Light diffraction pattern

 Light is a wave.  It is the fact that light is a wave that causes it to make a diffraction pattern.  The best way to understand why this works is to do a similar experiment with water in a bathtub.  First side with a small gap in between.  The idea is to block water from moving from one side of the tub to the other, except via the small gap, which should be a couple of inches wide.  Plywood also works great.  Then you can tap the surface of the water with one hand to create waves.  You can also try gently sloshing your hand back and forth.  As the waves go through the opening in the wood, they will create exactly the same types of patterns that light does.  The only difference is that the water will have a wavelength of a few centimeters, whereas light has a wavelength of only a half of a thousandth of a thousandth of a meter (really small!)

Light diffraction pattern

If you tap the water faster, you will see that the wavelength of the water is shorter. If you tap the water slowly, then you will see that the wavelength of the water is longer. Then, what you do is you look at the water pattern that bounces back from the far end of the bathtub. You will see diffraction pattern. It won't glow like light does, but it will have a similar kind of shape, at least until the water bounces around the tub a few times and the waves get confused.
If you do this experiment for a bit, you will notice that the opening is spreading the wave out. But the pattern that is generated depends not only on how the wave is spread out, but also the shape the wave had before it spread. This is because the pattern is caused by some parts of the wave interfering with other parts. That is, at one point the wave is higher than the other. When those two parts touch, then the wave disappears. But when two high points touch, they make a very high point.