We all know that light has constant speed, and variable wavelength, and frequency. Suppose I have a big red glass square sheet. When white sun rays pass through it, their colour changes from white to red. I am unable to understand this effect
"White light" contains all the colors and when the light passes through the red glass all other colors are absorbed but the red goes through.
First think, why is that glass sheet Red? Answer: it permits only certain wavelengths to pass through, absorbing all others. Those that passed are predominantly red, as perceived to our eyes.
White light being a combination of all colours contains the wavelength range of red, which is the only part of the spectrum that the glass lets pass through.
Essentially, white light is a combination of all colours in the visible spectrum. Now, when the light passes through the red glass, The glass absorbs all the colours of shorter wavelengths/higher frequencies and allows only red light(longer wavelength) to pass through it. I highly recommend you to study Color subtraction from this link: