Shining Light Along Hot Glue Sticks

I came across a pack of 8 x 19 cm sticks of hot glue in a discount store. I thought that it would be interesting to investigate the optical properties of these hot glue sticks. At a physics teachers' conference I had seen an experiment where ultraviolet light was shone through a rectangular block of gelatin jelly. The amount of light emerging varied exponentially with length of gelatin. I thought that I could replace the gelatin with different lengths of hot glue sticks.

Wishful thinking!

I found that both a blue LED and a blue/violet laser caused the near end to glow brightly with little light passing further than a couple of cm along the stick of glue. The light is strongly scattered and there is some fluorescence from the ultraviolet component of the incident light.

The photo shows the result when blue/violet laser light is shone into the glue stick from the right.

But all is not lost…

The next photo shows that red laser light is able to penetrate further than blue.

This means that hot glue sticks can be used to illustrate that many 'transparent' substances are not equally transparent to light of different wavelengths.

Usually this is hard to do since the larger variations in transparency are outside the range of visible light. Glass is opaque to UVB but is more transparent to infrared than visible light. That is why infrared is used to carry digital information along fibre-optic cables.

But, wait, there's more.

I was 'playing' with the short lengths of glue stick on the side bench of my lab, near the windows, and I noticed there was a color difference between the ends of each stick.
The end nearest the window was blue and the other end was a pale yellow/orange.

The photo shows this color variation. The glue stick lengths range from 2 to 5 cm.

Again, this shows that hot glue sticks scatter light at the blue end of the visible spectrum leaving the remaining light to be transmitted along the stick. This can be linked with how our atmosphere scatters blue light to form a blue sky and that same scattering causes reddish/golden sunrises and sunsets.