Film Interference and Feathers
Name: Sheng H.
We all see the colourful wings of butterflies and
beautiful feathers of ducks. How do u explain these "colour appearing"
phenomenons on those animals by using thin film interference?
When light hits two surfaces that have a separation (one behind the other)
of about a wavelength, there can be a big interference effect in the
reflected light. Some reflects from the front surface. Some reflects from
the back surface. Because light from the back surface travels a little bit
further than light from the front, it is shifted slightly. If back and
front light waves line up with each other, you get constructive interference
(peaks with peaks, valleys with valleys). That color is bright. If the
waves are exactly out of line, you get destructive interference (peaks with
valleys, valleys with peaks). That color is dim, possibly not visible at
all. Only some colors interfere constructively. All colors hit the wing
together in the sunlight. The film thickness determines which reflect
brightly and which reflect dimly. Your eye "interprets" the combination as
a specific color. Different parts of the wings have slightly different
Dr. Ken Mellendorf
Illinois Central College
I do not think that the colors in animals are created
by thin film interference. Thin film interference
occurs in transparent materials that have a thickness
of about a wavelength of light -- that's less than one
micrometer thick. I do not believe that butterfly
wings or duck feathers are transparent enough, or thin
enough, to do that.
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Update: June 2012