Why Care about Photons?

Photons (a term coined by Gilbert Lewis in 1926) carry the sun's energy to earth. But what are they, how do they work, and why should we care about them?



Photoelectric Effect

The main reason we care about photons in this web site is because of the photoelectric effect, which was first discovered in 1887 by Heinrich Hertz. In the photoelectric effect, it was observed that when visible light was aimed at (certain metals and non-metallic solids, liquids or gases) electrons were emitted.

The modern concept of the photon, namely that it had the dual characteristics of waves and particles, and thus could carry energy was developed gradually by Albert Einstein.

As the energy from electromagnetic radiation of very short wavelength, such as visible or ultraviolet light strikes certain materials electrons emitted in this manner may be referred to as "photoelectrons".

So What is Electromagnetic Radiation?

Electromagnetic radiation can be described in terms of a stream of photons, which are massless particles each traveling in a wave-like pattern and moving at the speed of light and containing a certain amount (or bundle) of energy. All electromagnetic radiation consists of them.

One primary difference between the various types of electromagnetic radiation is the amount of energy they contain. Radio waves have low energies, microwaves have a little more energy than radio waves, infrared has still more, then visible, ultraviolet, X-rays, and ... the most energetic of all ... are gamma-rays. Visible light is required for the photoelectric effect - and thus for solar cells to produce electricity.

The electromagnetic spectrum can be expressed in terms of energy, wavelength, or frequency. Each way of thinking about the EM spectrum is related to the others in a precise mathematical way.

Photovoltaic Cells

The solar cells you see on calculators, rooftops and satellites are also called photovoltaic (PV) cells, which as the name implies (photo meaning "light" and voltaic meaning "electricity"), convert sunlight directly into electricity.

Photovoltaic cells are made of special materials called semiconductors such as silicon, which is currently used most commonly. Basically, when the electromagnetic energy contained in visible light strikes the cell, a certain portion of it is absorbed within the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy knocks electrons loose, allowing them to flow freely.

PV cells also all have one or more electric field that acts to force electrons freed by light absorption to flow in a certain direction. This flow of electrons is a current, and by placing metal contacts on the top and bottom of the PV cell, we can draw that current off for external use, say, to power a calculator. This current, together with the cell's voltage (which is a result of its built-in electric field or fields), defines the power (or wattage) that the solar cell can produce.

The electricity produced from the visible light hitting the solar cells is collected from the various solar cells, panels / modules and is then converted from direct current to alternating current through the use of an inverter.

The following video demonstrates how the process works:

Source: US Department of Energy Photovoltaics Program

Fun Photon Facts

The photon is an elementary particle, despite the fact it has no mass. It also has the properties of waves and particles.

It cannot decay on its own, although the energy can transfer (or be created) upon interaction with other particles. They are electrically neutral and are one of the rare particles that are identical to their antiparticle, the antiphoton.

They are spin-1 particles (making them bosons), with a spin axis that is parallel to the direction of travel (either forward or backward, depending on whether it's a "left-hand" or "right-hand" photon). This feature is what allows for polarization of light.

The photon concept has led to momentous advances in experimental and theoretical physics, such as lasers, Bose–Einstein condensation, quantum field theory, and the probabilistic interpretation of quantum mechanics. It has been applied to photochemistry, high-resolution microscopy, and measurements of molecular distances.

Recently, they have been studied as elements of quantum computers and for sophisticated applications in optical communication such as quantum cryptography.

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