Is it true that the speed of light varies from medium to medium? Why or why not?

Yes, it is true that the speed of light varies from medium to medium. A lot of people have made so many mistakes about the constancy of the speed of light. Majority of them don't know that the constancy of the speed of light is only applicable to light travelling through a vacuum. This means the speed of light will not be constant when passing through some media like air, water, glass, etc.

The truth here is that the speed of light usually decreases as it enters into denser medium. For example, you should expect a decrease in the speed of light if it were to travel from air to a medium like glass or water. What is causing the decrease in the speed of light as it enters another medium is due to the refractive index of the new medium, but the frequency of the light is still constant.

Yes, the speed of light is going to vary depending on the medium present. The primary measurement is done in space. Since this is where the majority of the measurements are taken, there are now scientists who are taking a look at what happens when different mediums and constants are put into place. While there are changes and variants to the universe, these are not fast changes, meaning that they are somewhat constant.

The way that the speed of light travels through the air is going to change when it hits another medium, such as glass. Glass would cause the rate of speed to decrease. The rate of speed is also at its peak when used in a vacuum. The many different mediums are still be tested to see what changes it will have on the speed of light. There are many exciting discoveries yet to be made on the constants of space, the speed of light and its effect on the universe.