# Wave optics

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## The limits of geometric optics

### The narrowing single gap

Watch the following trial video. In this you can see a single slit that is illuminated by a laser. The width of the single gap is now reduced by means of an adjusting wheel until the single gap is completely closed. We observe the light spot on the viewing screen behind the single slit.

First of all, our observation corresponds completely to the expectation that we have if we use the model of the linear propagation of light rays (geometric optics) as a basis for the behavior of light: A light spot is visible on the viewing screen, the dimensions of which (apart from divergence effects of the laser beam) are those of the Corresponds to a single gap.

But as soon as the gap is screwed closer, we observe something completely unexpected!

The diameter of the light spot on the viewing screen does not decrease as we would expect if we assume the validity of the light ray model - it increases instead! And to top it all off, a light-dark structure is also visible.

What's that supposed to mean? Do some rays of light decide here to disappear without a trace, while their neighbors remain visible? And why does the laser beam evidently deviate from its straight-line propagation?

At least it is still clear to us why the light spot has completely disappeared in the end - the gap is now closed, no more light can shine through.

### The single slit that becomes the double slit

An experiment alone is of course not enough to completely call into question the generality of a proven model. So let's consider another experiment in which a single gap is halved by inserting a copper wire and thus made a double gap.

Well, our expectations based on the light ray model have been disappointed again!

Instead of seeing two half-diameter light spots lying next to each other after halving the single gap, we see how a light-dark structure is created when the wire is introduced, which is distributed over almost the entire width of the viewing screen.

We should slowly think about an extension of the light ray model known from geometric optics! Obviously, there seems to be a limited scope for this.

But how can we explain the propagation of light outside of this area of ​​validity? The answer to this question can be found in this tutorial, which introduces the wave model of light propagation. This is thematically closely related to the learning module on mechanical waves.