What is refraction of light?

Refraction of light is the bending of light when it passes from rarer(less dense) medium to a denser medium or vice versa.

Explanation:  The bending of light happens as light travels at different speed in different medium for example light travels slower in water than compared to air as water is denser than air. That is why when you put a pen in a glass cup you will notice that the pen is slightly shifted (the part of the pen above water is travelling at a different speed than the part of the pen underwater).

Hope this you understand it

B. 49 N

Explanation:

Friction Force

When an object is at rest on a rough surface, it encounters a friction force that opposes motion.

The friction force when an object is moving on a horizontal surface is calculated by:

Where is the coefficient of static friction and N is the normal force.  

If no forces other then the weight and the normal are acting upon the y-direction, then the weight and the normal are equal in magnitude:

N = W = m.g

Thus, the friction force is:

We are given the coefficient of static friction and the mass of the object m=10 Kg. The minimum force needed to make it start moving is the maximum friction force:

Fr=0.5*10*9.8

Fr = 49 N

B. 49 N