why does the point of a laser look pixelated when spread out?



why does the point of a laser look pixelated when spread out?

In: Physics

The thing about laser light that makes it special is that it’s ‘coherent’, which means it’s all coming out of the emitter at the same frequency and phase. But once it leaves the emitting element, it passes through some stuff. First, usually, a little bit of transparent plastic which is part of the laser diode package. Then, possibly, a bit of glass or plastic optics meant to bend or focus the light. Then some air. Each of these transitions bends the light beam a little. This means that different photons might be leaving the same spot on the emitter, and landing on the same spot on the wall (or whatever) but they travelled along different paths in between.

If those paths are not exactly the same length, then that means the photons are no longer guaranteed to be in phase with each other. If they are in phase, then they interfere constructively and are brightly visible. If they are out of phase, then they interfere destructively and will cancel each other out.

[a little diagram of how constructive and destructive interference works](https://cdn.britannica.com/07/62907-004-A3A6351E.jpg)

The speckled, mottled pattern you see around a laser pointer dot, is the result of those photons following slightly different paths and messily interfering with one another.

That principle – light from a single source travelling different paths and interfering with itself – is basically the same phenomenon they were exploiting in the [Michelson-Morley experiment,](https://simple.wikipedia.org/wiki/Michelson%E2%80%93Morley_experiment) which demonstrated some important facts about the speed of light and helped to confirm Einstein’s theory of relativity.

Light is actually tons of tiny packets of energy getting thrown at something. Think of a directed light source like a laser or flashlight as a shotgun. When it’s close to a wall, the spread is very very small, and the light looks like a single spot. When it gets further away, the pellets can go in a wider area, making it look more messy.

For a regular flashlight, all the energy getting thrown out isn’t identical, so in a way, this makes it less predictable. Lasers are super special and can throw out identical energy packets, so these will make more of an obvious pattern. While flashlights just get dim as they spread out, lasers will have clearer light and dark spots.

Going with the wave model of light, compare each bright speck of light the the tip of each wave on a body of water under “choppy” conditions. When the waves are regular and all coming from the same direction the top of each wave would be a line. Once the waves bounce off an irregular shoreline or rocks they start interfering with each other until no perceptible waves just thousands of random peaks.

Regular orderly waves are what is emitted by any laser but the wavelength of light is so short that there is no surface smooth enough to avoid acting like a rocky shoreline to the laser.

In a laser, the light is all the same color and all lined up. When it hits a rough surface the light waves can either add up to give you bright spots or interfere to give you dark spots.


It is called speckle noise, and it is fascinating.