Without light, the world would be completely dark and we wouldn’t be able to see. A luminous source is an object that emits light, such as the sun or a human made incandescent lamp. There are three types of media. Transparent media allows all light to pass through it, like glass for example. Translucent media allows some light to pass through it, like tinted glass. Opaque media allows absolutely no light to pass through it, such as a brick wall.
Light moves at the fastest known speed in the universe. It takes about eight minutes for light to get from the sun to earth and about 1.3 seconds to get from the moon to earth. Light travels in a straight path. Scientists have been trying to determine the speed of light for a long period of time. Galileo was the first scientist to attempt an experiment to determine the speed of light. In 1638, he and his partner, standing a far distance away from each other, were holding lamps covered with a piece of cloth.
Galileo will uncover his lamp and as soon as his partner saw the light he would uncover his. After the experiment, he reasoned that it was instantaneous and that light traveled ten times faster than sound, but he couldn’t determine a specific speed because the human reaction is slower. In 1675, the Danish astronomer Ole Roemer noticed, while observing Jupiter’s moons, that the times of the eclipses of the moons of Jupiter seemed to depend on the relative positions of Jupiter and earth. After doing deeper research, he concluded that light must have travelled at 200,000 km/s. In 1728, an English physicist named James Bradley estimated the speed of light in a vacuum to be around 301,000 km/s. He used stellar aberration to calculate the speed of light.
Stellar aberration causes the apparent position of stars to change due to the motion of earth around the sun. In 1862, a French physicist named Leon Foucault used a similar method to another French physicist named Louis Fizeau, but Leon’s data was more accurate. He shone a light to a rotating mirror, and then it bounced back to a remote fixed mirror and then back to the first rotating mirror. But because the first mirror was rotating, the light bounced back at an angle slightly different from the angle it initially hit the mirror with.
