Film Interference and Feathers ```Name: Sheng H. Status: student Age: 19 Location: N/A Country: N/A Date: 2000-2001 ``` Question: 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? Replies: Sheng, 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 surface thicknesses. 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. -Wil Click here to return to the Physics Archives

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