Question:

If a beam of red light (wavelength = 0.65 micrometres) passes through a pinhole with diameter of 4 micrometres, what is the diameter of the spot on a wall 3 m beyond the pinhole?

Ans: 0.5 m

See attached explanation.

My question is, why can you multiply D by the angle to get lspot?

If a beam of red light (wavelength = 0.65 micrometres) passes through a pinhole with diameter of 4 micrometres, what is the diameter of the spot on a wall 3 m beyond the pinhole?

Ans: 0.5 m

See attached explanation.

My question is, why can you multiply D by the angle to get lspot?