diffraction

Question

Which of the following settings in a ripplie tank will produce the most prominent diffraction pattern of water wave of wavelength of 2 cm?
A. The waves though an obstacle of length of 4cm
B. The waves though an obstacle of length of 2cm
C. The wave passes through a gap of 4cm
D. The wave passes through a gap of 1cm

Why the answer is B but NOT D?

Answer 1

First, we will cross out the answer D.

When we saw that the wave pass though the gap, we might think that the diffraction was the largest, but it was wrong. As the gap is smaller than the wavelength, although the pattern showed the largest diffraction, its ampitude was reduce a lot.That made a result as it became invisible.

But why the answer is B?

Of course it's due to the length of the obstacle. Since it is equal to the wavelength, the greatest diffraction will occur at the edge of the obstacle although it's only a abstacle , but not a gap.

Answer A and C are totally not acceptable, I think you ' ve known so I don't explain it.