Blue Sky &emdash; Hot melt glue

Why is the sky blue? That's a sticky question.


The stronger scattering of blue light by the atmosphere which creates the blue sky and red sunsets can be modeled when white light from a flashlight is shone through a clear glue stick.


A flashlight

4 hot-melt or cool-melt clear glue sticks (large diameter, clear sticks work best)

A dark background piece of paper or cloth

To Do and Notice

Hold the hot-melt glue stick in front of a dark background.

Shine the flashlight into one of the round ends of the glue stick.

Notice that the glue stick scatters light.

Notice that the light scattered near the flashlight is bluish, further from the flashlight the light is yellow.

Look into the end of the glue stick opposite the flashlight. Notice that yellow light shines out the end.

Put two glue sticks end to end. Shine the flashlight into the end of one. Notice how the light gets redder the farther it travels through the glue sticks.

Add more glue stick in line and notice the deepening of the red color.

What's Going On?

Blue light is scattered more strongly than red light by the hot-melt glue stick and by the atmosphere. (See below for why) The white light from the flashlight contains all wavelengths of visible light. The glue stick scatters the blue light out of the flashlight beam more strongly than it scatters the red light. The light scattered at first is thus blue. After the blue light is removed, the light that remains is yellow.

As more blue light is removed plus green light as well the remaining light becomes redder.

The glue stick is made from low melting point polymer. The polymer molecules are smaller than a wavelength of light. Blue light wavelengths are closer in size to the polymer molecules than red light wavelengths. The blue light is therefor scattered more strongly than the red light. (This same reasoning applies to the atmosphere where the molecules which scatter the light are smaller than a wavelength of blue light.)


The light scattered to the side by the glue stick is polarized. It is polarized perpendicular to the direction the light is moving and perpendicular to the glue stick as well. Look at the glue stick through a polarizing filter to confirm this.

Scientific Explorations with Paul Doherty

© 2003

5 May 2003