Comment by AlienRobot | Hacker News Reader

Comment by AlienRobot | Hacker News Reader

This is a great article about the topic! It covers everything.

But can someone explain this to me?

>Light is technically something called electromagnetic radiation and it has a frequency and wavelength. That wavelength can vary, depending on the energy of the wave. High energy waves have a higher frequency and shorter wavelength, and low energy waves have a lower frequency and longer wavelength.

>This means that the same amount of energy at different wavelengths will not be perceived as the same brightness. For example, a light with a wavelength of 555 nm (green) will appear brighter than a light with a wavelength of 450 nm (blue) even if they have the same energy.

The article asserts that the wavelength (thus color) changes with the amount of energy, but then it says that you can have light of different wavelengths (color) with "the same energy."

He confuses two different places there is energy. Light is made (this is a little bit of a cheat) of photons. Each photon has a wavelength λ, and a per photon energy E where E = hc/λ, h is Plank's constant and c is the speed of light constant. So energy and wavelength per photon completely determines each other (and the color of the light of that single photon).

These energies are very small. You can add a lot of photons per second, increasing the brightness of that color, and this now has the energy per second of all those photons. So you can have a lot of red photons which sum to some energy, or a different number of blue photons that sums to (very, very close) the total red energy.

These are the two energies he confuses in those two places.

The amount of energy in a wave depends on both its wavelength and amplitude.

So a "blue wave" has more energy than a "red wave" if both have the same amplitude (blue has a shorter wavelength, and energy is inversely proportional to wavelength). But you can have a "blue wave" with the same energy as a "red wave" if you increase the amplitude of the "red wave" to compensate for its longer wavelength.

Oh, that makes sense! Thanks.

So that means blue has more energy because it pulsates faster, and in spite of this we're less sensitive to it than we are to red, which pulsates slower. It's like our light sensitivity forms some sort of bell curve.

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