Recently in my studies, I was working with a microcontroller and some LEDs. While messing around with a circuit, I covered a red LED with my finger because I felt it was too bright to look at, and noticed that so much light passed though my finger it almost looked to be glowing. If I tried the same thing with a green LED, almost none of the light got through my finger into my eyes.
I was tempted to rationalize it to myself by saying that maybe my red LED was brighter, or that it was because my finger is more red than green and so it absorbs all the green light but not the red or something like that. However, later I watched Applied Science’s latest video about making rugate filters, and he mentioned that while germanium is opaque to visible light, it is transparent to infrared light.
Are these phenomena related? Why do different wavelengths of light pass through different materials in different amounts?
In: Physics
For your LED example, the simplest and most intuitive answer is that your fingers absorb every wavelength of visible light except for “skin colour” pluss the red inside your blood. So everything other than reddish. The red LED has plenty of red that can pass through, while the green one has virtually none.
On the subject of opacity, glass is transparent to visible light, but opaque to infrared (if you hold a glass pane in between you and a fire you won’t feel the heat). There are a couple of things that can be going on. For one certain wavelengths may be scattered, while other are not (that’s why the sky is blue). Another possibility is that the material absorbs the particular wavelength (like your finger and the LED). Lastly (that I can think of) there’s penetration. Some wavelengths are better at penetrating “stuff” than others, and some materials are harder to penetrate, though this his unlikely to be important in your case.
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