His early work was on the measurement of surface tension of liquids using oscillating fluid jets, for which he won a prize offered by the Copenhagen Academy of Sciences while still a student. He also got published by the Royal Society of London for related work.
While he was researching material for his doctoral thesis on the electron theory of metals, he came across Max Planck’s early work on quantum theory. There he read about how light was quantized into packets. After this, his work became a lot more abstract and eventually positively unworldly!
In 1912, Bohr met Ernest Rutherford, the discoverer of the atomic nucleus. Rutherford also worked on an early atomic model, for which he received the 1908 Nobel Prize. Bohr went on to make his own atomic model, under the guidance of Rutherford.
Bohr combined Rutherford’s description of the nucleus and Planck’s quantum theories into his own atomic model. For this, he received the Nobel Prize in 1922.
Bohr’s atomic model is considered one of his most significant contributions to modern physics. It shows the atom consisting of a positively charged nucleus with negatively charged electrons traveling around it in separate circular orbits with discrete radii. And that the electrons can transition between orbits by emitting or absorbing energy equal to the quanta of light.
His theoretical work was also used to help understand how nuclear fission works and was used in the first attempts to split uranium in the 1930s.
Bohr developed a concept of “complementarity” which would prove to be perhaps one of his most irrational and revealing philosophical notions. This concept held that there was seldom a single way to describe something and that often, seemingly contradictory, mutually-exclusive descriptions had to be embraced.
The most well-known and influential examples of complementarity at work is that of the particle-wave duality of light and other subatomic “particles”.
It is unfortunate that complementarity would come to lie at the heart of quantum mechanics. As we shall see, it is useful in revealing the deep irrationality of quantum mechanics and its philosophical influences.
Now, let’s look at some Bohr quotes that point to the pillars of his philosophy, the first one being, the rejection of reality and objective facts.
Bohr’s Rejection of Reality and Objectivity
“Isolated material particles are abstractions, their properties being definable and observable only through their interaction with other systems.” 
As we shall see, Bohr does not think that physical reality has any place in physics. He believes that knowledge is the process of forming relationships between abstractions, not the study of the concrete physical world.