Ray optics

How it works

Ray (geometric) optics treats light as straight lines. A mirror reflects a ray at the same angle at which it arrives. At the boundary between two materials the ray refracts according to Snell’s law. A convex lens focuses parallel rays to the focal point at distance f behind it; a concave lens spreads them as if they came from a focal point in front of it. Glass has a slightly higher refractive index for violet than for red light, which is why a prism splits white light into colours.

reflection: α = α′refraction: n₁ · sin α₁ = n₂ · sin α₂thin lens: 1/f = 1/a + 1/a′

Try it yourself

1. Open “Experiments” → “Bringing light into focus”. The lens sits at x = 0 and the rays meet at the focal point 180 mm behind it. Select the screen and move it to x = 180 mm: the spots of light merge.
2. Select the lens and change “Focal length” to 90 mm. Predict where the focal point moves, then refocus the screen.
3. In “The law of reflection”, turn the mirror using “Rotation” and check that the angle of reflection always equals the angle of incidence.
4. “The colours of white light” shows a prism splitting light: violet rays bend more than red ones.

Model limitations

The model is two-dimensional and uses rays only. It leaves out wave effects (interference, diffraction) and the brightness of reflected and refracted light. Lenses are thin and ideal, without aberrations; dispersion in the prism is simplified.

Related experiments

• Waves and interference
• The Solar System
• Atoms and chemical bonds