Diffraction

When starting out thinking about lasers, a lot of people think that they just go in a perfectly straight line forever. But no, physics is not as permissive as that. Light is a kind of wave, a wave made up of electric and magnetic fields but a wave nonetheless. And when waves encounter an obstacle, they can bend around the obstacle. Think of ripples in a still pond encountering a reed poking up through the water - the reed doesn’t cast a perfect shadow behind it. No, those ripples bend around and fill in the space behind the reed as well. In a similar way, if the ripples hit a wall with a hole in it, the ripples that go through the hole won’t march on in a perfectly straight column. Rather, they spread out as soon as they go through the hole. This bending and spreading out is called diffraction. Diffraction is a fundamental property of any wave in nature.