What Is the Difference Between a Spherical & Parabolic Mirror?

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The difference between a spherical and parabolic mirror is one that is relatively easy to express. Learn about the difference between a spherical and parabolic mirror with help from a research scientist and one of the world's leading experts on star formation in this free video clip.

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Video Transcript

Hello, my name is Harold Yorke, I'm a research scientist in Pasadena, California. And I'll be talking about the differences between spherical and parabolic mirrors. Curved mirrors are used n astronomy and also in communication, in microwave communication to focus light or microwaves. And two of the popular forms, are the so called spherical form and the parabolic form. The spherical form is, as if you had cut a hole in a sphere and taken out part of the sphere. And it will be curved like this, where each part of the mirror is the same distance from a given point. A parabolic mirror on the other hands, for short distances, looks like a spherical mirror. But the edges will resemble a, more of a parabola, which is a different geometric form. How these mirrors focus light, are shown in these examples. If you have light coming from a distance, a star, it bounces off this shiny mirror surface and is focused more or less into a point. The advantage of a parabolic mirror, is if your light is coming from a star, and is parallel to the so called optical axis, which I've drawn in red. Then, all of the light rays will indeed focus into one single point, and you have the sharpest object possible. The spherical mirror on the other side, on the other hand would have a fuzzy focus point. Because depending on where the ray hits the mirror, it doesn't quite focus in the same spot. The advantage of this however, is if we have light coming from a slightly different direction, the spherical mirror still looks spherical. And so, it will focus in a different point and it'll be just as fuzzy as before. This is called spherical aberration. But it's over a wide field and over large differences in direction, that you can get the same fuzzy spot. The parabolic mirror on the other hand, becomes much worse, if you try to look at an object which is not parallel to this optical axis. So, the summary of this is, this is good for the radio and spherical mirrors are good for optical UV and infrared telescopes, if you want to have a wide field of view. Now, in astronomy, there are special purpose mirrors that are being constructed to solve specific problems. And sometimes you may decide to choose a parabolic shape, a spherical shape or even something in between. Because there's a lot of corrections you can do with adaptive optics and with deformable mirrors that you put into the stream of, of light. Thank you.


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