An investigator is using a laser to illuminate a distant target. He decides that he needs a smaller beam, so he puts a pinhole directly in front of the laser. He finds that this actually spreads the beam out, making matters worse. Explain what is happening.

a. The pinhole has a size comparable with the laser wavelength, so the pinhole refracts the passing laser beam.
b. The pinhole has a size smaller than the laser wavelength, so the pinhole refracts the passing laser beam.
c. The pinhole has a size smaller than the laser wavelength, so the pinhole diffracts the passing laser beam.
d. The pinhole has a size comparable with the laser wavelength, so the pinhole diffracts the passing laser beam.

correct answer is d

The pinhole has a size comparable with the laser wavelength, so the pinhole diffracts the passing laser beam.

Explanation:

The researcher does not have a refraction problem since the medium on both sides of the pinhole is the same with the same refractive index, the problem he is having is with the diffraction of the laser beam through the pinhole, let's analyze the diffraction process that is described by the expression

          a sin θ = m λ

where a is the pinlole size,  λ the wavelength of the laser and m an integer.

The laser extends from the maximum of diffraction to the first zero (m = 1) of diffraction

         sin θ =  λ/a

when analyzing this expression we have some interesting cases

* when  λ « a. the sine approaches zero therefore we are in the case of optical geometry, in this case the laser passes through the hole without being diffracted

* when  λ ≈ a.  the sine function has values between 0 and 1, for which a diffraction of the laser beam occurs, which increases the diameter of the same

* when  λ> a.  The laser does not pass through the gap since the sine cannot have values greater than 1

After this analysis, we review the answers to the exercise to find that the correct answer is d