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Optics bench

Shine lasers, ray boxes and white light through lenses, mirrors, glass blocks and prisms. Every ray is traced live with real laws of reflection and refraction.

13 parts0/6 challenges

Presets

Parallel rays bend through a convex lens and meet at its focus, one focal length away. Slide the lens or change f.

Parts tray · tap to add

Settings

Tap a part on the bench to change it. Drag to move it, and drag the yellow handle to turn it.

Show angle readouts

Scale: 10 bench units = 1 cm.

Challenges

Build it yourself and the tick appears on its own. Presets do not count.

1Focus the ray box to one point on the screen
Add a convex lens between them and slide it until every beam lands on the same spot.
2Build a periscope to hit the target
Use two plane mirrors to lift the laser over the wall and onto the target.
3Make a rainbow with a prism
Shine white light into one face of a prism at a slant, about 50° to the normal works well.
4Trap light inside a glass block
Send the laser into the block so it meets an inside face at more than the critical angle.
5Hit a target hidden behind a wall, mirrors only
Bounce the laser around the wall with plane mirrors. No lenses or glass allowed.
6Make the beams spread out
Put a concave lens in front of the ray box so all its beams pass through it.

How light behaves

Reflection

When light bounces off a mirror, the angle of incidence equals the angle of reflection. Both are measured from the normal, an imaginary line at 90° to the surface.

Refraction

Light slows down in glass or water and bends towards the normal; it bends away again when it comes out. Snell's law links the angles: n₁ sin i = n₂ sin r. Beyond the critical angle (about 41.8° for glass) light cannot escape and is totally internally reflected, which is how optical fibres carry internet signals.

Lenses and curved mirrors

A convex lens or concave mirror brings parallel rays together at the principal focus, one focal length away. A concave lens or convex mirror spreads rays out, as if they came from a focus behind it. Shorter focal length means a more powerful lens.

Dispersion

Glass has a slightly higher refractive index for violet light than for red, so violet bends more. A prism splits white light into a spectrum, and raindrops do the same to make a rainbow.

India link: in 1928 the physicist C. V. Raman, working in Kolkata, discovered that a tiny fraction of light scattered by molecules changes its colour. This Raman effect won him the Nobel Prize in Physics in 1930, and India celebrates National Science Day on 28 February to mark his discovery.