Important Experiments in Physics for UPSC Optional Subject:

1. Michelson-Morley Experiment:
This experiment was conducted by Albert A. Michelson and Edward W. Morley in 1887 to detect the existence of the luminiferous ether, which was believed to be the medium through which light waves propagate. The outcome of this experiment was that it failed to detect any motion of Earth through the ether, leading to the rejection of the ether theory and providing support for Albert Einstein's theory of special relativity.

2. Millikan's Oil Drop Experiment:
Conducted by Robert A. Millikan in 1909, this experiment determined the charge of an electron and its mass. By observing the motion of charged oil droplets in an electric field, Millikan measured the charge of individual droplets and found that they were all multiples of a fundamental unit, which he identified as the charge of an electron. This experiment provided direct evidence for the quantization of electric charge and contributed to the development of atomic theory.

3. Rutherford's Gold Foil Experiment:
Ernest Rutherford conducted this experiment in 1911 to investigate the structure of the atom. He bombarded a thin gold foil with alpha particles and observed their scattering patterns. The unexpected result was that some alpha particles were deflected at large angles, while most passed straight through. Rutherford concluded that the atom has a small, dense, and positively charged nucleus, with electrons orbiting around it. This experiment laid the foundation for the nuclear model of the atom.

4. Photoelectric Effect Experiment:
The photoelectric effect experiment conducted by Heinrich Hertz, Philipp Lenard, and Albert Einstein demonstrated that light can behave as both a wave and a particle. It showed that when light shines on a metal surface, electrons are emitted only if the light has a minimum frequency. This experiment supported the concept of quantized energy and the particle-like behavior of light, leading to the development of quantum mechanics.

5. Davisson-Germer Experiment:
This experiment conducted by Clinton Davisson and Lester Germer in 1927 demonstrated the wave-like nature of electrons. They observed the diffraction of electrons by a crystal lattice, similar to how light waves diffract when passing through a narrow slit. This experiment provided further evidence for the wave-particle duality of matter and contributed to the development of quantum mechanics.

These experiments are important in the field of physics as they have significantly contributed to the understanding of fundamental concepts and theories. Familiarity with these experiments and their outcomes is crucial for the UPSC Physics optional subject, as they demonstrate the scientific method, experimental techniques, and the evolution of scientific knowledge.