I was sitting in Latin class (obviously somewhat bored) and got to wondering about the following:
Is there any way to alter the wavelength of light reflected back from an object? Would there be any way to, say, find a chemical compound or something that will take ANY visible light that hits it and alter the wavelength somehow into the non-visible range? I’m guessing no, but I don’t know and I don’t know why it could or could not be possible.
Also, assuming such a compound WAS around, say for instance it was made into a paint. Say, for instance you paint a small block with this special paint. Now, with the unaided eye, what exactly would happen? Would there be a white spot where the object should be? Would you be able to see directly through the object (I’m pretty sure this would be no)?
I’m fairly sure no substance exists, but if it did, think of the implications! Camoflauge, etc. . .
If the surface is moving away or towards you very fast, you’d get a Doppler shift. It would have to be travelling very fast to be noticeable, however.
Fluorescence is a bit similar to what you describe as well. Fluorescent substances absorb ultraviolet light and emit visible light. A blacklight is just an ultraviolet light source - it makes fluorescent substances glow.
Come to think of it, pretty much all substances absorb visible light and emit infrared light. What do they look like? They absorb visible light so they look dark to us. And since we can’t see the emitted infrared light, they don’t look any different from substances that absorb visible light and don’t emit anything.
If you painted something with perfect flat black paint, then you would not see any light reflected off of it. You could argue that this would make the object invisible, but it wouldn’t be a very useful invisibility: As long as the background wasn’t also flat black, you’d be able to tell where it was just fine. In order to make something effectively invisible, you’d need to first find some way to get light to go through or around it, and not just get absorbed, and secondly, make sure that the light getting to the other side of the object took exactly as long to get there as it would if it were travelling through the medium surrounding it (presumably air), or else it’d refract light and be apparent that way. The speed of light in glass, for instance, is almost exactly the same as through turpentine, so a piece of glass immersed in turpentine will be effectively invisible.