Absorption of Colored Light by Blood


In a darkened room, a red LED light is held up to your thumb. Your thumb glows bright red as most of the light passes through it. But when a green LED is held up to your thumb, almost all of the light is absorbed. 

Curriculum Notes 

This quick and easy demo can be used anytime the differential absorbtion of electromagnetic energy is being discussed. In general chemistry it can be used when electronic structure is being introduced. In organic chemistry it can be used to refresh the students on the principles underlying IR or UV/vis spectroscopy. This demo can be performed in one minute easily.

One day of lead time is required for this project.


Blood looks red because it absorbs the frequencies of visible light corresponding to all of the colors except red. Therefore red light passes through the thumb easily. Green light, however, is pretty much completely absorbed. Oxy-hemoglobin, the compound chiefly responsible for the red color of oxygenated blood, absorbs the shorter wavelengths of visible light strongly. Thanks to Prof. Mark Lonergan at the University of Oregon chemistry department for this demo. 


  • one red LED "mini-flashlight"

  • one green LED "mini-flashlight"


  • Darken the room.

  • Turn the red light on.

  • Hold it behind your thumb and shine it towards the class.

  • The light will shine through your thumb.

  • Repeat with the green light. Almost all of the light will be blocked.

Safety Precautions 

Don't trip over stuff in the dark.