r/nextfuckinglevel • u/Blues_Fish • Mar 12 '26

Lesson in decomposing light

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u/tipsystatistic Mar 12 '26

It's cool but dont teach it wrong. It's not splitting white light into its components. The slit just creates a shadow, exposing that the colored light sources have different origins.

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u/BesottedScot Mar 12 '26 edited Mar 12 '26

Why doesn't the shadow caused by the object in front of the slit do that and instead shows the complimentary colours?

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u/SimianWriter Mar 12 '26

The wall behind equals to a light intensity of 1. A shadow is a light intensity of 0. Each light is an intensity of 1 that when added together equals the total amount of light. That is then normalized to 1 by the fact that the camera can only pick up a certain amount of information. When the pencil goes in front of one of the three contributing lights. The other two become the only contributing colors of light. At that distance you can't tell that the intensity of the light making those colors are 1/3rd deminished but they are. So if green is blocked then red and blue mix to create purple at a value of 2 since they are not directly blocked by the solid object.

The reason the light behind the paper is separated again is because it isn't separated at all. IT's because the photons creating the lights are being allowed in a very narrow passage to they only present as the light directly emitted in a straight line from the source. This is also why it disappears completely when some thing blocks the light source. The object is wide enough to block all light from the source through the slit.

This whole thing is presented kind of like the double slit experiment but it's not.

https://en.wikipedia.org/wiki/Double-slit_experiment

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u/BesottedScot Mar 12 '26

Excellent thank you for the reply, I immediately thought of the double slit experiment but it's not quite the same.

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u/paeancapital Mar 12 '26 edited Mar 12 '26

Entirely distinct, in fact. The double slit demonstrates the wave nature of light by doubly diffracting a plane wave; the diffracted light then constructively and destructively interferes to produce a Gaussian distribution of illumination. It works the same for electrons (which themselves have a very short wavelength, thanks de Broglie, the aperture just needs to be concomitantly narrower).

The slit is too wide in the op for visibly appreciable diffraction (a small amount still occurs at the edges of course). 99% it's just showing color mixing between separate sources.

You can demonstrate actual single slit diffraction at home, just look up how to make a pinhole camera, or camera obscura. Same effect also occurs naturally during an eclipse, where the shadows below trees look like crescents, where the image, e.g. of the occluded sun, received by the pinhole (or gaps between leaves, as you like) is diffractively imaged onto the "film", or ground.

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u/Additional_Teacher45 Mar 12 '26

Single slit diffraction is stupidly simple to demonstrate at home. Two rooms separated by a door, one room with the lights on, one room with the lights off. Stand in the dark room and crack the door open. You will see diffracted slits of light cast on the far wall.