What is Light? How To Measure the Speed of Light?

Light is part of the electromagnetic spectrum, a wave and a particle. Let's look at what light is and how it behaves and the speed of light.

What is Light?

Physically, light (including X-rays, ultraviolet, visible light, infrared, and radio) is an electromagnetic wave and always appears in the form of finite quanta, photons. But what do these terms mean?

In an atom, there are a certain number of electrons, protons and neutrons. The protons and neutrons exist together in the nucleus, while the electrons "orbit" the nucleus at various distances. These distances are called electron levels or quantum states and are typically referenced as n. This is a simplified explanation of the Bohr model.

When an atom is excited, electrons jump from one level to a higher level, and each transition of an electron corresponds to the emission of one photon from the atom. This photon forms an electromagnetic wave.

The energy, E, of a photon, is given by the following formula.

Equation 46 - The energy of a photon

Where h is Planck's constant: 6.63x10^-34 J s, v is the frequency of the wave, c is the speed of light and λ is the wavelength of the electromagnetic wave.

The emitted photon has the same energy as the energy difference between the two quantum states in the atom.

When atoms are excited, electrons move from a lower to a higher quantum state releasing a photon as they do. For every electron in an atom, there is a certain threshold energy, such that if this energy is transferred to the electron, it can leave the atom. The remaining atom is positively charged and called a positive ion. This process is known as ionisation. Every further electron removed from the atom increases the level of ionisation of this atom. If a neutral atom gains an additional electron it becomes a negative ion and if it loses an electrum it becomes a positive ion.

If the electromagnetic wave loses energy, its wavelength becomes longer and the frequency lower. The wavelength may be altered in many circumstances. If the wavelength increases, we say it is redshifted; if it decreases, we say it is blue-shifted. Redshift and Blueshift is particularly useful for determining the speed and acceleration of distant objects.

Light is Waves and Particles?

Wave-particle duality is the concept that the electrons in the electromagnetic wave are both particles and waves at the same time, although paradoxically when measured it is only one of the two.

The phenomenon of diffraction is a well-known property of light waves. At the beginning of the 20th century, a problem was found with the theories of light waves emitted from hot objects, such as light from the sun. This light is called black-body radiation. These theories would always predict infinite energy for the light emitted beyond the blue end of the spectrum, and this is a contradiction of the principles of conservation of energy. Clearly, a new model for the behaviour of black bodies was needed.

The answer was to assume the energy of light waves was not continuous but came in fixed amounts as if it was composed of a large number of particles, or photons.

The strange thing about the diffraction experiment is that the electron wave doesn't deposit energy evenly over the entire surface of the detector, as you might expect with a sea wave crashing on the shore. Instead, the energy of the electron is deposited in points, just as if it was a particle. So while the electron propagates through space like a wave, it interacts at a point like a particle. This is known as wave-particle duality.

How To Measure the Speed of Light

Attempts to measure the speed of light have played an important part in the development of the theory of special relativity, and, indeed, the speed of light is central to the theory.

The idea of measuring the speed of light was first proposed in 1629 by Isaac Beeckman. He proposed an experiment when he observed the flash of a cannon about one mile away and heard the bang sometime later. Some years later, In 1638, Galileo Galilei devised an experiment to measure the speed of light by observing the delay between uncovering a lantern and its perception a distance away. While he was unable to distinguish whether light travel was instantaneous or not, he did conclude that it must be extraordinarily rapid.

The first quantitative estimate of the speed of light was made in 1676 by Romer. He did this from observations that the periods of Jupiter's innermost moon, Io, appeared to be shorter when the Earth was approaching Jupiter than when receding from it. He concluded that light must travel at a finite speed, and estimated that it takes light 22 minutes to cross the diameter of Earth's orbit. Christiaan Huygens combined this estimate with an estimate for the diameter of the Earth's orbit to obtain an estimate of the speed of light of 220,000 km/s, a value 26% lower than the modern measured value.

Throughout the 19th century, several other experiments were devised to refine this value until in 1862, Léon Foucault improving on the works of Hippolyte Fizeau, came to a value of 298,000 km/s. The resulting Fizeau-Foucault apparatus involves a rotating mirror which reflects light onto a distant mirror. This distant mirror, in turn, reflects light toward the viewer. If mirror the mirror is stationary, the slit image will reform at the viewer regardless of the mirror's angle. As the mirror rotates, the angle will have moved slightly in the time it takes for the light to bounce from the slit to the mirror and back, so the light will be deflected away from the source by a small angle. The speed of light could be calculated based on the timing of the mirror and the angle the light is reflected.

In Foucault's experiment, lens L forms an image of slit S at spherical mirror M. If mirror R is stationary, the reflected image of the slit reforms at the original position of slit S regardless of how R is tilted, as shown in the lower annotated figure. However, if R rotates rapidly, the time delay due to the finite speed of light travelling from R to M and back to R results in the reflected image of the slit at S becoming displaced.

Since then, the speed of light has been accurately measured at a fixed value of 299,792,458 metres per second in a vacuum.

Upper Limit on the Speed of Light

The Special theory of relativity has many counterintuitive, yet experimentally verified implications including placing an upper limit on the velocity of the speed of light. You see, the energy of an object with rest mass m and speed v is given by γmc2, where γ is the Lorentz factor. The Lorentz factor is the factor by which time, length, and relativistic mass change for an object while that object is moving.

Equation 48 - Lorentz factor

When v is zero, γ is equal to one, giving rise to the famous E = mc² formula for mass-energy equivalence.

The γ factor approaches infinity as v approaches c, and it would take an infinite amount of energy to accelerate an object with mass to the speed of light, thus the speed of light is the upper limit for the speeds of objects with positive rest mass. Since mass is linked to time, as mass approaches infinity so time slows down, therefore an object travelling close to the speed of light would require infinite energy, have infinite mass and time would stop for the object.