Colors   Leave a comment

It’s not surprising that things appear very different under high magnification. However, it’s less appreciated that the color of an object can change. Here’s two examples- terra-cotta and 400-grit sandpaper. Do I really need to warn you not to try imaging sandpaper under a microscope?

Here’s the sequence of images of terra-cotta:

The first image was taken with a 4X NA 0.1 lens, the second with a 16X NA 0.35, the third with a 40X NA 0.85, and the last with a 80X NA 0.95 lens.

Clearly, the color of the material changes from a brick-red to a bright orange. Here’s the sandpaper, using the same lenses:

Again, the color changes from a neutral grey to something else- the alumina particles are bright (and display interference fringes) and the background gets washed out.

Why does this happen?

The illumination used to make these images is called axial ‘epi-illumination‘. The light passes down through the objective lens, bounces off the object, and back into the lens. This is completely different than how your vision works- it would be as if your eyes illuminated what you are looking at.

Color in objects comes from a combination of absorption and scattering. White objects, like paper and t-shirts, scatter light very well but don’t absorb much. Black objects, like…. black t-shirts, scatter and absorb the light. Very smooth objects (freshly waxed cars), don’t scatter much light, and so a black car can reflect a blue sky.

By imaging using axial epi-illumination, I control the amount of scattered light that gives the object it’s color.

Just for fun, I used a water-immersion lens to image the sandpaper- here’s a low-magnification image of wet sandpaper followed by the high magnification (100X, NA 1.2) image:

Notice, the low-magnification image has a much darker color (and brighter highlights) than the dry image. Also, the high-magnification image has a much larger range of color: I’ve reduced the scattering by covering the material with water.

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Posted September 3, 2010 by resnicklab in Physics, pic of the moment, Science

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