Question

When a wave of wavelength λ encounters an obstacle with size ℓ, under what circumstances does the wave diffracted by the obstacle carry information as to the size and shape of the obstacle?

1. diffraction does not occur at all in such circumstances.

2. λ > ℓ.

3. λ ≫ ℓ.

4. λ ≃ ℓ.

Answer 1

Ans. (4) λ ≃ ℓ

Actually, diffraction occurs all the time. But when the aperture/wavelength ratio is large, the spacing between fringes becomes too small, and the diffractive effects are unnoticeable. If ℓ≫λ, the angular spread of light leaving the slit will be very small. When λ∼ℓ., the angular spread becomes significant. Finally, when λ≫ℓ., the slit acts like a point source - light coming out of it spreads out over the entire area on the far slide of the slit.