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Your first experiment

Nothing here needs any theory. Follow along, and by the end you will have measured chromatic aberration — the reason cheap lenses have coloured edges — on a lens you placed yourself.

1. Put a lens in the beam

1. Start Photonica. You have a white laser, a beam, and a screen.
2. In the Build panel on the left, click Convex lens. It drops into the beam, and the beam immediately pinches together behind it.
3. Drag the lens with the left mouse button. Watch the cone of light move with it.

That pinch point is the focus. The lens is bending every ray towards the axis; they all cross at one place.

2. Measure it

Click the Measure tab at the top right. This is the instrument panel — the same numbers an optics bench would give you:

• Focal distance — how far behind the lens the light comes to its tightest point.
• Red / green / blue focus — the same measurement, for three separate colours.
• Best-focus spot — how small that point actually is, as an RMS radius in millimetres.

Now press F, or click AUTO FOCUS: screen. The screen slides along the beam to sit exactly at best focus, and the spot on it collapses to a point.

3. Split white light

1. Press Del to remove the lens, then click Prism in the Build panel.
2. Rotate it with Q and E until a spectrum fans out of the far side.
3. Drag the screen into the fan to catch the rainbow.

This is the same experiment Newton did in 1666, and the same physics as the cover of The Dark Side of the Moon — which is, in fact, demo 3.

4. Make it beautiful

Press P. The bench switches to a spectral path tracer: caustics pool where the light concentrates, the beam glows through the haze in the air, and the image keeps refining while you leave it alone. Press P again to go back to the fast view.