I was just wondering what happens when you see a visible light, for example violet, and that light slowly increases/decreases until it's not visible to the human eye. Will it fade or just become instantly invisible? Could it even injure and cause blindness?
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1$\begingroup$ Welcome! Saying that the "light slowly increases/decreases" can mean two different things: a gradual change of the light's intensity or of its wavelength. Both such processes can appear to be the same but are physically quite different. $\endgroup$– AmitJun 1 at 17:36
2 Answers
Will it fade or just become instantly invisible?
It will fade out as its wavelength gets further from the peak sensitivity wavelength of the eye (or of a specific photoreceptor in the eye).
Could it even injure and cause blindness?
Yes, light at invisible wavelengths can cause injuries or blindness.
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$\begingroup$ Sunburn might be the most common example of invisible wavelengths (ultraviolet) causing injury. $\endgroup$ Jun 1 at 18:24
It will fade in or out and a good example can be seen when you slowly heat something up until it starts to radiate from inferred to red visible light.