Refraction through a rectangular glass block sim
Set a ray box or pins, mark the ray, measure i and r with a protractor, and find n from the gradient of sin i against sin r.
Light: refraction · covers the O-Level, IGCSE, GCSE topic
What the sim shows
Light from the ray box (the red filter gives one colour, so there is no spread into colours) meets the block at angle i to the normal. It slows down and bends towards the normal, to angle r. Snell's law at the first face: n = sin i ÷ sin r. At the second face it speeds up and bends away by the same amount, so the emergent ray is parallel to the incident ray, shifted sideways by d = t sin(i − r) ÷ cos r, where t = 60 mm is the block's thickness. For i = 40° and n = 1.50: r = 25.4° and d = 16.8 mm. A parallel-sided block never totally internally reflects the ray at the second face. Some light is reflected at each face (faint rays), more at larger i; each ray is drawn as bright as the share of light it carries (Fresnel equations). Plotting sin i against sin r gives a straight line through the origin, and its gradient is n. Data-book values: crown glass 1.50 (typical soda-lime glass 1.52), Perspex 1.49, water 1.33. Nothing moves on its own: each button press is one step. Model limits: air is taken as n = 1.00; the water box's thin plastic walls are ignored, because thin parallel walls do not change the final direction; the ray is drawn thin, and a seeded scatter of up to ±0.25 mm in each pencil cross (about ±1° in i, more in r at large angles) stands in for the width of a real beam and for parallax.
- Red ray: incident, refracted and emergent; faint rays are reflected
- The block: glass, Perspex or water in a thin plastic box
- Pencil: the normal at N, your crosses and the lines you draw
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
- Size ladder: atom to Earth simDrag a zoom slider along a powers-of-ten rail from 10⁻¹⁰ m to 10⁷ m and record the real objects that come into view, from a hydrogen atom to the Earth, with their sizes in prefixes and standard form.
- Leslie cube: emitters and absorbers of infrared simPour hot water into a Leslie cube, read a thermopile 10 cm from each face over three rounds as it cools, then time which waxed pin drops first beside a radiant heater.
- Black-body radiation: temperature and colour simTurn up the supply to a lamp and watch its filament go from giving out only infrared to dull red, orange, yellow and yellowish white while its emission curve's peak moves to shorter wavelengths and its total power climbs; then warm a black or shiny can in a lamp's light until it absorbs as fast as it gives out, and see the same balance set the Earth's temperature.
- Charge flow and current simClose the switch on a real series loop and watch the lamp light at once while every ammeter reads the same; drag a magnifier over any lead to see the electrons drift from − to + against the conventional current, time a run, work out Q = I × t yourself (✗ reasons for mA and ms) and check it against a coulomb counter.
- Combining resistors simDrag real resistors into the clips of a resistance board to wire them in series or in parallel, measure V and I with real digital meters, work out R = V ÷ I from your own readings and check it against the series and parallel formulas, and see that adding a parallel branch lowers the total while adding in series raises it.
- Efficiency: a motor lifting a load simSet the supply's voltage and the load, then time a small geared motor winding a slotted-mass load up beside a half-metre rule: read the energy in from a joulemeter (or V × I × t), work out the useful energy m g Δh and the efficiency, watch a Sankey diagram of where the energy went draw itself, and plot how the efficiency changes with the load.