I shone a laser at a wall. Note the circular dot on the wall (and ignore the reflection below it from the table top).
Then I fired the laser beam through the gap between 2 razor blades.
When the gap gets small enough, the beam gets spread out. Note that it does not spread out evenly but that there is a line of dots with blanks between them. This is just caused by a SINGLE slit.
When the slit is really narrow, the dots are stretched out even more sideways. Here you can see the central dot smeared out into a line and a very faint one to the right and to the left of it. Maximum spread is when the width of the gap is the same size as the wavelength. It this case, the wavelength is about 600 nanometres. That is 0.0006 of a millimetre. I don't think I got the razor blades quite that close together.
Here the razor blade gap was somewhere between that shown in the first two pictures.
