Is the speed of light really constant?
In a particular medium, yes, the speed of light is a constant. If you switch from one medium (say air to glass) you will affect the speed of light (decreasing it). Now for the second part of your question. Let’s look at any old wave. There is a relationship between the wavelength, frequency and speed of said wave. That relationship is (forgive the math) v=f*y where v=speed of the wave f=frequency of the wave y=wavelength If v were a constant and not allowed to change, but wavelengths were, how could we say that the speed (v) is constant? Simultaneously, as the wavelength of light decreases, the frequency increases. So, each wavelength of light has a corresponding frequency– not a range. Since there is only one frequency for each wavelength, the speed of light can be a constant. Moreover, if you multiply the wavelength and frequency of light together you will always get the speed of light. Neat, eh?
