Huygens’s Principle by Christian Huygens

Microscopes existed in Holland by 1620. Christian Huygens and Robert Hooke were the first two scientists to make scientific use of microscopes.

Both designed and built two-lens microscopes (two ground glass lenses inside a thin metal barrel). Christian Huygens (1629-1695) was a Dutch Physicist who proposed a wave theory of light long before Maxwell's incredible discoveries.

He suggested that each point on a wavefront acts as the source of tiny wavelets that move forward with the same speed as the original wave. All these wavelets at a later time will combine to form the subsequent wavefront.

In 1678, Huygens published his Traité de la Lumière (Treatise on Light), in which he described this wave theory, and the principle named after him comes from this text. It was published in 1690 showed his need for ultimate mechanical explanations in his discussion of the nature of light.
Huygens’s Principle by Christian Huygens

Galileo Galilei and Law of Free Fall Object

The Italian Galileo Galilei (1564-1642) originated the present day ideas concerning falling objects.  He was one of the first scientists to assert that all objects fall with the same acceleration.

Galileo a 24 year old mathematics professor at the University of Pisa, Italy often sat in local cathedral when some nagging problem weighed on his mind.

Lamps gently swung on long chains to illuminate the cathedral. One day in the summer of 1598, Galileo realized that those lamps always at the same speed.

He decided to time them. He used pulse in his neck to measure the period of each swing of one of the lamps. Then he times a larger lamp and found that it swung at the same rate. He immediately set up an experiment that revealed the pendulum principle.

Over many days he timed the lamps and found that they always took exactly the same amount of time to travel through one compete arc.

On his essay On Motion, Galileo gave an argument in support of the idea that objects of different weights fall at the same rate – that is, that their speed of falling does not depend on their weight.

He stated that heavy objects would fall faster than lighter objects.
Galileo Galilei and Law of Free Fall Object