This chapter is from the book
This chapter is from the book
This chapter is from the book
How a Lens Focuses an Image
You must remember boring, sunny summer days in your childhood when you took a magnifying glass outside looking for anything flammable. Having spied a dry leaf, you held your magnifying glass between the leaf and the summer sun. Moving the magnifying glass back and forth with the precision of a diamond cutter, you found the exact point where the magnifier’s lens concentrated the sunlight to form an image of the sun on the hapless leaf. Within seconds, the concentrated rays of the sun caused a thin wisp of smoke to rise from the leaf. Then, suddenly, it burst into flame. You had just done the work involved in focusing a camera’s lens, though with more dramatic results.
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The Bend in the Rainbow
Not all colors of light bend at an angle when they pass through a lens. Different colors have different wavelengths, the distance between the high point of one wave in a beam of light to the high point in the next wave. That makes each color bend at a different angle, as shown in the drawing.
When light passes through raindrops, the drops act as lenses and break sky light into the colors of the spectrum. We call that a rainbow; when something similar happens in a camera, we call it chromatic aberration. In a photograph, it shows up as fringes or halos of color along the edges of objects in the picture, as in the photo shown here.
The aberration is prevented by an achromatic doublet, or achromat, a single lens made of two materials that are bonded together. Because each material has a different refractive index, their chromatic errors tend to cancel each other. The aberration is also corrected with image editing software, as was used on the second photo shown here.
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