When kids ask the age-old question “why is the sky blue,” try this simple science experiment that draws upon John Tyndall’s work from Ireland in the 1860s.
Tyndall used a clear tube and smoke particles to demonstrate how blue light scatters more than other colors; this phenomenon explains why our atmosphere gives the sky its hue.
Rayleigh Scattering
As sunlight passes through the atmosphere, its shorter wavelengths – blue and violet light – become scattered by gas molecules in the air. Since these particles are much smaller than light wavelengths, collisions between photons passing by them and passing photons tend to be relatively “elastic”, leading to greater scattering of blue light than red light; ultimately causing its total path through to appear bluer than expected.
Rayleigh scattering, named for British physicist Lord Rayleigh who first described it in 1870, decreases with altitude because the molecules in the atmosphere become less dense over time and therefore less blue light reaches your eyes – hence why skies directly overhead may appear paler or whiter due to reduced blue light reaching that height in the atmosphere.
Blue light scatters more than red when projected through water with some milk or soap mixed in; this will reveal Rayleigh scattering very clearly as you’ll witness an array of blue and red dots demonstrating its effects.
An essential factor in this process is that electrons within atoms and molecules have natural resonant frequencies that match up closely with blue portions of electromagnetic spectrum, making hydrogen molecules much more likely to absorb that area than red parts of spectrum.
This is why the sky appears blue; had its electrons had different natural resonant frequencies, then we might also perceive other colors, including green and yellow hues in our skies.
Tyndall Scattering
An opaque sky occurs when sunlight passes through scattered water droplets, scattering into our eyes in different wavelengths separated by a color sensitivity gradient – shorter wavelengths (violet) reach our eyes before longer wavelengths (red). Blue wavelengths spread more widely than any other hue, giving an impression of an overall blue-tinged cloudy sky.
Irish scientist John Tyndall first observed this phenomenon in 1859 and named it after himself: The Tyndall Effect. To do this experiment, he shone a beam of white light through fluid with floating particles before looking from the side at what happened next. He noticed that blue light bounced more off particles due to shorter wavelengths.
Tyndall observed the same effect when sunlight enters an open window containing dust or smoke-strewn air. As it passed through it, Tyndall observed how its colour changed as the beam passed over it; he believed this to be caused by certain frequencies of light reflecting off dust particles differently and more blue reflected off than red did.
The Tyndall Effect can be demonstrated in a lab by filling a glass with distilled water and shining a laser pointer through it. You won’t see the laser beam because the water is clear, but adding milk molecules causes light scattering similar to Rayleigh scattering but at higher frequencies than air would do, turning blue the colour of its water molecules.
Tyndall Effect-exhibiting solutions are known as colloidal suspensions. To create your own colloidal suspension at home, mix milk into water and shine a flashlight through it; its solution should light up in blue as its milk particles scatter light much like Rayleigh cones do – note that smaller particle sizes generate stronger Tyndall effects.
Red Cones
Due to this effect, light that bounces off of red objects such as flowers or the surface of your car stimulates both M and L cones equally, leading the brain to interpret this as yellowish color – hence why yellow flowers often look more vibrant under fluorescent lighting than when photographed under daylight conditions.
Under normal conditions, M and L cones are approximately equally sensitive to red, green and blue light. However, our eyes can detect long wavelengths which fall beyond the range of our M and L cones – this explains why we can detect infrared and ultraviolet radiation.
Short wavelengths within our M and S cone ranges are equally essential, since objects reflecting many of these wavelengths will appear blue to us; in fact, this ability allows us to detect many colors of the rainbow.
Photobleaching allows us to utilize our eyes in order to determine the relative arrangement of M and S cones on the retina, using dark-adapted retina as a basis and exposure to wavelengths that paralyze cone cells most sensitive to that wavelength – up to thirty minutes may pass before your retina can dark-adapt again.
So if we gaze on a red object before looking at a white area, our afterimage won’t be red but rather blue-green because our red cone cells have become paralyzed while our brains only use signals from green and blue cones.
Monochromatism occurs when genetic mutations have compromised one of the three primary colors, leaving people only seeing black and white with faint colors appearing dimly visible. This condition has no cure.
Sometimes people have mutations that cause extra M and/or S cones, creating what are known as tetrachromats – people with four times the sensitivity of an average person and the ability to perceive brighter colors under certain conditions than average. A doctor can test for this gene by taking a low light picture of their retina and looking for evidence of its fourth color receptor receptor.
Blue Cones
As light passes through air and Earth’s atmosphere, different wavelengths are scattered differently. Blue wavelengths tend to be redirected more than red ones due to Raleigh scattering – this phenomenon also accounts for why the sky appears bluer. Raleigh scattering also explains why ocean appears bluer since molecules in it tend to scatter short wavelengths like blue more readily than long ones such as red.
Human eyes contain 6 to 7 million cone photoreceptor cells that allow us to perceive fine detail and distinguish colors. This network of color sensitive cones, divided into red (Rd), green (Gr) and blue (Bb) types, are located mostly within an area called fovea centralis on our retinas – this area hosts most Rd and Gr cones while most Bb ones reside outside it; this explains why blue skies appear less distinct because our eyes cannot detect as much detail from less densely packed blue cones as other colors do.
In the early 1970s, physicists began to understand how visual systems process spectral input in order to produce accurate color perception. It became evident that there are three classes of cone cells and their sensitivity determined by which wavelength of light their response cell responded to; further evidenced by membrane potential fluctuations when exposed to more photons of certain wavelengths than others; further supported by increased membrane potential when receiving multiple photons at once while decreasing when only exposed to few.
After discovering this interplay between light physics and vision physiology, researchers created a model illustrating how humans extract color information from spectral stimuli. Although not fully accounting for phenomena like why the sky is blue or other subtleties of human vision, this theory has become the go-to explanation.