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Why do waves that were traveling in a straight direction change direction when passing through an opening?

I thought that the waves (red arrow) when colliding with the wall bounce in the opposite direction (green arrow).

And the waves that pass through the aperture follow its path normally as shown in the image on the right.

The waves that go in a straight direction should follow traveling straight line like a car that goes under a bridge the car is straight on the road.
But this is not so.

Why does the direction of the waves change?

How is the direction of the waves calculated?

For the full math, you can look up 'diffraction' and 'Huygens Principle' but here I will just post a quick observation that is enough to get a good physical intuition.

Suppose we are considering water waves, and imagine yourself sitting behind the barrier in the 'harbour' (at the lower part of your diagram), watching the waves approaching from 'out at sea' (i.e. the top of your diagram). As the waves reach the 'harbour mouth' (i.e. the small opening in your diagram) the water there is caused to go up and down. So there is this water bobbing up and down in the small opening. Now the surface of the water nearby is going to bob up and down too, isn't it? And the ripples will spread out from there. It doesn't really matter in what direction you consider: the waves will spread out into the 'harbour' because the water at the harbour mouth is moving.

From this way of thinking, you begin to wonder why the waves out at sea are so straight! Ultimately it is because in that case you have oscillating water all along a long line, and so the water all along that long line is caused to move in synchrony.

As I say, this is not a full mathematical answer, just an attempt to give you some intuition about the physics.

A quick answer would be that they are not changing direction.
Each point in the plane is the source of a single wave. Single waves expand in circles, but as you put many single waves together you sum them and get a plane wave.
The aperture if small enough simply blocks the other waves allowing only one to pass and thus it re-takes circular shape.

This is a simplification of diffraction and huygens' principle but it might help you get an idea.

Your aperture only allows a very short segment of the incoming plane wave to pass through. As the aperture becomes smaller, the segment looks more and more like a point source. A point source emits spherical waves like you show in your lower right figure. (this is almost intuitively obvious because of symmetry--what other shape of wave would a point emit?).

This is usually explained more formally via diffraction:

https://isaacphysics.org/concepts/cp_diffraction

"Diffraction is the spreading out of waves as they pass through an aperture or around objects. ... In an aperture with width smaller than the wavelength, the wave transmitted through the aperture spreads all the way round and behaves like a point source of waves (they spread out below)"

When you want to know why the waves move in circular path when the initial waves are planner in nature then you have to go through huygens wave theory you will they understand diffraction better

huygens wave theory

In the aperture the wave us no longer plane : it us the product of a rect function, which is unity in the aperture and zero outside it, and a plane wave. You can inspect which wave vectors are present by Fourier transforming this product. The result is a convolution of the transform of the rect, the so called sinc function, and the plane wave. The message is that the result is a sum of plane waves of varying direction. For a point aperture all plane waves are present with equal amplitude and phase, that is, a spherical wave. Alas this requires some elementary math to understand.

Tausif commented:

I think OP wants to know why the diffraction occurs and why the waves don't just continue like they pointed out in the diagram.

In any elastic medium, a pressure effect not only leads to material displacement in this direction, but also to lateral displacement. (In an inelastic medium the material gets simply punched out.) So the awaited longitudinal wave is accompanied always by a transversal wave.

This transversal wave spread out in isotropic media as a spherical wave. The obstacle with the slit limiting the isotropy and instead of a spherical wave on get only have of a spherecal wave.