When I shine a laser pointer at something, the point looks like it is made of a bunch of tiny dots, how is the laser diode different from a normal LED?
In: Technology
No one should do this: it looks the same if you stare into the beam of a laser pointer.
It also looks the same if you use a DSLR camera.
It’s fascinating.
If you stare into the beam of a laser at least put a lens in front of the laser so it gets spread out to the point where it is a very weak flashlight.
Light from a laser is coherent, which means that it is all one big wave. When it reflects off of a surface that is not perfectly smooth some spots will create reflections that interfere constructively with each other, creating points that are brighter
laser lets light out of a small hole. the light has a narrow range of wavelengths. small hole with the same colour of light gives you the speckly interference pattern
[https://en.wikipedia.org/wiki/Diffraction#Single-slit_diffraction](https://en.wikipedia.org/wiki/Diffraction#Single-slit_diffraction)
For a proper ELI5 go and look at water waves passing through a single gap in a sea wall.
Speckle is the name of this phenomenon. It’s an inherent property of coherent light. My 1978 student job at the Optical Sciences Center in Tucson was helping a grad student understand it. I don’t think he ever got that far.
Not an explanation but a fun fact, the speckle effect can be used as a form of eye test. Here’s an article copied from Wikipedia.
Laser speckle also known as eye testing using speckle can be employed as a method for conducting a very sensitive eye test.[1]
When a surface is illuminated by a laser beam and is viewed by an observer, a speckle pattern is formed on the retina.[2][3] If the observer has perfect vision, the image of the surface is also formed on the retina, and movement of the head will result in the speckle pattern and the surface moving together so that the speckle pattern remains stationary with respect to the background.[4]
If the observer is near-sighted, the image of the surface is formed in front of the retina. Since the speckle pattern is perceived by the brain to be on the retina, the effect is of parallax; the speckle pattern appears to be nearer to the eye than the surface and hence moves in the same direction as the surface, but faster than the surface. If the observer is far-sighted, the speckles appear to move in the opposite direction as the surface, since in this case the surface image is focused behind the retina. The apparent speed of motion of the speckles increases with the magnitude of the defect of the eye.
This technique is so sensitive that it can be used to determine changes in the ability of someone to focus through the day.
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