The colour of the sky is often explained with reference to the fact that the air (or the atmosphere) scatters blue light more strongly than it scatters red. Consequently, if you do not look directly at the Sun, the sky will appear blue. At sunrise (or sunset) the light from the Sun has to travel through more of the atmosphere to reach us than it does when the Sun is higher in the sky. The blue light gets ‘scattered out’ at these times, leaving the red that gives us the beautiful sunsets and sunrises.
While this is true, doesn’t it open up more questions? What exactly is scattering the light? If blue light is scattered more than red, why is violet light not scattered more than blue (after all, the order of the rainbow goes from red through blue to violet, why not with the scattering from the sky)? Was the musician Prince onto something with his song Purple Rain? Most importantly, what has this got to do with coffee?
These questions are more easily addressed in reverse. It is the scattering of light by molecules in the air that causes the sky to appear blue but this scattering also occurs for small particles held in suspension in a clear fluid. First investigated by John Tyndall (1820-1893), we can explore the effect with milk diluted with water as is done in the video below. As it concerns particles being held in suspension, the effect should also occur in coffee, but for coffee other effects such as chemical absorption of light dominate, making the coffee appear that delightful blackish-brown colour rather than shades of blue.
The particles in the milk-y water scatter the light exactly analogously to the way that the molecules in the atmosphere scatter sunlight. The scattering of the light depends on the wavelength of the incident light (click here for why) and so blue light, with a short wavelength, is scattered far more effectively than red light, with a longer wavelength. Holding the glass of milky water away from the light source (see the video) makes the liquid appear slightly blue.
Looking at the light source through the milky water gives you a ‘sunset’ effect:
We can establish that the blue colour (which is more vivid in life than in the photograph) is from scattered light and not from any reflections from the sky with a couple more experiments.
The sky does not appear violet due to a combination of the response of our eye and the emission of light from the Sun. Firstly, the Sun does not emit all light equally. Green/Yellow is emitted with greater intensity than violet. So there is less violet to get scattered in the first place. Secondly, the human eye does not detect all colours equally either. Colour detection is achieved using the ‘cones’ in the retina. There are three types of cone cell, each one having a peak response in a different region of the visible spectrum (roughly red, green and blue). Taken together, they are far less sensitive to violet light than to the other wavelengths and so again, it will be far harder to see the violet light. For us, blue light will appear far more intense than any violet that is scattered. The sky would look very different to a honey bee.
Lastly to the question of what exactly is causing the scattering. For this sort of light scattering to occur, the particles doing the scattering have to be smaller than the wavelength of the light that is scattered. For visible light, the particles therefore have to have a diameter smaller than approximately 500nm (0.0005 mm). Milk is made up of fat globules (which are large compared to the wavelength of light) and groupings of proteins (which are about 0.0001 mm diameter). Light scattering in diluted milk is therefore a mix of wavelength independent (ie. white) scattering from the fats and preferential scattering of shorter wavelength light by the smaller particles which gives us the blue colour. In the atmosphere, it is the molecules of air (nitrogen, oxygen) that act as scattering sites. As each molecule is much smaller than the wavelength of light, the light scattering from the sky is dominated by the wavelength dependent component (unless there are clouds in the sky). Although each molecule is tiny compared to the size of the particle in the milk, even a scattering site this small can cause enough scattering that we get the blue sky effect.
Sketch of John Tyndall by Meilin Sancho.