object spreads over the whole lens, so some still passes through the uncovered half and focuses at the image point. The image stays complete but gets dimmer. Tick “Cover the top half of the lens” on the bench to see it.

How do I measure a convex lens’s focal length?

Either point the lens at the Sun and move paper behind it until the spot is smallest and brightest; the paper-to-lens distance is f. Or adjust the bench until the screen shows an inverted image the same size as the object; then u = v = 2f, so f is half the object distance.

Why do short-sighted people wear concave lenses?

A short-sighted eye’s lens is too strong, so distant objects focus in front of the retina. A concave lens spreads the light slightly so the image moves back onto the retina. Long sight is the opposite and uses a convex lens. Glasses strength in diopters is 1 ÷ focal length in meters; Chinese “degrees” are 100 times that, with concave lenses negative.

🔍

🔍Convex Lens Lab - optical bench, imaging rules, short and long sight

Q: Why can’t I find an image on the screen?

Usually one of three reasons: the candle is at or inside the focal length, so there is no real image; the flame, lens and screen centers aren’t level, so the image misses the screen; or the image distance is longer than the bench. The tool tells you which one applies to your setup.

Q: If I cover half the lens, do I lose half the image?

No. Light from every point of the object spreads over the whole lens, so some still passes through the uncovered half and focuses at the image point. The image stays complete but gets dimmer. Tick “Cover the top half of the lens” on the bench to see it.

Q: How do I measure a convex lens’s focal length?

Either point the lens at the Sun and move paper behind it until the spot is smallest and brightest; the paper-to-lens distance is f. Or adjust the bench until the screen shows an inverted image the same size as the object; then u = v = 2f, so f is half the object distance.

Q: Why do short-sighted people wear concave lenses?

A short-sighted eye’s lens is too strong, so distant objects focus in front of the retina. A concave lens spreads the light slightly so the image moves back onto the retina. Long sight is the opposite and uses a convex lens. Glasses strength in diopters is 1 ÷ focal length in meters; Chinese “degrees” are 100 times that, with concave lenses negative.

The school-lab version of convex lens imaging. Drag a candle, lens and screen along an optical bench marked in centimeters, and the image on the screen blurs according to how far it is from focus. Once it is sharp you can record the object distance, image distance and image type. You can also find an unknown focal length with sunlight, summarize the rules in a table and graph, see what happens when half the lens is covered or the candle burns down, and correct short and long sight with glasses.

Free · No sign-up · Runs in your browserLast updated: 2026-10-29

Loading tool...

How to use Convex Lens Lab

1On the Optical bench tab choose a focal length and drag the candle, lens and screen (or nudge with ◀ ▶)
2Move the screen until the image in the screen view is sharpest, press Record, and repeat for all five cases
3Compare with the Find the focal length, Imaging rules, Short & long sight and Concave lens tabs
4Try randomized exam-style questions on the Practice tab with instant marking and worked answers

FAQ

What are the convex lens rules?: One focal length separates real from virtual, two focal lengths separate smaller from bigger. u > 2f gives an inverted, diminished real image (camera). u = 2f gives an inverted, same-size real image. f < u < 2f gives an inverted, magnified real image (projector). u = f gives no image. u < f gives an upright, magnified virtual image (magnifying glass). For real images, a closer object gives a farther, bigger image.
Why can’t I find an image on the screen?: Usually one of three reasons: the candle is at or inside the focal length, so there is no real image; the flame, lens and screen centers aren’t level, so the image misses the screen; or the image distance is longer than the bench. The tool tells you which one applies to your setup.
If I cover half the lens, do I lose half the image?: No. Light from every point of the object spreads over the whole lens, so some still passes through the uncovered half and focuses at the image point. The image stays complete but gets dimmer. Tick “Cover the top half of the lens” on the bench to see it.
How do I measure a convex lens’s focal length?: Either point the lens at the Sun and move paper behind it until the spot is smallest and brightest; the paper-to-lens distance is f. Or adjust the bench until the screen shows an inverted image the same size as the object; then u = v = 2f, so f is half the object distance.
Why do short-sighted people wear concave lenses?: A short-sighted eye’s lens is too strong, so distant objects focus in front of the retina. A concave lens spreads the light slightly so the image moves back onto the retina. Long sight is the opposite and uses a convex lens. Glasses strength in diopters is 1 ÷ focal length in meters; Chinese “degrees” are 100 times that, with concave lenses negative.

Features

0–100 cm optical bench with draggable candle, lens and screen, and f = 5, 10, 15 or 20 cm
Front view of the screen blurs with defocus and dims with magnification, and sharp images can be recorded with u, v, type and use
Optional special rays, plus demos of a half-covered lens, a burned-down candle and the virtual image when u < f
Find an unknown f with parallel light, and a rules table with v and image-height graphs plus an animation of the object approaching
Short and long sight ray diagrams with adjustable glasses and a simulated eye chart showing what the person sees
Concave lens demo and 11 kinds of practice questions

Related tools

View all 196 Physics Tools →