What is the power of a plane mirror?
It's a straightforward concept: the power of a plane mirror is zero. But you might hear some people get this confused when talking about the magnification of a plane mirror. The magnification ($m$) of a plane mirror is actually $+1$. * The '1' means the image size is the same as the object size (no magnification or minification). * The '+' sign indicates that the image is virtual and erect (upright). So, remember: Power is $0$ (zero) because it doesn't converge or diverge light, but Magnification is $+1$ because the image is the same size and orientation as the object. Don't mix up the two!
The power of a plane mirror is zero. This is because power in optics is defined as the reciprocal of the focal length (P = 100/f in cm), and a plane mirror has an infinite focal length. Hence, it does not converge or diverge light rays, making its power equal to zero.
The power of a plane mirror is zero (P=0). In optics, the power ($P$) of a mirror or a lens is defined as the reciprocal of its focal length ($f$) when the focal length is measured in meters (i.e., $P = 1/f$). The power is typically measured in dioptres (D). A plane mirror forms an image that is virtual and at the same distance behind the mirror as the object is in front of it. Essentially, it behaves like a lens or mirror with an infinite focal length ($f = \infty$). Since $P = 1/f$, if $f$ is infinite, then: $$P = \frac{1}{\infty} = 0$$ Therefore, a plane mirror has zero power. This means it doesn't converge or diverge light rays in the way that a curved mirror or lens does.
Think of it this way: the concept of "power" in optics relates to how much a surface bends light. * A convex lens/mirror diverges light, giving it negative power (or converging power). * A concave lens/mirror converges light, giving it positive power (or diverging power). A plane mirror is perfectly flat. When light hits it, the light rays reflect off at the exact same angle they came in (the Law of Reflection). The mirror doesn't bring the rays closer together or spread them farther apart; it simply redirects them. Because it causes no change in the convergence or divergence of the light (i.e., its focal point is infinitely far away), its optical power is defined as zero dioptres (0 D). It has no optical power to magnify or minify an image.