As a physicist, Newton worked in three fundamental areas of modern science. Optics, Calculus and Gravity.
In 1666 he first observed the effect of light passing thought a prism leading to the splitting of white light it to the range of rainbow color, like on the album cover of Pink Floyd ‘‘dark side of the moon’’. This lead to a proof that color is an intrinsic property of light and would go on to be the basses for the discovery of Ultra-Violet and Infrared light many years later. His contribution to optics also supported the world of astrophysics. While building a telescope that doesn't uses lens so that the light reaching the telescope would be unaffected by the equipment, he designed the first functional mirror based telescope. However his understanding of light was still deeply floored in places. He was one of the first to bring forward the idea of the ‘’ether’’ - a plan of reality in which interactions between particles occur. Now we would understand this effect in many different way, whether that be thought common forces such as electromagnetism all the way to the complication of quantum mechanics.
Calculus, a form of mathematics that is based around calculating the gradient or area underneath curved lines on a graph, was developed by newton between 1666 and 1668 in a series of letters. There was, at the time, some debate between himself and Gottfried Leibniz, a German polymath of the same era, over who had invented calculus as both of them had been sending letter to a range of people on there findings. While the Royal society of mathematics, which was incidentally chaired by Newton, decided that Isaac had been the first to come up with the idea - modern historians say that both men came up with the theory at the same time, independent of one another.
Now on to gravity - by far this most well know work. Having worked on celestial mechanics (a field we would now call astrophysics) for the early part of his career, newton returned to the subject in 1679 after a range of correspondence with Hooke, a leading observational astronomer. His discovery of gravity was a multi-stage process, starting with a understanding that some sort of centripetal force would be needed to for Kepler's Elliptical orbits to be maintained and that this force would be proportional to the radius squared. This work would go on to be published in the Principia - where the three universal laws of motion where introduced. Interestingly, the name of the force was coined as a result of the uses of Latin in all scientific papers published at the time as 'gravitas' is the translation for 'weighted'. This would go on to be the invisible force that act over the long distance of space - gravity. This work was by far his biggest yet - gaining recognition across Europe and the main reason for his knighthood.
But just like all physicists - there is a lot more to someone then just the science they studied. He was also a theologian, with a strong yet unorthodox belief in Christianity, rejecting the Church Of England and its belief in the holy trinity. He also dedicated most of his time to alchemy - with over two thirds of his publish work dedicated to the subject. In this latter life he served as warden and protector of the royal mint, a role that would put him charge of prosecution forgery. He died on the 31st of March 1727 and is now buried at Westminster abbey.
A true polymath, Sir Issacs Newtons affect on the modern world is monumental. While he may have claimed that he only stood on the shoulders of giants, it is fair to say that the last 300 years of scientists have, in one way or another, stood on his.