A great way to learn about light is to know the ideas and experiments that have boost our knowledge and deeper understanding on this outstanding phenomena over the centuries. Moreover, everyone of us, with little effort and motivation to be engaged in scientific experimentation can discover the magic side of light choosing to be part in experiential activities able to underline what we can call the specific historical context that is connected with the concept we are interested to learn and able to quickly explore. This post aim to provide a little historical context for some important concepts related to light, along with links to fun and experiential activities.
Ancient Ideas about Light
In 1750, the American scientist Benjamin Franklin wrote, “About light, I am in the dark.” Yet, all of us know that different theories and ideas about light have existed since antiquity. So it does not surprise us too much to remember that at the time we were pupils at junior secondary school, several teachers underlined a sensitive relationship between one name, Democritus, and its theory, which in fact introduce us to the atomic world. The Greek philosopher stated that “The Universe is composed of two elements: the atoms and the void, where they move” without the possibility to be destroyed, differ in size or shape and temperature. As every tiny atom is identical and our fallible senses aren’t able to see it, is pretty simple to infer that in this framework even the most advanced microscope that we can extol today (i.e. the AFM or the STM) couldn’t allow to learn its features.
Being invisible was a fact, despite any relevant philosophical adjectival phrase.
Later on, in 55 BC, it was the Roman poet and philosopher Lucretius Carus who wrote, “The light and heat of the sun; these are composed of minute atoms which, when they are shoved off, lose no time in shooting right across the inter space of air in the direction imparted by the shove.” Around 1000 CE the Arab scholar Ibn al-Haytham, also known as Al-Alhazen (965–1040 CE), published his seven-volume Book of Optics. His ideas about light and color, perception, and our visual system (based on multiple experiments and accurate, meticulous observations) were enormously influenced on later scientific developments in Europe. Here are some of his better ideas:
- Light travels in straight lines.
- Light speed is fast but not infinite.
- Light refracts when it enters water because it changes speed.
- Light is made of particles from the sun or fire, which bounce off things and enter the eye.
- Afterimages show that light comes into the eye and not out of it.
Years passed, and scientists tried to move a bit further the knowledge of their time crowned by practical science, as a courageous attempt to change the world.
Thus, the triumph of science was connected to its practical utility, technique was the standard approach to overcome the “doctrine” who pretend to explain the nature of the world. In fact, technique conferred a sense of power: man is now much less at the mercy of his environment than he was in former times. But who was enraptured by its “power” know that co-operation was needed to reach a common direction in scientific discovery, “protecting” its meaning.
Differently from any type of credo or religion, science is ethically neutral: it assures men that they have the right and the intelligence to perform wonders, avoiding a sort of normative laws able to tell how and why pursue “a set of allowed wonders” to perform. Thus, the power impulse has a scope which it never had before.
A first resounding event was when the Galileo tried to measure the speed of light in 1638. He stationed two people with covered lamps on two different hilltops. One person uncovered his lamp and when the other person saw the light he uncovered his lamp. Then, the first person noted how much time had elapsed. Of course, Galileo knew about human reaction time. So he repeated the experiment with one change to separate out the time it took light to travel between the people taking care of this biological limitation.
Galileo concluded that light traveled so fast he could not measure it. Today, the speed of light is defined as precisely 299,792,458 meters per second. The symbol for the speed of light, defined by c, comes in fact from celeritas: the Latin word for speed.
The speed of light was first measured by the danish astronomer Ole Roemer that in 1676 made the first quantitative measurements of the speed of light using a technique similar to Galileo’s. He waited the right moment, derived from having framed all the controlled variables he wanted to analyze and used an eclipse of one of the Jupiter’s moons to replace the more artificial effect of a lantern. His time imposed a certain degree of creativity and idealized solutions. From this point on, Roemer calculated the speed of light as 220,000,000 meters per second; a pretty good first measurement.
In 1704, it was the turn of Newton (1642-1727) who assert that light traveled from the sun to the Earth in seven or eight minutes. Following the way prepared by Copernicus, Kepler and Galileo, he achieved triumph. He showed that acceleration toward the Earth and the Sun explain the moon’s motion, and that acceleration of falling bodies on the Earth’s surface is again related, according to the inverse square law, to the one of the moon. The concept of force implied a change of motion, as it was its primary cause: think on acceleration. A first step towards the announcement of the laws of universal gravitation.
Every body attracts every other with a force directly proportional to the product of their masses and inversely proportional to the product of their masses and inversely proportional to the square of the distance between them.
This formula was a real success, imposing a firm of originality and sufficient authority to push over research and progress. Newton, focused and gifted, was able to explain (or at least letting scientists deduce) through demonstrations almost everything in planetary theory: the motion of the planets and their satellites, the tides and the orbit of comets.
An here we are towards the additional dilemma: wave or particle?
