Meghnad Saha is known for his contribution in the field of Physics. He was an Indian astrophysicist who made significant contributions to the field of astrophysics and spectroscopy. Saha is best known for his concept of "Saha ionization equation," which is used to understand the behavior of ionized gases in stellar atmospheres.

His equation played a crucial role in explaining the spectral lines observed in the light emitted by stars and understanding the process of ionization in stellar atmospheres. Saha`s work laid the foundation for our understanding of the physical and chemical properties of stars, and his research continues to be influential in the field of astrophysics.

It is important to note that while Meghnad Saha is recognized for his contributions in Physics, he did not make significant contributions to the other fields mentioned in the options (Medical Science, Environmental Science, and History).