# Question Video: Describing the Interference of Diffracted Light Rays Physics • 9th Grade

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. [B] Interference of light waves only occurs when the light waves pass through narrow gaps.

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### 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.