Critical angle, total internal reflection, prisms and optical fibres sim
Drag a ray box round a semicircular glass, Perspex or water block on a protractor, aim it at the centre, read i and r into an empty table and plot sin r against sin i, find the critical angle three times from each side and work out n = 1 ÷ sin c, then turn light through 90° and 180° with prisms, build a periscope, and bend a light guide until light leaks out.
Light · covers the O-Level, IGCSE, GCSE topic
What the sim shows
Model. Rays in the plane of the paper, traced from boundary to boundary: Snell's law n₁ sin θ₁ = n₂ sin θ₂ at each surface, and total internal reflection when sin θ₂ would be more than 1. Nothing tells a ray where to go except these two rules. The 2 mm beam is 7 parallel rays; the share of light reflected and refracted at each surface comes from the Fresnel equations (beyond the syllabus). Semicircular block 100 mm across on a clear protractor read at 75 mm from the centre; ray box 160 mm from the centre. The refractive indices are data-book values for glass, Perspex and water, shown once you have found your own. Nothing moves on its own: each drag or press is one step.
- The semicircular block (or the prisms, or the light guide)
- Ray box with a single slit and a red filter, clear protractor, 12 V 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
- Series and parallel rules simWire the same lamps in series or in parallel, drag ammeters and voltmeters into place, unscrew a lamp, and find the rules for current, p.d. and resistance from your own readings.
- Simple pendulum: timing, and finding g simSet the length, time 20 oscillations, fill the table and find g from the gradient of T² against l.
- Sound: vibrations, a medium and the oscilloscope simDrag the microphone and turn the oscilloscope's time-base and Y-gain to see louder = taller and higher pitch = more waves; read n waves over d divisions, work out T and f = 1 ÷ T and have each step checked; watch the air's compressions and rarefactions; pump the air out of a bell jar and graph the fading sound; and race a pulse through steel, water and air.
- Speed of sound by echo simClap in front of a wall, get in time with the echoes and time a run of them with a stopwatch, work out v = 2d ÷ t and see why one echo is too short to time by hand; then time a pulse between two microphones in air, water and steel with v = d ÷ t and compare the three speeds on one graph.
- Stability and toppling simTilt a hinged board with a model bus, a bottle of sand or a wooden cone on it and watch the weight line from G leave the base as it topples; then measure the toppling angle of six wooden blocks on the protractor and plot tan θ against (b ÷ 2) ÷ h.
- States of matter: the particle model simDrag a heater slider to take 50 g of ice to water to steam and back while a magnified box shows its particles go from a vibrating lattice, to sliding past each other, to flying apart and hitting the walls; record the heating curve with its flat parts, then squash sealed syringes of air, water and wax.