Light Isn't Constant in Certain Media?

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Liger20
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Hello, I was recently browsing through on of my physics books, and I came upon the following statement:

"Newton was right, light does travel at a constant speed, but it does not travel at the same speed in all media. For example, light travels at a speed in water that is about three fourths the speed of light in a vacuum. The speed of light in air is slightly less than the speed in a vacuum."

I found this statement disturbing, because I've always been taught that light always travels at a constant speed. It not that I don't believe that it travels slower in water than in a vacuum, but my question is why does light behave like this?

Also, what is the medium through which light travels, if any?
 

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Liger20 said:
but my question is why does light behave like this?
In vacuum light just travel and its speed is maximum possible.

In water, glass or air light not only travels, but also being absorbed and reemitted by media atoms. This process takes some time. So, it has less time to travel. The result is slower traveling in media other than vacuum.