Focusing a Laser

Tiny spots, huge intensities

An interactive lesson with 6 steps, about 5 minutes: you predict, drag and solve, with instant feedback on every answer. It runs in your browser and needs JavaScript.

What you’ll learn

• Focused spot radius $w_0 \approx \lambda f / (\pi w)$: wider beams focus tighter.
• Small spots give enormous intensities, used for cutting and welding.
• Depth of focus $z_R = \pi w_0^2/\lambda$ shrinks with the square of the spot size.

Steps

1. How small a spot?
2. Intensity
3. Choose the lens
4. Even smaller
5. Depth of focus
6. Halving the spot