018 Why did the shadow disappear?

It is funny that the first explanation that comes to many A-level students’ mind is diffraction grating and slit separation. The actual explanation is something much more elementary.

The changing shadow comes about because the light source was an elongated fluorescent tube. So the shadow cast depends on the orientation of the tube with respect to the comb.

Picture1

In one orientation, the light source is just narrow enough to cast a thin umbra. In the other orientation, the light source is so wide it casts only the penumbra.

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