Color is an interesting thing, it’s both quantitative and qualitative. Today, I’m going to talk about its quantitative nature. I’m sure most people know that visible light comes from a specific region of the electromagnetic spectrum, but they probably don’t understand why certain things are specific colors. Therefore, I’m going to ask some specific questions about the physical nature of color and answer them. My hope is that you’ll have a greater appreciation of our colorful world.
1. Why is the sky blue?
The sky is blue in color because of a process known as Rayleigh scattering. This is the elastic scattering of light by particles smaller than its wavelength and it’s caused by the electric polarizability of particles. When the oscillating electric field component of electromagnetic waves act on the partial charges of molecules, these electrets (electrostatic analog of magnet) begin to move at the same frequency, thus radiating the light again. There is also some non-elastic Raman scattering of light, which causes the particles to rotate. Molecules in the air, such as nitrogen and oxygen, make Rayleigh scattering most efficient for smaller wavelengths. That’s why the sky is blue. This kind of scattering is also angle dependent though, which is why the colors of the sky change so drastically at sunrise and sunset.
2. Why are clouds white?
Clouds are white because of Mie scattering. This is what happens when electromagnetic radiation acts on particles larger than its wavelength. In the case of clouds, these particles are water droplets. Incoming light reflects off of the droplet surfaces and scatters throughout the volume of the cloud. This type of scattering is wavelength independent and that’s why clouds are white. It’s also responsible for mist and fog.
3. Why are plants green?
There’s a green pigment in plants known as chlorophyll. The primary function of chlorophyll is absorption of light and its transfer via resonance energy transfer. Thus, it is vital to photosynthesis. There are several types of chlorophyll that differ in their absorption spectrum. Some types are more suitable for marine plants. Plants of varying colors have accessory pigments. For example, brown algae has fucoxanthin which is responsible for its brown color.
[3B] Specifically, which part of this structure?
The magnesium²⁺ ion center is the primary cause of the green color. The emission spectra of molecules comes from their electronic structure. When an electron in an atom gains enough energy from a photon, it’ll jump to a higher energy level. It will then drop down to a lower energy level, which may be lower than the one it started at, because it cannot maintain that energy state due to the inherent electronic instability. The resulting photon that’s emitted is dependent on the difference of these energy levels. This is why magnesium²⁺ is used in green fluorescent indicators and why magnesium deficiency in plants causes a yellowing in color.
4. Why is blood red?
Blood is red for a similar reason that plants are green. Blood has a red pigment known as hemoglobin. The primary purpose of hemoglobin is to transport oxygen from the lungs to the tissues. Thus, hemoglobin is vital to aerobic respiration.
[4B] Specifically, which part of this structure?
Iron(II) and iron(III) are responsible for the dark red color. Inside a hemoglobin, iron may be in either of those oxidation states. This is because oxygen which binds to iron(II) temporarily becomes iron(III). The red brownish color of rust comes from hydrated iron(III) oxide in fact! Iron(II) is will appear black. This is why venous blood is so dark! It’s deoxygenated!
5. Why is silicon black?
Band gap refers to the energy difference between the top of the valence band and the bottom of the conduction band in a solid. Silicon has a low optical band gap at 1.1 eV at 300K. In the case of silicon, its electronic band gap is identical to its optical one. This makes silicon a great semiconductor material. The low optical band gap means that silicon absorbs light in the visible spectrum which is radiated back as infrared light.
[5B] If somebody replaced their windows with silicon panes, what technology would you use to see through them?
You would use thermal imaging goggles because silicon won’t absorb infrared light. The black body radiation from heat on people’s bodies creates infrared light that will pass right through the silicon panes and can be picked up with thermal imaging devices.
6. Why is outer space black?
Space is immensely huge and there are lots of stars. Because of this, you’d expect the night sky to be brilliantly white because there’s a star in every minute direction that you’d look. Despite this, we only see various specks of light. This is known as Obler’s paradox. So, how is it that outer space is black? Well, it turns out that the universe is very dynamic. The universe is stretching outward and because of this, light distant in time becomes so stretched that it’s not visible to us. This is known as the Doppler effect. That stretched light now exists as microwaves every direction. We call this the Cosmic Microwave Background Radiation. If you had microwave vision, the universe would be very bright indeed. The stars which you see in the night sky are, relatively speaking, not too far off in time and so they’re visible. They appear as twinkly little specks because they’re kind of like lasers. You don’t see the beam of a laser, unless there are particulates in the air, but you see its spot because its light doesn’t disperse. Star light doesn’t disperse when traveling through the vacuum and so it travels in a local straight line until it meets our eyes.