Since its early years of scientific research, the natural philosopher (Sir) Isaac Newton modeled light as a particle. He bought his first prism in 1666, one year after the Italian natural philosopher Francesco Grimaldi’s work on diffraction was published. Newton claimed that the “colleague” Grimaldi’s diffraction was a different kind of refraction. Regarding this particular concept, let me make a small parenthesis about it. Every professional hunter, or passionate about this wonderful sport, know that if you want to spear a fish with an harpoon, you’ll miss it if you point where the fish appears to be. Anyone can see the spear bend when it enters the water. Nowadays we frame this concept in other words: is light that allow us to see the spear “bending” as it leaves the water. In other words, the bending of light as it enters (or leaves) water is called refraction. If you are curious, one of the oldest tables of data that we could study to understand more about refraction is a Greek stone tablet, into which are both sculpted the angle of incidence and angle of refraction of light going from air to water.
The index of refraction, n, of a material is the ratio of the speed of light in a vacuum to the speed of light in a material:
- air n = 1.0003
- water n = 1.33
- diamond n = 2.54
Hoping that the concept is clearer now, let’s go back to Newton and his empirical research. He argued that the geometric nature of the laws of reflection and refraction could only be explained if light was made of particles, which he defined “corpuscles” since waves are not so accustomed to travel in straight lines. Newton introduced the term of ‘color spectrum’, which was continuous and with no distinct boundaries between the colors that were divided into seven: red, orange, yellow, green, blue, indigo, and violet. The order is indicative of their frequency that human eyes are able to recognize, from the lowest to the highest one. As we know, spectral colors are infinite, since, between the two extremes of the optical spectrum, the possible wavelength values are infinite. Then, if our eye is able to distinguish them all, well, this is all a different matter!
So, if we have the radiation of a “white” source (Sun, flames, simple desk lamps, all kind of incandescent sources, and so on and so forth along with your experience), we can decompose it in its pure colors with a laboratory instrument: the spectroscope. We can even achieve the goal by using a narrow beam from a distant source (i.e. a Sun beam coming from the tiny spaces of our house shutters) by fragmenting it with a glass prism. Newton showed that every color has a unique angle of refraction that can be calculated using a prism.
He saw that all objects appear to be the same color as the beam of colored light illuminates them, no matter how many times it should had been reflected or refracted. This led him to conclude that color is a property of the light that reflects from objects, not a property of the objects themselves. It In 1672, Newton accepted to join the Royal Society of London, claiming that the 44th trail in a series of experiments conducted could be a valid proof that light is made of particles and not waves. Yes, you read me good. Through demonstrations, papers, matured studies and empirical tests, Newton instilled a seed of a scientific revolution in one of the most recognized scientific realities of that time. Then, some years afterwards, the dutch physicists, astronomer and inventor Christian Huygens modeled light as a wave. Newton believed that the refraction of light bending into water was due to the particles of light speeding up in a direction perpendicular to the surface of the water as they entered, while Huygens argued that if light were a wave, it would decelerate its motion as it was curved.
Years passed negotiating, but without giving up the first principles that set-up the experimental conditions of their ideas. Then in 1850, a french engineer, Augustin-Jean Fresnel, designed an experiment with the help of Léon Foucault to measure the speed of light in water. What they found was that the speed of light there was slower than in air.
The experiment made by Thomas Young in 1801 (establishing the scientific proof of light interference) represent another proof of the phenomena: a light beam spread apart and overlap, and, in the area of this overlap (observing it passing through two closely set pinholes onto a screen), bands of bright light alternated with bands of darkness. Young found that the light beams spread apart and overlapped, and, in the area of overlap, bands of bright light alternated with bands of darkness. Demonstrating the interference of light, Young was able to establish that it had to be modeled as a wave.
Intensity of Light and the Inverse-Square Law
Human perception of brightness depends on the color of the light viewed, along with the intensity of the light. The parameter of the Intensity is power per m^2. If brightness is directly connected with human perception, intensity can be physically measurable.
Since light spreads out in straight lines from a point source in a vacuum and it does not lose energy as it travels in a vacuum, energy spreads out on the surface of a sphere with an area that increases proportional to the radius of the sphere squared. But let’s make it more simple, right? Since the intensity is power divided by the area of the sphere, we say that light intensity originated from a source follows an inverse-square law. A “scientific snack” can allow you to explore the concept of Inverse-Square Law and use the inverse-square law to compare the brightness of two different light sources with an Oil-Spot Photometer easily doable with your friends. In fact, this oil spot photometer is simple but a unique occasion to allow a quantitatively comparison of the output connected with any two light sources, such as an incandescent lamp, a compact fluorescent lamp or a LED lamp – all of which claim to produce the same amount of light. It is also possible to use it to calculate the intensity of the Sun, even trying few other variations.
