Limits of the visible spectrum

[Lionel Mandrake]

Last edited Tue Aug 7, 2018, 08:02 PM - Edit history (1)

What determines the limiting wavelengths of visible light? Below about 400 nm., light is usually called ultraviolet (UV). Above about 700 nm., it's called infrared (IR). But why?

The link below shows a chromaticity diagram, the boundary of which has a curved part (corresponding to spectrally pure visible light) and a straight part (the purple line, corresponding to mixtures of red and violet).

In a normal adult human, the lens filters out most of the UV. Babies can see 300-400 nm. light, which is well into the UV. But they don't see new colors in this range. Spectrally pure UV produces the same sensation as visible light within the chromaticity diagram, i.e., not spectrally pure and not on the purple line.

The cutoff at the upper wavelength limit is more gradual. Retinal photoreceptors become less and less efficient at converting light into nerve excitations as the wavelength moves toward IR. Again, although we can sometimes see IR, we don't see new colors in this range.

https://scc.ustc.edu.cn/zlsc/sugon/intel/ipp/ipp_manual/IPPI/ippi_ch6/ch6_cie_diagram.htm

[PJMcK]

1. Electromagnetism is very broad

Visible light- for humans- is a relatively narrow band in the EM spectrum.

Imagine if, (like the Suliban in Star Trek: Enterprise), we were able to see in a broader bandwidth. The things we would see on a daily basis would be awesome as our visual sensations would be multiplied.

Thanks to the brilliance and creativity of scientists, we have machines that can perceive and articulate the parts of the spectrum that we cannot see unaided.

Thanks for the link, Lionel. It's very informing.

[Lionel Mandrake]

2. You're welcome! /nt

[LakeSuperiorView]

3. UV is similar, but different than visible light.

Some substance absorb UV light but not visible light. And vice versa. This video discusses UV light.

[Lionel Mandrake]

4. Very interesting and informative video.

Thanks for posting it. Differences in the way materials interact with visible and UV explain a lot of things we take for granted.

A similar video could be made about visible vs. IR light. It turns out that IR is (1) absorbed by our atmosphere but (2) penetrates the gas and dust in space. These facts explain why IR is good (1) for TV remote controls and (2) for observing the interior of our galaxy, where some stars have been observed to be in elliptical orbits around the central black hole. But I digress.