7. Why is bismuth multicolored?
Bismuth is a very interesting mineral. In its naturally occurring form, it always has an oxide tarnish. This tarnish is key to the brilliant hues you see on bismuth. The oxide layer causes a phenomenon known as iridescence. Soap bubbles, sea shells, butterfly wings, CDs, DVDs, and even clouds. Anyways, when incident light strikes the semi-transparent oxide layer, part of it goes through to be reflected by bismuth. The reflected light then modulates the incoming light. The oxide layer varies in thickness, causing a variety of phase shifts and thus a plethora of colors, all of which change depending on the viewing angle.
8. Why is opal multicolored?
Silicon dioxide in normally white. Yet, opal, which is composed of the same stuff is multicolored. This has to due with the structure of the silicon dioxide. Specifically, opal is hydrated amorphous silica. An amorphous material you’re probably familiar with is glass. Amorphous materials have no ordered crystalline structure. Opal has spheres of silica about 150 to 300 nm in diameter. The silica itself is crystalline with a hexagonal or cubic close-packed lattice. Light passing through opal is diffracted through these microstructures, causing the colored facets.
9. Why is Cherenkov radiation blue?
Nothing can travel faster travel than light, right? Wrong. Light speed in a vacuum cannot be surpassed. However, light in a medium is often slower. In fact, light has been slowed down to the speed of a bicycle! Furthermore, there are different kinds of velocity for waves, namely group and phase velocity. Group velocity is the speed at which the envelope of a wave travels through space. Phase velocity is the velocity that the phase (top of a crest or bottom of a trough) propagates at. These things can even go in opposite directions! Cherenkov radiation happens when a charged particle passes through a dialectric material faster than the phase velocity of light in that medium. The phase velocity in the medium depends on its refractive index. The greater the refractive index, the lower the phase velocity. Once a charged particle moves faster than the phase velocity, it effectively breaks the light barrier and creates an electromagnetic shock-wave, just like a supersonic jet breaking the sound barrier. Normally, when a charged particle passes through a medium, it polarizes the medium which then elastically relaxes back to equilibrium. When the light barrier is broken, the medium cannot relax and it’s shock-wave is manifested as radiation. The brilliant blue color is due to the following facts: (1) Cherenkov radiation has a continuous spectra with no characteristic peaks, (2) higher frequencies (such as blue) can carry more energy and so they’re more intense, (3) human eyes are more sensitive to blue than violet. So even though violet is more intense, we see the blue better. Most Cherenkov radiation is in the UV spectrum, which we cannot see.
10. What’s the colorful creature in the following picture?
This creature is known as peacock mantis shrimp and it’s as beautiful as it is frightening. Their raptorial appendages in the front of their body, which are used to dismember their prey, move at the velocity of a gunshot from a .22 caliber rifle. This allows them to strike prey with 1500 newtons of force in less than 0.003 seconds. If a human could move their arm a tenth of that speed, they’d be able to throw a baseball into orbit. That’s not the only thing that’s fast about them either. Their limbs move so fast that the water boils due to supercavitation. These cavitation bubbles collapse with a pressures high enough to reach thousands of Kelvin and cause emissions of light, a mysterious effect known as sonoluminescence. So even if these guys miss their prey, they can still steam cook their food.
So, why did I include this question in color trivia? Yeah, this animal is violent and certainly colorful. But what more does it have to do with color? Well, these guys’ eyes are pretty dang special. Eyes have rod and cone cells. Rods allow us to see light and motion. Cones allow us to see color. Dogs have two types of cone cells, blue and green. So, they can see blue, green, and their brains come up with yellow (an interpolation of blue and green). Humans have blue, green, and red cone photoreceptors. This allows us to see these primary colors, secondary colors (blue+green=yellow), and tertiary colors (red+yellow=orange). Honeybees are similar to humans in that they have three cones, but their spectrum is shifted. They have green, blue, and UV (ultraviolet). Butterflies have 5 types of cones, red, green, blue, violet, and UV. Guess how many types of cones the peacock mantis shrimp has? They have 16 that range from IR (infrared) to EUV (extreme ultraviolet). This allows them to see hyperspectral colors, but they also use a signal technique known as differentiation to see linear and circular polarized light. They need this ability because that’s how they communicate, with polarized light. It also allows them to spot prey behind rocks.
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