“Oh Color, what is your sincere Nature?”
Newton gave light to Optics in 1730, declaring on its very first page that the “design in this book is not to explain the properties of light by hypotheses, but to propose and prove them by reason and experiments.”
One of the subjects Newton examined in his book was the nature of the colors of light.
In particular, after further experiments, he wrote in 1733:
The homogeneous Light and Rays which appear red, or rather make Objects appear so, I call Rubrifick or Red-making; those which make Objects appear yellow, green, blue, and violet, I call Yellow-making, Green-making, Blue-making, Violet-making, and so of the rest. And if at any time I speak of Light and Rays as colored or endued with Colors, I would be understood to speak not philosophically and properly, but grossly, and accordingly to such Conceptions as vulgar People in seeing all these Experiments would be apt to frame. For the Rays to speak properly are not colored. In them there is nothing else than a certain Power and Disposition to stir up a Sensation of this or that Color. For as Sound in a Bell or musical String, or other sounding Body, is nothing but a trembling Motion, and in the Air nothing but that Motion propagated from the Object, and in the Sensorium ‘its a Sense of that Motion under the Form of Sound; so Colors in the Object are nothing but a Disposition to reflect this or that sort of Rays more copiously than the rest; in the Rays they are nothing but their Dispositions to propagate this or that Motion into the Sensorium, and in the Sensorium they are Sensations of those Motions under the Forms of Colors.
That’s pretty hard to read, right? I’ll summarize its content: “Without a human eye and brain there is no color.” One of Newton’s classic earlier experiments was determining whether or not color is added to light passing through a prism or a component of the light itself. I’m sure you remember the experiment!
Newton used one prism to break a beam of sunlight into its component colors, using a second prism to recombine the divided colors and remake white light again: white light could be broken up into colors and could be made by adding together colors.
Light is a Wave in Electromagnetism
In 1865, James Clerk Maxwell proposed that not only was light a wave, it was a wave in electromagnetism. He knew the properties of electric and magnetic fields and that both electric fields and magnetic fields can cross a vacuum.
Explore this phenomena in a kitchen or in a classroom is pretty hard. But we can explore how electric forces can pass through air with an activity that show the behavior of an object that is electrically charged. It is sufficient to take two pieces of tape and then wiggle one (when charged) at a distance from the other, making the second tape move without touching it. This shows that the electric field can carry the force across an air gap. Yes, I’m agree with you 🙂 this experiment is pretty explanatory, as it also works across a vacuum.
In fact, when Sir Maxwell combined the equations for electric fields and magnetic fields under the set of laws that everyone remember, he created an equation for a traveling wave: able to travel at the speed of light, creating a new model for light as an electromagnetic wave.
A Pinch of Math
The most important relationship for a wave of light relates the frequency of light, f, the wavelength L, and the speed of light, c.
c = Lf
so that L = c/f and f = c/L
As you can see, wavelength and frequency of light are inversely proportional.
And here we go again: Light as a Photon
After all these interesting earlier debates about light being a particle vs. a wave the question was raised once again in the early 20th century when Albert Einstein came up with an explanation of the photoelectric effect in 1905 that once again up-ended our understanding of light.
The photoelectric effect was first observed by Heinrich Hertz in 1887, who discovered that certain frequencies of light hitting particular metals would cause electrons to be released while others would not. For example, when a high-intensity red light was shined onto a negatively charged zinc plate, the metal would hold its charge, but when replaced by a faded ultraviolet light, the plate discharged. These results did not fit with existing theories of electromagnetism…
Einstein proposed that light traveled as quanta of energy and that the energy of each quantum was proportional to the frequency of the light wave, and so inversely proportional to the wavelength. Now, I should clarify that this quanta of light energy was named a photon by Gilbert N. Lewis in 1926, it was his “linguistic privilege” having changed our vision of physics. So bear that in mind!
Einstein was prophetic to spark the field of quantum mechanics: it was him to propose that an electron could absorb all the energy from one particle of light, and there was an energy barrier that an electron had to surmount to escape a metal. One photon of red light could not transfer enough energy to an electron to escape, while one photon of ultraviolet could. Thus by 1905, we understood light to have both the properties of waves and particles: it traveled as a wave and was created or destroyed as a quanta. In fact, you could interact with a photon of light completely or not at all.
If you want to test by yourself Einstein’s experiment that proved how light waves are made up of quanti(zed) energy, recognizable as photons you can test your skills having an active role in the Photoelectricity snack.
I’m concluding now with a message to whom is trying to be sensible and empirical, starting to develop a critical sense on the possible inferences that require to do not take nothing on trust, but seeking to achieve acuteness in the analysis and the patience to follow and recognize the instructions obtained from observation and variables is the first step to learn something from scientific experience: Keep going to achieve a BRAVO!