This lesson explores how scientists established that light travels at a finite, measurable speed. Early measurements used astronomical observations, while later experiments used equipment on Earth.
In 1675, Danish astronomer Ole Roemer studied Io, one of Jupiter’s large moons. Io repeatedly passes into Jupiter’s shadow, producing eclipses that can be timed from Earth. Roemer found that the interval between observed eclipses varied by a few seconds depending on Earth’s position in its orbit.
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How did Ole Roemer infer that light has a finite speed?
He compared the observed timing of Io’s eclipses as Earth’s distance from Jupiter changed; the varying travel time of the light explained the timing differences.
How does the index of refraction relate to light’s speed?
The index is the ratio of vacuum speed to material speed: n=c/v, so v=c/n.
What does a larger index of refraction imply about light’s speed?
For a given vacuum speed, a larger index means a lower speed in the material.
What does a light ray represent?
A straight-line model of light’s path and direction when wave effects can be neglected.
What is geometric optics?
The study of light paths using rays, geometry, and trigonometry.
Quiz questions
In a material with refractive index n=1.50, what is the speed of light if c=3.00×108m/s?Answer: 2.00×108m/s
Which statement describes the refractive index of gases at ordinary conditions?Answer: Their indices are usually very close to 1.
When is the ray model generally useful for describing light?Answer: When the objects involved are much larger than light’s wavelength.
What does refraction describe?Answer: A change in light’s direction as it passes between materials.
How does the speed of light in a vacuum compare with its speed in ordinary matter?Answer: It is greater than or equal to its speed in the matter.
Based on University Physics Volume 3 by Samuel J. Ling, Jeff Sanny, William Moebs (OpenStax), licensed CC BY 4.0. Adapted into a summary, flashcards and quizzes.