1
$\begingroup$

Why is it that wherever we see spherical mirrors, we mainly find concave and convex mirrors? Why don't we use parabolic mirrors as extensively as the other two types of mirrors? It even has an advantage that any type of incident parallel light ray passes through its focus, unlike concave and convex mirrors where the rays have to be paraxial.

$\endgroup$
1
  • 1
    $\begingroup$ Parabolic mirrors do exist. You can buy one. $\endgroup$
    – garyp
    Commented Oct 28, 2021 at 19:45

1 Answer 1

3
$\begingroup$

Besides the obvious reason that a parabolic mirror is much more difficult and expensive to manufacture than a spherical one, a parabolic mirror has the worst coma imaginable (ok, this may be an exaggeration) for off-axis rays, see https://en.wikipedia.org/wiki/Coma_(optics). Your statement that "[...] has an advantage that any type of parallel light ray on it passes through its focus" is true only for rays parallel with the mirror axis, not ones that fall obliquely. Becuase of its symmetry spherical mirror has the same aberration for oblique rays as for rays that are parallel with the axis.

$\endgroup$
2
  • $\begingroup$ But why do we use parabolic mirrors to avoid spherical aberrations if it has "THE WORST COMA" ?I mean that using a mirror to avoid an aberration which already gives its own aberration. $\endgroup$ Commented Nov 4, 2021 at 9:32
  • 2
    $\begingroup$ Because of its very high 3rd order aberration an "optical quality" parabolic mirror is useful for very (very) narrow field of view only. It is possible to correct for the distortion but if you do so then might as well change the shape of the mirror itself to something easier manufacturable. A fixed parabolic mirror looking at a fixed target is often used as a microwave antenna but if it needs to track a moving target then it must receive off-axis echoes, as well, and the distortion beyond a couple of beamwidths is rarely acceptable amount. $\endgroup$
    – hyportnex
    Commented Nov 4, 2021 at 15:26

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.