Apparent depth / seismic waves

[kingwinner]

1)

In this figure explaning apparent depth and true depth, why are two rays shown instead of one ray? (In cases of refractions, like air->water, usually showing one ray's refraction is enough to explan and show its bending!)

 

2) "Seismic waves speed up as waves from the crust reach the mantle, this indicates that the mantle is denser than its crust"

Why do seismic waves speed up as they travel through denser layers?

 

Thanks for explaning!