If it's a mechanical wave, then its speed depends on the physical characteristics of the medium.
If it's an electromagnetic wave, then its speed depends on the
electrical characteristics of the medium.
Either way, the properties of the medium determine the wave speed.
You want to change the speed ? You have to change the properties
of the medium.
Answer:

Explanation:
By Snell's law we know at the left surface




now we have


now on the other surface we know that
angle of incidence = 

so again we have

so we have


also we know that


By solving above equation we have

Answer:
Magnetic field, B = 0.042 T
Explanation:
It is given that,
Speed of charged particle, 
Angle between velocity and the magnetic field, 
Charge, 
Magnetic force, F = 0.002 N
The magnetic force is given by :

B is the magnetic field


B = 0.042 T
So, the strength of the magnetic field is 0.042 Tesla. Hence, this is the required solution.