Video Transcript
The following figure shows light
rays diffracted when they pass around an obstacle. Which of the following most
correctly describes the interference of the diffracted light? (A) The light waves from either
side of the obstacle will overlap after traveling some distance from the obstacle
and will interfere. Or (B) interference of light waves
only occurs when the light waves pass through narrow gaps.
To begin, let’s recall that light
waves interfere when they occupy the same region of space at the same time. One of the most notable examples of
this phenomenon can be seen when we shine light through two narrow gaps that are
nearby one another. Light diffracts through each of the
gaps. And the diffracted light rays from
both of the gaps will overlap and thus interfere.
But it’s important to understand
that diffraction and interference don’t only occur when light passes through
gaps. Diffraction will also occur when
light waves travel parallel to a surface and reach the end of the surface, like
what’s being shown in the diagram given to us. We can see how these light rays
spread out after reaching the end of the surface. So we can tell that the wave fronts
of these light rays will eventually meet. And where the light rays do meet,
we know interference will occur.
Thus, we know that wave
interference does not only occur when light passes through narrow gaps. So we should eliminate answer
choice (B). Answer choice (A) is correct. The light waves from either side of
the obstacle will overlap after traveling some distance from the obstacle and will
interfere.
