Simple Model of How the Sky Changes Color at Sunset

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).


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

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.

sun earth diagram

The yellow sunset and the daytime blue sky are caused by different path lengths through the atmosphere. The longer the path, the less blue light remains.

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.


Sunlight that has lost its blue looks yellow.

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.

egg in light

The blue sky effect clearly shown on the top half of the egg. Note that the scattered blue light (moving to your eye) is perpendicular to the incoming light. The light that passes all the way through the egg is very yellow.

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.


A similar blue sky effect can be achieved by using an aquarium full of water with a little coffee creamer. When light passes through the aquarium it gets scattered by the tiny coffee creamer particles. But blue light gets scattered more frequently, making the aquarium look blue over all.

Sunset effect

The light that passes through the aquarium has less blue in it and so it looks yellow. This causes a sunset effect. It is not just an effect however; this is the real cause.

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.

Egg polishing

Cooking oil provides polish for a dull egg.

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.

Sunset Egg

In Stock SKU: P2-1000

Sunset Egg 6 Pack

In Stock SKU: P2-1001

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Comments (3)

  • Robert Blair Hewitt Reply

    The eggs look great. We will have 720 children to look after and would almost certainly purchase your products. Blair

    May 11, 2016 at 10:56 pm
  • Scott Schneider Reply

    Is there are certain length of the fishtank that works best (that is, can it be done with small tanks, or does it have to be long/narrow ones? Presumably rectangular glass is best – curved plastic would probably not work as well.

    May 26, 2016 at 8:11 pm

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