Light is the common name for electromagnetic radiation with wavelengths of 400-750 nm. This term is sometimes expanded to include ultraviolet and infrared radiation. The human eye is capable of detecting light, which distinguishes it from other regions of the electromagnetic spectrum (despite having shared properties with light, most notably speed).

Both the electric field component and the magnetic field component of electromagnetic radiation travel in space, much like a wave travels in water. In fact, these two components are perpendicular to each other, and are both also at a right angle to the direction of propagation of the wave (e.g. if the wave travels along a z-axis, then the electric field might be on the x-axis and the magnetic field on the y-axis). The speed of travel of the wave is constant, equal to about 300,000 kilometers per second in vacuum. According to Einstein's theory of relativity, this speed is unique in that it is not affected by the speed of the observer.

Light obviously has a crucial role in our lives; allowing us to see, delivering us the sun's radiation, warming the surface of the Earth, and more. Light is characterized by several very interesting properties, the observation of which has led to the development of the most prominent theories in contemporary physics – the theory of relativity and quantum mechanics. In this article I will briefly review some of these properties – the duality of light, light polarization and the interaction between light and matter.

The duality of light

The concept of the duality of matter stands at the heart of quantum theory, and it is well established today that matter can possess properties of both waves and particles. With respect to light, this has been recognized for many years, and even during the time of Newton it was realized that light can be described using either linear optics (like a beam of particles), and also using wave-related phenomena, such as interference and diffraction.

Today we can describe light at the macroscopic level using Maxwell's wave equations which describes the propagation of the electromagnetic field in space, while at the microscopic level we know that light is composed of energy quanta in the form of photons.

Each photon's energy is determined solely by its frequency/wavelength (i.e. color). This was first speculated by Max Planck in his theoretical studies regarding the electromagnetic spectrum emitted by a "black body", thus laying the most important cornerstone to the development of quantum mechanics.

The polarity of light

Another interesting property, which can also be associated with electrons (as I will soon explain), is the polarity of light.

As mentioned earlier, light consists of electric field and magnetic field components, both propagating in space in the form of a wave. Let us distinguish between two different situations. In the first one, the direction of the electric field (and hence of the magnetic field) is constant. This is called linear polarity. In the second situation, termed circular polarity, the electric field rotates constantly around the axis along which the light wave propagates. In this case, if the wave travels along the -axis, the electric field rotates continuously between the x- and y- axes.

This feature of light can be exploited in the making of polarizers, which are devices that block out light waves of a specific linear polarity. The light coming from the sun or from incandescent light bulbs is polarized in a completely random manner. By passing it through a polarizer, one can reduce its intensity (as is done in sunglasses) and obtain polarized light.

Polarity can also be discussed with respect to quantum theory. Since photons possess a spin that can take on one of two possible values (one or negative one), we can make a polarizer that would completely block photons with a spin of negative one and let through only photons with a spin of one. In fact, this property of photons is utilized today as a means to encrypt information. This type of encryption is extremely difficult to decipher without being discovered.

This type of quantum polarity can be described for electric current much like it is described for light. Like photons, electrons also possess a spin, which can take on a value of either half or negative half. Hence, one can think of a similar use of electron spins in order to produce a polarized electric current. However, the implementation of this is much more complicated.

The interaction of light and matter

One of the most interesting questions in the field is how light and matter interact with each other. For example, why are some materials transparent while others are not? Can light change the properties of materials? And so on.

Following the suggestion that light is composed of energy quanta in the form of photons, it was Albert Einstein who proposed a mechanism by which photons that hit a material can be absorbed by it and excite its electrons to a higher energy state, but only if the photons possess the required amount of energy. If not, no matter how many photons hit the material, they would not be able to excite the electrons and be absorbed. It was this photoelectric effect that endowed Einstein with a Nobel Prize in Physics in 1921. When this effect was later demonstrated experimentally, it provided a practical proof for the very existence of photons.

The photoelectric effect was an important milestone in our understanding of the interaction between light and matter. A photon can be absorbed only if it can transfer all its energy to the material, exciting its electrons from their ground state to a higher energy level. In doing so the photon is absorbed in the material, which is then said to be opaque to light of the color of the photon. If the photon does not get absorbed, it simply passes through the material. 

Since a photon excites electrons when it gets absorbed, it can change some of the properties of the material. For example, a photon can transfer electrons between different energy levels, thus transforming the material from a non-conductive to a conductive state. This phenomenon is exploited in devices such as night vision instruments, photoelectric cells and more.

Einstein's suggestion that photons get absorbed and emitted when electrons are moving between different energy levels led to the realization that this process can be controlled and exploited to produce a unique light beam, the laser. This invention brought about a true technological revolution.

Yaron Gross
Department of Condensed Matter Physics
Weizmann Institute of Science


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