History of understanding "light"
Waves in the sea propagate through water as their medium, and sound propagates through air as its medium.
The physicians in 17th to 19th centuries believed in light medium and called it "ether".
- Romer(1644-1710) first measured and induced velocity of light by observing the Jupiter's satellite Io and applying optical Doppler effect to these observations.
- In 17th century, Newton believed that ether as a medium will exist in air and vacuum and that light will allow ether to vibrate so as to propagate through solid material.
- In 18th century, Euler famous for his mathematics believed that ether will transmit magnetic force, electric force and gravity as well as light and heat.
- Around in 1850, Fizeau made a measuring device with gears to solve velocity of light.
- Maxwell (1831-1879) introduced an electromagnetic wave theory saying "light is a wave made up of electric and magnetic waves each rolling with respect to its propagating direction".
- In 1887, Michelson and Morley carried out their experiment to observe ether drift in vain. Nonetheless, they found that velocity of light is always constant irrespective of direction.
- In 1905, Einstein found out "photoelectric effect" leading to his discovery "light
is both wave and particle".
Now that light itself is particle, it became no use assuming ether as a medium transmitting light.