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The search for the speed of light has persisted across centuries, providing a vital part of the field of classical and modern physics that is taught and understood today. The speed of light is now determined as 299,792,458 m/s and serves as a universal constant that is applied across various fields of sciences. Throughout the course of modern history, there have been scholars of various eras that contributed to this magnificent search, each bringing breakthroughs of different extent.
Galileo Galilei (1638)
Galileo conducted one of the earliest experiments to determine the speed of light. This required 2 people, Galileo and his assistant, to be standing on top of 2 different hills each holding a lantern. Galileo would then first unveil his lantern, while at the same time start off a timer, revealing the light released from his lantern to be detected by his assistant on the other side. As soon as his assistant detected the light, the assistant would then also reveal his lantern, which Galileo would detect and stop the timer. This experiment aims to measure the speed of light through the distance required for light to travel back and forth to Galileo with a recorded time travelled. Given by the equation 2d/t where:
d = distance between Galileo and his assistant
t = time recorded by Galileo.
This experiment, however, holds many flaws, as human reaction and the process of unveiling the lantern all possess human errors.
Ole Romer (1676)
Ole Romer, a Danish astronomer, approached his way of measuring speed with periodic movements of Jupiter's moon “Io”. The periodic movement was found to be changing based on the position of the Earth relative to the moon. Through calculations, Ole Romer determined the speed to be approximately 220,000km/s (or 220,000,000m/s), which is still quite inaccurate compared to the current constant.
James Bradley (1728)
Somewhat similar to Ole Romer, James Bradley focuses on stellar aberration to determine the speed of light. Stellar aberration refers to the relative change in motion of stars relative to Earth’s position. James Bradley determined the speed of light to be approximately 308,000 km/s (or 308,000,000m/s).
Hippolyte Fizeau (1851)
Fizeau developed a mechanism requiring a light source to be reflected through an angled mirror, which passes through a rotating cogwheel that rotates at different frequencies. Another mirror is then placed 8km away to reflect back to the cogwheel and then to an observer. The light ray produced by the light source is placed directly to pass through the teeths of the cog wheel. This allowed Fizeau to determine that when the cogwheel spins at specific frequencies, the light that passes through the tooth of a cogwheel will be blocked off by its rotated adjacent tooth when reflected back. With the frequencies determined, Fizeau measured the speed of light to be around 314,000 km/s (or 314,000,000m/s).
Leon Foucault (1862)
Leon Foucalt similarly utilised light’s properties of reflection. By reflecting light through a rotating mirror and measuring the time taken for the light to reflect back to the observer with the changed angle from rotation, he determined the speed of light to be approximately 298,000 km/s (or 298,000,000m/s).
Louis Essen (1950)
Essen utilised a cavity resonator to determine the speed of light to be about 299,792,500 m/s. A cavity resonator provides constructive and destructive interference of waves, which provides standing waves that could be measured with known frequencies and wavelengths.
More Modern Methods
As technology improves over time, better calibrated and precise technologies are able to pave the way for scientists and scholars to find a more accurate measure of the speed of light. Such includes and are not limited to:
Interferometer
Oscilloscopes
LIGO gravitational wave detectors
Time of Flight experiments
Reference list
Impey, C. (n.d.). Have we made an object that could travel 1% the speed of light? [online] The Conversation. Available at: https://theconversation.com/have-we-made-an-object-that-could-travel-1-the-speed-of-light-170849.
lco.global. (n.d.). The Speed of Light | Las Cumbres Observatory. [online] Available at: https://lco.global/spacebook/light/speed-light/.
Lecture 21 Cavity Resonators. (n.d.). Available at: https://engineering.purdue.edu/wcchew/ece604f20/Lecture%20Notes/Lect21.pdf.
Soter, S. and deGrasse Tyson, N. (2019). Ole Roemer Profile: First to Measure the Speed of Light | AMNH. [online] American Museum of Natural History. Available at: https://www.amnh.org/learn-teach/curriculum-collections/cosmic-horizons-book/ole-roemer-speed-of-light.
Wikipedia. (2021). Fizeau experiment. [online] Available at: https://en.wikipedia.org/wiki/Fizeau_experiment.
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