Introduction to Research: Light

I’d like to share aspects of my research, but it requires a little scientific background to understand. For those of you whose ears went numb or eyes glazed over with that statement, do not stray away just yet! This information is intended for a general audience. I will try not to make any assumptions about reader’s knowledge and will include definitions of new words. It will be sparse with esoteric knowledge and abundant with useful information and interesting history. Most importantly, it will allow an insight into the research process, such as how long it takes for research to reach application and why scientific discovery should be highly valued. I started off with a post about atoms and now will introduce light and absorption.

  • Light ray – Narrow beam of light; an idealized model
  • Reflection – A change in direction of a light ray off of a surface
  • Refraction – A change in direction of a light ray into a surface due to light changing speed in different substances

Light sources definitely have a storied background, but the science of light also has a unique history. The ancient Greeks argued whether light rays emerged from a person’s eye and hit an object or if objects themselves produce light rays. Arab scholars parsed the optics of the eye and studied light properties like reflection and refraction. As early as the 5th century, Indian Buddhists equated light with energy, which is known to be true today.

Schematic of reflection and refraction, two basic properties of light.

Schematic of reflection and refraction, two basic properties of light.

Real example of refraction and reflection. The post in the water is not bent, but refraction makes it look that way.

Real example of refraction and reflection. The post in the water is not bent, but refraction makes it look that way.

  • Photophysics – The study of the physics of light.

In the 1600s, it was proposed that the behavior of light would be best modeled as a wave. Some scientists, including Isaac Newton, argued that light must be small particles. It wasn’t until the 1800s that there was evidence that the wave-like behavior of light might be accurate; this experiment, known as Young’s double-slit experiment, is now well-regarded by physicists as a turning point in photophysics. A beam of light shined through two tiny slits onto a screen did not form two bright spots as would be expected if light were a particle. Instead, the two slits produced a distinct pattern that indicated that light behaves like a wave.

The two-slit experiment produced interesting results. On the left would be the expected results if a beam of light was a bunch of particles. On the right is what actually happened. The light waves interacted after passing through slits, and where they overlapped, they produced discrete patterns on the screen.

The two-slit experiment produced interesting results. On the left would be the expected results if a beam of light was a bunch of particles. On the right is what actually happened. The light waves interacted after passing through slits, and where they overlapped, they produced discrete patterns on the screen.

  • Electromagnetism – Describes the interactions of electricity and magnetism.
  • Wavelength – Distance between two similar points on a wave (e.g., peak-to-peak distance)
  • Frequency – How often a wave cycles through a wavelength. The frequency and wavelength are related by the velocity (frequency*wavelength=velocity).
"Electromagnetic wave" - The blue portion is a wave of electricity, which interacts with the magnetic red portion. The red portion should be imagined as perpendicular to the blue portion and extending into and out of the screen.

“Electromagnetic wave” – The blue portion is a wave of electricity, which interacts with the magnetic red portion. The red portion should be imagined as perpendicular to the blue portion and extending into and out of the screen.

The light-wave model paved the way for the eminent James Clerk Maxwell to develop a theory of electromagnetism. Electromagnetism was fundamental for understanding how light interacts with matter. Through this theory, light gained an alternative name: electromagnetic radiation, which is a catch-all term to describe all light – even light that is not visible. Since light is a wave, it can be described by a wavelength or frequency. Visible light has wavelengths ranging from 400 nanometers to 700 nanometers with each wavelength representative of a different color, but the entire electromagnetic radiation spans from several kilometers (radio waves) to trillionths of a meter (gamma rays given off by nuclear reactions). The light we see is just a small fraction of the light waves propagating around.

The entire "electromagnetic spectrum", or the lengths of waves that describe all light.

The entire “electromagnetic spectrum”, or the lengths of waves that describe all light.

Do you see light as a particle or as a wave?

Do you see light as a particle or as a wave?

As it turns out, both sides of the arguments aligned with modern thinking. Through experimentation and observation, physicists have concluded that light is of a strange nature; it behaves as a particle and a wave. This might be counterintuitive, but if light was only modeled as a particle, then the two-slit experiment mentioned above would be paradoxically. If light was only modeled as a wave, then discrete energy levels of electrons described by the Bohr atom wouldn’t be true.

Schematic of Bohr Model

Schematic of Bohr Model

  • Subatomic – Anything that occurs within an atom

The Bohr atom model stated that electrons spin around the nucleus in discrete orbits. As previously mentioned, light interacts with matter by reflecting off of or refracting into the material (or through transmission, which is light that passes through unreflected and unrefracted). Light can also be absorbed. To jump to a higher orbit, an electron must have enough energy. Another aspect of particle-wave experiments was that the energy of light was determined to be related to its frequency (I.e., energy = (6.626*10-34 joules*seconds)*frequency). That strange, seemingly random number is called Planck’s constant and was first determined from experimental data. However, it started popping up in various places and is now defined as a subatomic-scale constant.

  • Resonant Frequency – Natural frequency where the wave response is large. A swing exhibits a resonant frequency. Someone pushing the swing will push at a certain frequency no matter how high the swing is.

The electrons rotating around the Bohr model atom can be thought of like a plucked guitar string. If you pluck a guitar string, it will vibrate at a natural frequency determined by the string’s properties. Absorption of light is similar to plucking a guitar with varying force. An electron is naturally at its lowest orbit, or vibrating at a low frequency, when a light wave comes in. The electron and light can interact. If the light has enough energy, the electron will absorb the energy to enter an “excited state”, and begin to vibrate at a high frequency. This is said to occur when the light frequency matches the electrons resonant frequency.

If the energy of light and the energy to excite an electron are mismatched, the light will reflect or refract. The light and electron still interact, but the electron begins vibrating only to quickly re-emit the light. This property is what gives objects their color. Contrary to what the ancient Greeks proposed, our eyes do not emit light rays to perceive objects, nor do objects create light rays to send to our eyes. Instead, a green object will absorb red, orange, yellow, blue, indigo, and violet, and it will reflect green to our eyes.

Absorption is extremely important for solar cells. Solar cells function by converting the sun’s energy into electrical energy. The only way this is possible is by having something that absorbs the sun’s energy. As mentioned, a single atom, as described by the Bohr atom, would only absorb at its single resonant frequency. This is a simplified case though. As it turns out, the interactions that occur between atoms in a solid, like electrons from one atom experiencing repulsion from electrons from a different atom, alters the resonant frequency. Instead of a single wavelength of absorption, solid materials likes those used in solar cells have a broad absorption band. You can compare the approximate relevant absorption band of the most commonly used solar cell material, silicon, with the spectrum of sunlight that irradiates the earth. Ideally, the more sunlight a material can absorb, the better, but there is a wide array of other important properties, too, that will be discussed later.

The radiation spectrum of the sun with the estimated absorption of silicon added in (blue lines).

The radiation spectrum of the sun with a hypothetical absorption spectrum added in (blue lines).

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