Overview:
For the UPSC Physics optional subject, it is important to be familiar with significant scientific experiments and inventions. Here are a few key experiments and inventions that you should know:

1. Michelson-Morley Experiment:
The Michelson-Morley experiment aimed to detect the existence of the luminiferous aether, a hypothetical medium through which light waves were believed to propagate. It involved an interferometer that split a beam of light into two perpendicular paths to measure the interference pattern. The experiment's null result led to the development of Einstein's theory of special relativity.

2. Rutherford's Gold Foil Experiment:
Ernest Rutherford's experiment involved bombarding a thin gold foil with alpha particles. The unexpected results showed that most of the alpha particles passed through the foil, while a small fraction experienced large deflections. This led to the conclusion that atoms have a small, dense, and positively charged nucleus, thus revolutionizing the understanding of atomic structure.

3. Millikan's Oil Drop Experiment:
Robert Millikan's experiment determined the charge of an electron using tiny oil droplets suspended in an electric field. By measuring the droplets' motion, he was able to calculate the charge on each droplet and determine the charge of an electron, contributing to the understanding of atomic structure.

4. Hertz's Experiment on Electromagnetic Waves:
Heinrich Hertz's experiments confirmed the existence and properties of electromagnetic waves. He generated and detected electromagnetic waves using a spark gap oscillator and a receiver circuit. Hertz's experiments provided experimental evidence for James Clerk Maxwell's theory of electromagnetism.

5. Photoelectric Effect:
The photoelectric effect experiment demonstrated that light can exhibit particle-like behavior. Albert Einstein explained this phenomenon by proposing that light is composed of particles called photons. The experiment involved shining light on a metal surface and observing the emission of electrons.

6. Invention of the Transistor:
The invention of the transistor by John Bardeen, Walter Brattain, and William Shockley revolutionized electronics. The transistor is a semiconductor device that amplifies or switches electronic signals. It paved the way for the development of modern computers, communication systems, and electronic devices.

7. Discovery of the Higgs Boson:
The discovery of the Higgs boson at the Large Hadron Collider (LHC) was a significant achievement in particle physics. The Higgs boson is associated with the Higgs field, which gives particles mass. Its discovery confirmed the existence of the Higgs mechanism and completed the Standard Model of particle physics.

Conclusion:
Familiarity with these experiments and inventions is crucial for the UPSC Physics optional subject. These experiments have played significant roles in shaping our understanding of the physical world and have paved the way for scientific advancements.