Mirrors: Difference Between Plane & Spherical

Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

A mirror surface can produce clear reflections due to its smooth surface. Explore the difference between the two types of mirrors, plane and spherical, and the problem with aberration that is solved using parabolic mirrors. Updated: 10/28/2021

What Is a Mirror?

When you look at yourself in a mirror, you see a nice, clear image. It's quite different to your reflection on a shiny car. And most surfaces you see in everyday life aren't reflective at all. You certainly don't see your reflection in the kitchen table. That's because, while the table might feel smooth, it actually isn't. If you could look at most surfaces under a microscope, you would see a landscape of peaks and valleys.

A mirror is a surface that is smooth enough that it produces specular reflection - clear reflections - so you can see an image. But mirrors can be made into different shapes for different purposes. Two of those shapes are planes and spheres. So what's the difference between a plane mirror and a spherical mirror?

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  • 0:05 What is a Mirror?
  • 0:49 Plane Mirrors vs…
  • 2:03 Aberration & Parabolic Mirrors
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Plane Mirrors vs. Spherical Mirrors

A plane is a flat surface. So a plane mirror is just a smooth, mirrored surface that is completely flat. This stops the image you see from being distorted. The opposite of this might be a fun-house mirror, where the various bends and shapes can make the image look ridiculous.

A sphere is the 3D version of a perfect circle - it's a shape with a consistent curve all the way around and a constant radius. So a spherical mirror is a mirror that has a consistent curve and a constant radius of curvature - a sphere-shaped mirror.

Spherical mirrors can be convex or concave, depending on which side you put the mirrored surface on. A convex mirror is a spherical mirror where the mirrored surface is on the outside of the spherical curve. And a concave mirror is a spherical mirror where the mirrored surface is on the inside of the spherical curve.

Plane mirrors produce virtual, upright images that are the same size as the object. Virtual, by the way, just means the image is formed behind the mirror instead of in front of it.

Concave mirrors produce different kinds of images, depending on whether the object is placed further away from the mirror than the focal point or inside the focal point. And convex mirrors always produce images that are upright, virtual and smaller than the object.

Aberration & Parabolic Mirrors

One problem with spherical mirrors is that they produce distortion effects called aberrations. Spherical aberration is the effect that causes spherical mirrors to not focus parallel rays to the same point, producing a blurry image. This is especially apparent in astronomy because all objects in the sky are far enough away for the light rays to be parallel.

This would make spherical mirrors useless, especially for astronomy. But thankfully, there's a solution! When spherical aberration becomes too much, we make the mirrors parabolic in shape - the shape of a parabola or x-squared curve. This causes parallel rays to focus at a particular point, sharpening the image. All modern mirror-based telescopes use parabolic mirrors.

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