Simple Model of How the Sky Changes Color at Sunset

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Simple Model of How the Sky Changes Color at Sunset

The Sunset Egg is a fun and engaging demonstration on the science of light. The egg is made of "opalescent" glass. This refers to the way it looks different at different viewing angles (similar to the gem's properties).

To use the egg hold it in one hand and close your hand around it. The egg will appear blue. Now hold it up to a source of white light, such as overhead lights. The egg will appear yellow. What's going on? How can this be used to explain the blue sky and the yellow sunset?

The Sunset Egg responds differently to light based on its wavelength (The sky does the same thing). This process is called Rayleigh Scattering and when light is scattered, the shorter wavelengths are scattered more often.

When light hits the egg, more blue light is scattered than red and yellow, thus the egg usually looks blue. But the light that passes through the egg has had its blue light scattered away. The remaining light is yellow and red. Looking through the long end of the egg or using multiple eggs can also increase the effect.

But how can the egg help explain the sky?

During the day the light we see in the sky comes from light being scattered by air molecules (mostly oxygen and nitrogen). Since shorter wavelengths get scattered more often, the blue light is more frequently scattered. During sunset, the light has to pass through a more of the sky and that journey causes the blue light to get scattered out sideways on its way. The result is yellow and orange sunsets.

This is easily seen in the egg. The light scattered sideways is blue, but the light traveling all the way through is yellow. In the case of the sky, the light is being scattered on air molecules, mostly oxygen and nitrogen, but also dust and other particulates. In the case of the egg, the light is being scattered on fine dye particles inside of the glass.

The egg behaves like a little piece of the sky, and it looks like one for the correct reason – scattering. When the light passes through a small bit of it, the egg or sky looks blue, but when light passes through a lot of it, whether it is the egg or the sky, it looks yellow.

When you first get the egg, it can be used immediately for these experiments. However, it might have a sheen of white dust. This can be washed off somewhat, but it is helpful to wipe cooking oil over it and then dry it off with a paper towel. This will give the egg a smooth surface and improve the demonstrations that follow.

The reason the cooking oil smooths out the opalescent glass egg is because oil and glass have nearly the same index of refraction; they bend light by the same amount.

The Sunset Egg may be the best science gift ever because it is so much fun and can teach us so much.

An oil-polished Sunset Egg, lit from below, shows both blue sky and yellow sun.

The yellow sunset and the daytime blue sky are caused by different path lengths through the atmosphere.

The blue sky effect clearly shown on the top half of the egg.

A similar blue sky effect can be achieved by using an aquarium full of water with a little coffee creamer.

Cooking oil provides polish for a dull egg.

James Lincoln

James Lincoln is an experienced physics teacher with graduate degrees in education and applied physics. He has become known nationally as a physics education expert specializing in original demonstrations, the history of physics, and innovative hands-on instruction.

The American Association of Physics Teachers and the Brown Foundation have funded his prior physics film series and SCAAPT's New Physics Teacher Workshops.

Lincoln currently serves as the Chair of AAPT's Committee on Apparatus and has served as President of the Southern California Chapter of the AAPT, as a member of the California State Advisory for the Next Generation Science Standards, and as an AP Physics Exam Reader. He has also produced Videos Series for UCLA's Physics Demos Project, Arbor Scientific,,, and

Collin Wassilak

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