Focal length of a converging lens sim
Slide a screen, a lens and a lamp box along an optical bench, judge for yourself when the image is sharpest, read u and v off the metre rule into an empty table, and find f three ways: a distant window, a plane mirror behind the lens, and a 1/v against 1/u graph; then measure the image to find the magnification.
Light: thin converging lenses and their focal length · covers the O-Level, IGCSE, GCSE topic
What the sim shows
Model. A thin converging lens (f = 10, 15 or 20 cm, or an unknown lens), traced ray by ray: rays from each point of the 20 mm arrow through every part of the lens's 40 mm clear aperture land on the screen as a blurred circle A·|1 + s/u − s/f| across (s: lens to screen), round a point scaled by −s/u (upside down). Nothing moves the screen for you, and the image is never snapped sharp: the blur changes smoothly with the screen's position, and the image looks sharp only where the blur is under about 0.5 mm (chosen). Distant window 10 m away; plane mirror 5 cm behind the lens. No aberrations. The lens equation is used only for the 1/v against 1/u graph method, which is beyond the syllabus.
- The white screen (in the mirror method, the plane mirror)
- Lamp box (arrow and cross-wire), converging lens, metre rule, bench rail, lamp supply
Use it in your lessons
- Try it here: anyone can unlock 2 figures free, and a free account unlocks up to 5. Runs on a laptop, a Chromebook or a phone; nothing to install.
- Present it and share student links with the teacher plan, from US$5 a month, billed yearly.
- Put it on your own website or LMS for US$9, paid once, or every sim from US$29 a month.
Free guide: 12 practicals as sims, and how to run one in a lesson →
Need a version for your exact set-up? Have one made: US$49 a figure in an order of 5.
More physics sims
- Cylinder rolling down a ramp: v² against x simMeasure sin θ with a half-metre rule and set square, time a rolling cylinder (or a trolley) over eight distances, fill the table, plot v² against x and find G ÷ (2 sin θ).
- Adding vectors by scale drawing simSet two forces or two velocities, pick a scale, draw them head to tail or as a parallelogram, then measure the resultant with a ruler and a protractor and compare it with the calculated value.
- Atom and nuclide builder simDrag protons, neutrons and electrons into an atom; read Z, A, the element, the nuclide notation and the charge, see whether the nucleus is stable or radioactive, and compare isotopes side by side.
- Centre of gravity of a plane lamina simHang a card from a pin at three holes, mark the plumb line each time, find the centre of gravity where the lines cross, and check it by balancing on a pencil point.
- Charging by rubbing: only electrons move simRub a polythene, acetate or glass rod with wool or silk, watch electrons move from one to the other, then bring the rod near a hanging charged rod or scraps of paper: like charges repel, unlike attract, and a charged rod pulls a neutral one.
- Circular motion: the whirling bung simWhirl a rubber bung on a string: set its speed, radius and mass and read the pull on a newton-meter, let go and watch it leave along the tangent, then hang slotted masses on the string, whirl until the paper clip holds steady, time ten turns and plot F against v².