Imagine you have a flashlight and you shine it onto a wall. The light will create a bright spot where it hits the wall and it will gradually become dimmer the further away from the spot you look. This is because the light is spreading out in all directions as it travels away from the flashlight.
Now imagine you have a different kind of flashlight that emits light in a very specific way, so that the light only spreads out in one direction. If you shine this flashlight onto the wall, you will see a very different pattern. Instead of a bright spot and a gradual fading, you will see a sharp line where the light hits the wall, and on either side of the line, you will see areas of alternating brightness and darkness.
This is a near-field diffraction pattern. It happens when light waves pass through a small opening or around an obstacle that is close to the source of the light. The pattern is created because the waves interfere with each other as they pass through the opening or around the obstacle. The interference causes some areas to be brighter or darker than others, creating a pattern.
The pattern that is created depends on the size and shape of the opening or obstacle, as well as the distance between the source of the light and the opening or obstacle. Scientists and engineers use near-field diffraction patterns to study and manipulate light, for example in microscopes or in the design of computer screens.