The Visible Light Spectrum

White light includes all primary colors (ROYGBV). Remember prisms from elementary school? This prism separates Red, Orange, Yellow, Green, Blue, and Violet from white light! I do want to point out, Violet does not mean purple! It is the "roses are red, violets are blue" color.
Violet is a flower. Click here to see some. https://en.wikipedia.org/wiki/Viola_(plant)
Picture source: ​http://blogs.nature.com/ofschemesandmemes/files/2015/02/Week-2-Blog-Dispersive-Prism.jpg

ROYGBV is just the visible light wavelengths! 

Light penetration

You can see red light penetrate through your finger if you put it in front of a flashlight. You see red on the other side because most of the other colors were absorbed in your finger. Try this at home! Put your phone flashlight, or any white flashlight, on one side of your finger. Then look at what color makes it to the other side! 
www.researchgate.net/publication/316712743/figure/fig13/AS:668385360744448@1536366896067/a-Tissue-penetration-depth-of-light-with-different-wavelengths-NIR-light-penetrates.png
https://images.theconversation.com/files/201297/original/file-20180109-83559-19kt3yi.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip

Red light can be used to see inside your body. 

Something is not right with this finger!
Got an X-ray and yep, something is there.
Images are sourced from: ​http://www.eatonhand.com/img/img00087.htm
Let's shine red light on it. Yep, it's darker where something is sitting. That means there is not good blood flow there.
​There is more deoxygenated blood in that spot.  
Click this button see what was taken out after surgery. ​Warning: somewhat graphic image. 

Seeing your Heartbeat with Light! 

The color of blood

Have you ever heard that blood gets darker if it does not have oxygen in it? Well, that's true. The more oxygen blood has, the more deep red light it absorbs so it reflects less deep red. 
Picture
All sketches for this section come from this source: ​https://www.youtube.com/watch?v=MHPgamGQmDY
The oxygenated hemoglobin (HbO2) in the arterial blood has the characteristic of absorbing IR light. The redder the blood (the higher the hemoglobin), the more IR light is absorbed. As the blood is pumped through the finger with each heartbeat, the amount of reflected light changes, creating a changing waveform at the output of the photodetector. As you continue to shine light and take photodetector readings, you quickly start to get a heart-beat (HR) pulse reading.

Graphing blood's light absorption by wavelength

This is a rough sketch showing how much hemoglobin (Hb) absorbs different light wavelengths  depending on if it is oxygenated.  

Red light at a 660 nm wavelength is used in pulse oximeters. This is because it is the max difference between light absorbance.
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