Let's consider an air-glass interface. When light passes from the air to the glass its frequency stays the same, but the velocity and wavelength change. Now the problem starts. If the colour stays the same then, as I was taught in school, the frequency as well as wavelength should stay the same, because they both equally determine the colour of light. But just one of them is constant! So what determines the colour of light: frequency or wavelength? Or maybe there is a mistake in my way of thinking, namely the spectrum changes when the light changes the medium? Or maybe something else?

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    $\begingroup$ It doesn't matter if the wavelength changes in glass; when you see the light, it's in your eye, so the frequency/wavelength relation is fixed. $\endgroup$ – knzhou Jan 20 '16 at 22:20
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    $\begingroup$ Something to think about: is the wavelength of the light that reaches your retina (where the color is "measured") going to be different depending on whether the light traveled through water or air before it got to your eye? $\endgroup$ – Floris Jan 20 '16 at 22:20
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    $\begingroup$ @knzhou - bingo. We were writing the same idea at the same time... $\endgroup$ – Floris Jan 20 '16 at 22:21
  • $\begingroup$ @Floris and knzhou, from this you can deduce that frequency is the reliable indicator, as it doesn't matter where on the light's path you measure it (but wavelength does, which requires you to know the index of refraction at the measurement site). $\endgroup$ – orion Jan 20 '16 at 23:04
  • $\begingroup$ @orion - that was already well explained in the duplicate. I was trying to add some intuition for the OP, and let them reach their own conclusion. $\endgroup$ – Floris Jan 20 '16 at 23:06

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