Physics Photo of the Week

February 10, 2023

Milk: Red, White, Blue, and "Fire"

Photos by Donald Collins

This image of "fire" is actually a squirt of milk immersed into a vessel of clear water and illuminated from the rear by a bright flashlight.  What we see is various light scattered from the milk particles in the water before the solution was stirred.  It resembles a campfire on which we can toast marshmallows!  But it is merely cold white milk and water.  The colors are explained below.

 

The next image (right) is a plastic refrigerator storage bin with fresh water beside a small amount of  white milk before the milk was added to the water. Only about 4 ml of milk (about 2 teaspoons) was added to the water.

 

The next image (left) shows the water and milk after they have been thoroughly mixed.  The mixture is illuminated by a flashlight shining from the back.  The flashlight beam is strongly diffused by the mixture of milk and water (about 0.2% milk).  The diffused light looks yellow.

 

 

 

 

The final image is looking down through the top of the mixture, and we can see the beam of the flashlight progressing partly through the solution.  The flashlight penetrates the solution from the left.  Notice that it appears blue then progressively becomes reddish as it tries to penetrate further into the solution.  

 

All these images demonstrate how the scattering of light depends on its color.  The white light in the flashlight consists of all colors of the rainbow.  Each color of the rainbow spectrum is determined by the wavelength of light for that color.  The blue has the shortest wavelength (about 400 nm).  Red has the longest wavelength of visible light (about 600 nm).  Yellow and green consist of the middle colors (about 500 nm).  The units (nm) are "nano-meters" (1 nm = 10^-9 m).

 

Since light is predominately a wave (it is also particle-like, but that is an entirely different - yet interesting - feature).  Imagine small ripples of water striking a pebble that is slightly larger than the wavelength.  The short wavelength ripples will be scattered strongly by the pebble.  Next imagine ripples with a wavelength that is much larger then the size of the pebble.  The long-wavelength ripples will pass right through the pebble and hardly be scattered at all.  For waves to be strongly scattered by objects the wavelength of the waves must be shorter than the size of the objects.

Back to the milk mixture:  The the light that penetrates the milk mixture the furthest is red - the longest wavelength.  The light that is scattered sideways the most is blue - the shortest wavelength.  The new James Webb Space Telescope uses infrared (very long wavelengths of light - much longer that the wavelengths of visible light) so that it can see through the interstellar dust and thus image galaxies much further than is accessible with shorter wavelength visible light.

In the "fire" image above that is back-lighted by the flashlight, the red "flames" are regions where the blue light has been scattered out and the penetrating light appears red.  The blue "flames" are regions where we see reflected light from regions of higher concentrations of milk particles. 

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  Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.

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