When the Great Plague of London ravaged through the British city beginning in 1665, Issac Newton was a student at Trinity College, Cambridge. As described in Gale Christianson's Isaac Newton, a few months after acquiring his undergraduate degree in the spring of that year, the 23-year-old retreated to his family farm of Woolsthorpe Manor, some 60 miles northwest of Cambridge. Along with being located a safe distance from the carriers of the horrific disease that was wiping out the population of the city, Woolsthorpe provided the sort of quiet, serene environment that allowed a mind like Newton's to journey, uninterrupted, to the farthest reaches of the imagination. This period is now known as annus mirabilis – the "year of wonders."
Newton helped develop calculus
First, he continued the work on mathematics that had engaged his mental acuities until being shut out of Trinity. The issue at hand was determining universal equations involving fluctuating quantities, an issue that had been tackled, on a limited scale, by the French mathematicians René Descartes and Pierre de Fermat.
By the end of 1666, Newton had effectively solved this problem with a series of papers on the rules of "fluxions," now known as calculus.
He analyzed color, light and the spectrum
Newton also turned his attention to the study of optics, and the prevailing wisdom that every color on the spectrum was a mix of dark and white light. He conducted an experiment in which he drilled a tiny hole in the shutter of his bedroom window, intercepted the ensuing light beam with a prism, and then placed a second prism in the path of those refracted beams.
The resulting panorama allowed Newton to calculate the angle of each refracted color. More importantly, it revealed the stream of colors as unchanged – proof that colors were not modifications of white light, but that white light is comprised of all components of the spectrum.
Newton studied gravity, which aided in the creation of his laws of motion
Finally, this was the period that birthed the Newtonian legend of the falling apple and the thump on the head that led to the deduction of gravity. Things didn't exactly unfold in that manner, but Newton did get to thinking about the principles of inertia and how an airborne apple, or any object, is prevented from flying off the rotating Earth into space.
The force that pulls the apple down must be the same one that pulls the moon to the Earth, he decided. Furthermore, the moon must also apply that same attracting force toward the Earth, albeit on a lesser scale. This led to the law of universal gravitation, which holds that those forces are proportional to the product of their masses and inversely proportional to the square of the distance between them.
He didn't quite get his calculations to work out at the time – he was more successful in this endeavor years later, before the 1687 publication of his fame-cementing Principia.
Meanwhile, the deadly plague abated by spring 1667, paving the way for Newton to return to Cambridge and demonstrate that the unexpected changes to his lifestyle during those dark days of England would, in turn, change the rest of the world forever.