By Professor Don Lincoln, Ph.D., Fermi National Accelerator Laboratory (Fermilab)
We have always accepted the statement that the speed of light is a constant. But is this assumption really valid under all circumstances. Does the speed of light vary when it travels through mediums such as water, glass or plastic? If so, why does the speed vary and what is the principle behind this variation?
What Is Light?
In common parlance, light is something that comes out of a flashlight or brightens the sky on a summer morning. In physics, light refers to the electromagnetic field of any wavelength traveling through space, either visible or invisible to the eye. Different types of radiations such as radio waves, microwaves, infrared waves, ultraviolet rays, x-rays and gamma rays are also considered as light. Within the range of visible light, the ROYGBIV spectrum of red, orange, yellow, green, blue, indigo, and violet colors are also electromagnetic waves traveling at the speed of light. Thus, light is an electric field that is changing both in position and in time. For a major part, this article will assume light as a wave most of the time.
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Measuring the Speed of Light
Light travels at an incredible speed of 1,86,000 miles per second or 2,99,792,458 meters per second in vacuum. This means that light travels so fast that it could circle the Earth seven and half times in one second.
In 1676, Danish astronomer Olaf Rømer calculated the orbits of the moons of Jupiter. He noticed the variations in the eclipse times of Jupiter’s moons depending on the distance of Jupiter from the Earth in their orbits. He calculated the speed of light as 2,14,000,000 meters per second or about two-thirds of the modern value. Quite a feat about 350 years ago!
The first measurement of the speed of light on Earth was by French physicist Armand Fizeau. Using a spinning gear, bright light, and a mirror located about five miles away, he estimated the speed of light to an accuracy of about 5% at 3,15,000,000 meters per second. Further experiments by physicists nudged scientists closer to accuracy in measurement of light. For instance, Frenchman Léon Foucault, who altered Fizeau experiment using rotating mirrors and achieved an accuracy of less than a percent.
In the late 1860s, physicist James Clerk Maxwell introduced the famous Maxwell’s equations to calculate the speed of light. These equations provided a sufficiently accurate measurement of the speed of light in vacuum.
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Different Speeds of Light
It is also important to emphasize that light traveled at different speeds in different objects. The speed of light is highest in vacuum and it slows down when it passes through transparent materials such as glass, water, and plastic. The bending of a light ray as it passes from a transparent medium to another is called refraction. The path of light bends when it enters a transparent medium due to the index of refraction and is easily visible to the eye.
The four basic properties of light frequency of oscillation, wavelength (color), velocity, and amplitude (brightness) aid in the understanding of the mechanism. Frequency refers to the number of oscillations per second while amplitude is the strength of the electric field. Since light exists both inside and outside the medium, the frequency of light will also be the same. If the velocity slows down, it means that the waves move a smaller amount for every period of time. Since there has to be the same number of oscillations per second, it means that the wavelength of light gets shorter inside the glass or water. Thus, when you see the light bending, it is an evidence of its slowing down in water. There are several explanations to this mechanism, but the one in classical physics is close to accuracy.
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What Slows Down the Speed of Light?
Classical physics assumes light as an electric field changing both in position and in time. Light inside an object is a mix of electric fields, including electric fields originating in the material itself. All materials are made up of electrons or moving particles with electric charge, which in turn cause the electric fields to move. There are now two electric fields, one of the electrons and the other of the beam of light. When these two electric fields move at a different velocity within the object, the combined speed of the mix of electric fields is lesser than the original speed of light. This is the fundamental reason why light moves more slowly in material than in a vacuum contrary to the common misconception that the photons in the light are absorbed, re-emitted and hence responsible for slowing down of the speed. The energy levels of atoms and electrons are radically different in extended solids and liquids. The photons traveling through transparent matter do not hit individual atoms or electrons but the electric field of the photon moves and changes the energy of many electrons. So, it can be concluded that light travels slower in transparent materials than it does in vacuum.
Common Questions about Speed of Light
The definition of a meter is the distance traveled by light in 1/ 299,792,458th of a second.
Armand Fizeau used a spinning gear, a bright light, and a mirror located about 5 miles away to measure the speed of light. He shined the light at a distant mirror, from where it was reflected back to him through the gears that spun.
The velocity is about two-thirds the speed of light in glass and plastic, while in water it is about three-fourths the speed of light. However, light travels at just 40% of its normal speed in diamond.
It is a common misconception that the photons in the light jump from atom to atom, electron to electron to get absorbed. It is assumed that there is a tiny fraction of time involved before it travels from one atom to another and hence the decrease in effective speed of light. However, this idea of light being absorbed, waiting for a bit and getting re-emitted is incorrect.