Reflection
When light rays travelling through a medium reaches the boundary of another medium, they turn back to the first medium.
Travels in Straight Light
As long as light stays in the one medium (air or glass) and does not hit anything like a mirror, it will travel in a straight line.
If a spring or wire which is fixed at one end is stretched by hanging a load to the other end, then Hooke's law can be defined as:
The instrument which is used to measure the weight is called spring-balance or Newton-meter.
Balanced and Unbalanced forces are discussed below.