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Every electromagnetic radiation contains particular frequency(f), Wavelength(l) and Amplitude (a). Frequency and Wavelength categories the wave (Infrared , Radio etc) and will be constant. But Amplitude decreases over time. It can be because of propagation or losses.

So i want to know how the colours in visible spectrum look with different amplitudes. Can we observe it with naked eye?

Can i get Some examples of different amplitudes in visible spectrum

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  • $\begingroup$ Check out Feynman Lectures volume one chapter 35-Colour Vision, $\endgroup$
    – SK Dash
    May 25, 2020 at 12:42
  • $\begingroup$ there is also color perception, mny frequencies build up the perceived light color seemy answer here physics.stackexchange.com/questions/552840/… $\endgroup$
    – anna v
    May 25, 2020 at 13:08

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Amplitude is just how strong the light is. If you take a red LED and you hold it close to a wall, the amplitude is "high". If you have it far away, the amplitude is low, and equivalently, it looks less bright. (see also the comments below).

The color does not change. They look the same.

(Simple answer above, the harder answer has to do on the fact that very low amplitude visible light looks grey. If you think on how it looks like to go around a room at night, you can see things, but everything is just grey. Not only that, but you can see better with your peripheral vision than with your central visual point. That's because we have 2 different light sensitive cells in our retina: the cones and the rods. The cones are responsible for color vision (there are 3 different types, each sensitive to a different part of the visible spectrum), but are less sensitive to light than rods are. Rods are used only to detect visible light, independent of color and are more sensitive. But as humans are diurnal, our main focus point (fauvea) in the retina has a much bigger proportion of cones than rods. But the peripheral region has much more rods, and so at night, you can see better on the sides of the middle than directly on the focus of your vision. Also, at night the cones are not sensitive enough for the amplitude of the light, only the rods are, and so things look more black and white (grey) than during the day.)

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    $\begingroup$ Re, "If you take a red LED and you hold it to your eye..." I don't know about that. When I view a light source from far away, it doesn't look any less bright to me than when I view it from up close. It just looks smaller. It's true that less light reaches my eye when the source is distant, but the lens in my eye focuses that smaller amount of light onto a proportionally smaller area of my retina, resulting in approximately the same irradiance within that smaller spot. $\endgroup$
    – Solomon Slow
    May 25, 2020 at 14:51
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    $\begingroup$ IMO, a better example would be to point the light at a white wall, and look at the spot that it casts there. When the light source is further from the wall, the spot that it casts will be larger, and less bright. $\endgroup$
    – Solomon Slow
    May 25, 2020 at 14:51
  • $\begingroup$ You are right in that sense, I will change it to the wall example which is correct as scattered light. $\endgroup$
    – José Andrade
    May 25, 2020 at 17:05
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    $\begingroup$ "The color does not change. They look the same." Possible nitpick here, but as the intensity is one component of color, the color will change. Maybe not the hue, but the color will. Bright red is not the same color as dim red. $\endgroup$
    – Not_Einstein
    May 25, 2020 at 17:36
  • $\begingroup$ Yes, you are right. I also dabble a lot in color theory and of course, color is defined by hue, saturation and lightness. But I think that in this case, from the question posed by OP, I interpreted it as if they entail hue shifts. If I would have to go full color theory, additive/subtractive, etc, then the answer would be very very long, for such a more "frequency" (which I interpret as pure hue) question. Then amplitude will both change saturation and lightness, but the field of color vision is too vast. $\endgroup$
    – José Andrade
    May 25, 2020 at 18:29
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As José Andrade pointed out,

Amplitude is just how strong the light is.

However, there is a problem with José's assumption that:

the color does not change. They look the same.

And @Not_Einstein is right when he writes:

as the intensity is one component of color, the color will change. Maybe not the hue, but the color will. Bright red is not the same color as dim red.

But the problem is actually even more complex than that.

Very few monochromatic lights conserve the same hue when their intensity changes. You can see that with LEDs: in a white room with no other light source, have a look at the color of the light projected by a blue LED situated, say, at 5 cm of a white wall. Then look at the color of the light projected in the room: they are different. The high-intensity blue light is blue, the low intensity light is violet. This difference in hue exists for most wavelengths. As far as I can tell by my own experience with monochromatic LEDs, the only "hue stable" lights will be violet (between 380 and 430 nm), blue-green (around 490~500nm, and probably slightly different for each individual), yellow (around 570~575nm - probably slightly different for each individual) and far red (between 640~650 and 700nm). Other monochromatic lights tend to change in hue when the intensity of the light changes.

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