What makes mirrors reflective

Peter N. Saeta, an assistant professor of physics at Harvey Mudd College, responds: "The oscillating electrical field in the incoming light wave produces a force on the charges inside the mirror.

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Create Account See Subscription Options. Continue reading with a Scientific American subscription. Subscribe Now You may cancel at any time. Most objects absorb some colors and reflect others, giving rise to our perception of the color properties of things. For example, when light hits a banana, it absorbs every color except for yellow, which it reflects, making the banana appear yellow.

You might also remember from school that, much like mirrors, white objects like a piece of printer paper or a white wall reflect all the colors of the visible spectrum.

When rays of light hit rough surfaces, they bounce the light back in all directions. This is called diffuse reflection. Metal and glass, meanwhile, are very smooth, and reflect light back more directly. This is called specular reflection. If the mirror is convex, or curved outward, it will reflect a wider area, in which images appear smaller and farther away than those from a flat mirror.

These mirrors are often used for outside rearview mirrors on cars and for keeping large areas under surveillance in stores. If the surface is concave, or curved inward, a group of light rays from a distant source is reflected back toward a single location known as the focal point. This generally produces a magnifying effect, such as that seen in a makeup mirror. The radius of curvature of a mirror determines its magnification factor and its focal length.

Newton used a concave spherical mirror to make his reflecting telescope , a design that is still popular with amateur astronomers due to its simplicity, low cost and high degree of image quality. In a Newtonian reflecting telescope, light rays from distant objects, which are essentially parallel because they come from so far away , strike the concave main mirror at the same angle. The rays are then reflected back up through the telescope tube toward the focal point.

However, before they reach the focal point, they strike a secondary, flat mirror that is tilted at a degree angle. The secondary mirror diverts the light out through a hole in the side of the tube. The eyepiece lens then focuses the light. This produces a magnified image. Also, the image appears much brighter than it does to the naked eye because the mirror gathers and concentrates the light. The shape of a spherical mirror affects the image that is reflected. Light striking near the edge of the mirror does not focus at the exact same spot as light striking nearer to the center.

This results in what is called spherical aberration. This phenomenon is often corrected by using a combination of lenses, or in the case of large telescopes, by using parabolic mirrors, which are shaped like rounded cones that focus all the light from a source to a single point.

Refraction is the bending of light rays. Normally, light travels in a straight line, and changes direction and speed when it passes from one transparent medium to another, such as from air into glass.

In a vacuum, the speed of light , denoted as "c," is constant. However, when light encounters a transparent material, it slows down.