Why can’t light resolve details smaller than about half its wavelength? — SJ, Philadelphia, PA
Suppose that you have a white card with what appears to be a black line on it. That line might actually be two very closely spaced lines; you’re not sure. To find out, you focus a beam of light to the smallest possible spot and then move this tiny spot of light across the line. You realize that if there are two separate lines on the card, then the spot of light should cross first one line and then the other, and you should see two changes in the reflected light rather than just one.
It turns out that, however, that no matter how hard you try you can’t focus the light to a spot much smaller than the wavelength of the light. An equivalent problem would occur if you tried to use water waves to create a narrow spike of water above the surface—no matter how you worked with the water waves, you would be unable to make them to merge together into a spike that’s much narrower than the wavelength of the water waves. Because of his limitation, your spot of light can’t be much smaller than the wavelength of light and you can’t distinguish between one line or two if those lines are much closer than a wavelength of the light you’re using. Since visible light has a wavelength of 400 nanometers or more, you can’t use it to resolve details much smaller than 400 nanometers wide.
Actually, there is an exception to this general rule—near-field scanning optical microscopy or NSOM uses light emerging from the tiny tip of a glass fiber to resolve details far smaller than the light’s wavelength. In NSOM, the resolution is determined by the tip size and not the light’s wavelength.