It's velocity is not constant as direction is changing.
We know, velocity is speed with direction, so if direction is changing, velocity can't be constant, doesn't matter that speed is constant.
Hope this helps!
D ............................
The refractive index of water is

. This means that the speed of the light in the water is:

The relationship between frequency f and wavelength

of a wave is given by:

where v is the speed of the wave in the medium. The frequency of the light does not change when it moves from one medium to the other one, so we can compute the ratio between the wavelength of the light in water

to that in air

as

where v is the speed of light in water and c is the speed of light in air. Re-arranging this formula and by using

, we find

which is the wavelength of light in water.
Increase .... decrease .... presumably it's the "best shape" for a body which has been formed by the gravitational force