Visible Light and the Electromagnetic Spectrum

We already know the basics of light: it’s electromagnetic energy, carried through space as a wave, in discrete packets called photons. But photons come in a variety of energies, and different energy photons can be used for different real-world applications. The energy of a photon determines, among other things, how quickly the electromagnetic wave oscillates. Higher energy photons oscillate more quickly than lower energy photons, so we say that high-energy photons have a higher frequency.

This frequency isn’t related to the speed that the photons travel, though. They can oscillate more or fewer times over a given distance, but still traverse that distance in the same amount of time. And as we know, the speed of light is given by Maxwell’s Equations for electromagnetism, and is constant regardless of reference frame! But another way to look at frequency is by considering the wavelength of light. Picture two photons which are traveling through space, at the same speed, but with one oscillating faster than the other. Thus one photon is high-frequency and one is low-frequency. While traversing the same distance, the high-frequency photon will oscillate more times than the low-frequency photon, so the distance covered by each cycle is smaller. We call this distance for a single cycle the wavelength, and it’s inversely proportional to the frequency. Long-wavelength photons are low-frequency, and short-wavelength photons are high frequency. Overall the range of photon frequencies is called the electromagnetic spectrum.

On Earth, photons come from an external source, often the sun, and are reflected off various objects in the world. The photons of a specific color may be absorbed, and thus not seen by an observer, which will make the absorbing object look like the other non-absorbed colors. If there are many absorbed photons or few photons to begin with, an object may just look dark. Our eyes contain molecules capable of detecting photons in the wavelength range 400-700 nanometers and passing that signal to our brains, so this is called the visible wavelength range of the electromagnetic spectrum. But it’s the interaction of photons with the world around us, and then with the sensing apparatus in our eyes, that determines what we see. Other creatures that have sensors for different frequencies of light, or who have more or less than three types of cones, may perceive the color and shape of things to be totally different. And, the visible spectrum is only a small slice of the total range of photon frequencies, as you can see in the image below!

Photons that are slightly lower energy than the visible range are called infrared, and our skin and other warm objects emit photons in the infrared. Night-vision goggles often work using infrared photons, and some kinds of snakes can see infrared. Even lower energy photons have a lot of practical uses: microwave photons can be used to heat material, and radio waves are photons with such low energy that they’re useful for long-range communication! Long wavelength photons are difficult to absorb or alter, so they’re also really useful for astronomy, for example to observe distant planets and stars.

The sun emits photons in the visible range, but it also emits a lot of photons with a slightly higher energy, called ultraviolet or UV. Sunscreen blocks UV frequency photons because they can cause biological tissue to heat up or even burn slightly, and that is sunburn! At even higher frequencies, x-rays are a type of photon that are widely used in biomedical imaging, because they can penetrate tissue and show a basic map of a person’s bones and organs without surgery. And very high energy gamma rays are photons which result from chemical processes in the nuclei of atoms, which can pass through most material. I’ll talk a bit more about x-rays and gamma rays soon, as part of a larger discussion of radiation.

There is a lot more to light than visible light, and the various parts of the electromagnetic spectrum are used in many applications. Each wavelength gives us different information about the world, and we can use technology to extend the view that we’re biologically capable of to include x-rays, infrared, and many other parts of the electromagnetic spectrum!

Entropy and Life

Entropy is a measure of how many configurations could yield the same macrostate, and thus how probable the macrostate is. It can be a measure of information, or a measure of disorder in a physical system. But what about the entropy of biological systems?

Relatively few configurations yield life, compared to the many that don’t. Life is highly ordered, so living organisms should have much lower entropy than their non-living constituents. In fact, using energy to create and maintain order is one of the key signatures of life! One implication of life having low entropy is that life is improbable, which so far seems to be true based on the limited observations we have of other planets. But another implication is that living things act to reduce entropy locally, in the organism, there must be a corresponding increase in entropy somewhere else to offset that reduction. This is required because of the second law of thermodynamics, which says that by far the most statistically probable outcome is an entropy increase. But the second law applies to ‘closed systems’, which means a system that cannot exchange heat or energy with its surroundings. An organism that can interact with its surroundings can expel entropy via heat, to gain local order and reduce local entropy. Global disorder still increases, but for that organism, the ability to locally reduce entropy is literally a matter of life and death.

An obvious example of this principle is humans. Our human bodies are very highly ordered compared to inanimate things like air and water. Even compared to dirt, which has a whole ecosystem of microbiota and larger organisms like worms, a human represents many times more order. But this doesn’t contradict the second law, because the way we maintain life is to take in food and expel very disordered waste products. Humans can extract the chemical energy in the food and use it to maintain or decrease local entropy levels, and thus stay alive. Obviously other animals do this too, though they may eat different things than we do or digest them in different ways. And actually, the plants that we and other animals eat have done something similar, except that instead of getting chemical energy from combustion they are able to extract it directly from the sun’s light. Plants maintain their low entropy by releasing heat and high-entropy waste products, and anything that eats plants (or that eats something that eats plants) is converting solar energy into local order as well as expelled heat.

If you’re really feeling clever, you might ask, what about the planet Earth? If we’re receiving all this sunlight, and making life from it, shouldn’t there be a corresponding buildup of entropy on the planet, in the form of waste heat or some other disordering? Isn’t the Earth a closed system, isolated in space, whose order is constantly increasing?

But if a closed system is one which exchanges no heat with its surroundings, then the Earth doesn’t qualify because it is obviously exchanging heat with the Sun! The Earth receives a massive number of photons from the sun, which is where plants, and by extension the rest of us, get energy to create order. But in addition, the Earth is also radiating energy and heat into space, as all objects do. The incident energy from the sun is directional, high-energy, and highly ordered, but the energy the Earth radiates into space is in all directions, low-energy, and very disordered. That’s where the excess entropy is going!

Thus, life on our dear planet is not a violation of the second law of thermodynamics at all, because living organisms and even huge ecosystems are not closed systems. What’s more, the creation of order from chaos actually requires a net increase in entropy: it requires a reconfiguration of atoms and microstates, and the most likely outcome of any reconfiguration is an increase in entropy.  Many of chemical reactions necessary for life are entropically driven, where the outcome has many more available states than the inputs so the reaction is statistically favored to occur. Organisms that do work to create order must also create entropy, and the organisms most likely to survive are often those with the most clever control of entropy generation. So the proliferation of life is not threatened by entropy, as in the popular conception, but actually depends on entropy generation!