in refraction, the wavelength of the light changes. but the frequency remains the same. but wavelength and velocity are inversely proportional to each other. so, when the wavelength changes, the frequency should also change. please explain.

 

two things change when light goes from one medium to another: the *speed* of the light and the*wavelength* of the light. but the frequency of the series of the light waves, passing a given point per second, does not change; thus the color does not change.

let's say laser light of a certain color (for instance, green) is aimed at an aquarium full of water. the light enters one side, passes through the water, and emerges from the other side. if you are at the top of the aquarium looking in, you will see that the beam of light is the same color outside of the aquarium, as well as inside of the glass/water. the reason for this is because of the following important formula:

speed of the light = frequency x wavelength
 
as the light enters the glass and water, it *slows down*. at the same time, the wavelength of the light also *decreases*. but the frequency (and, therefore, the color) remains the same, because the same number of waves are passing the same point in space per second both inside the glass/water and in the air outside. here is a graphic depiction of this:
 
 
 
 
 
 
![](http://www.tedmontgomery.com/remarks/08.1-12/optics/graphic.jpg)
 
as you can see, the wavelength of the green light shortens in the water, and so does the speed of the light. but the frequency remains the same; therefore, so does the color.