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### Course: AP®︎/College Physics 2>Unit 5

Lesson 1: Reflection and mirrors

# Spherical mirrors, radius of curvature & focal length

In this video, we will introduce the terms radius of curvature, and see its connection with focal length. We will also introduce the term principal axis and see the need for it.  Created by Mahesh Shenoy.

## Want to join the conversation?

• If the curvature of mirror is increased, will its focal length will also be increased?
• if curvatur is increased, focal length will decrease
• Great explanation! Physicists are wonderful at math and are logical thinkers. Not so great at writing. This chapter in my textbooks had my head spinning in circles!
• Mahesh shenoy
in video to you say that f, focal point is between centre of the circle and pole. why can't it be after the centre of the circle.
(1 vote)
• Think logically! The only way to make the focal length longer is making the mirror less curved. If we make it less curved, then the sphere which it is a part of, will also get bigger and hence keeping the center of curvature further than the focal point.
• How can a spherical mirror have a small width/aperture that is 2 or 3 times smaller than the focal length?
• a mirror can have an aperture smaller than the focal length , there are mirrors present where the f=10cm, and the aperture is 8cm!!
• At , is aperture the diameter of mirror from one end to another or the length of circumference of the mirror? I hope you get my point.
(1 vote)
• Aperture is the diameter of mirror/lens from one end to another.

Hope you get my point. :)
• I still don't get how to visualize the radius of curvature and center of aperture in a mirror. Can somebody please explain?
Thank You.
(1 vote)
• The centre of curvature is the centre of the sphere from which the mirror is cut.
The midpoint of the aperture is the pole of the mirror.
• how focal length decreases if curvature increases?
(1 vote)
• If the spherical mirror starts curving in then the radius of the sphere decreases which then affects the focal length too.
• if we decrease the curve of the mirror does it brings focal length nearer to the pole. Or it remains the same.
(1 vote)
• It brings focal point towards the pole.
(1 vote)
• @ How is the point on mirror is the pole of the mirror? Is the pole should be on the boundary of the mirror?
(1 vote)
• The centre of the reflecting surface of a mirror is called the Pole(P). It lies ON the mirror. This is different from the Centre of curvature(R). Each spherical mirror forms a part of an imaginary circle. The centre of this circle is called Centre of curvature. It DOES NOT lie on the mirror.
(1 vote)
• He mentioned that if a beam of light is parallel to the principal axis then its reflected beam would always reach the focus and he also said that if the beams of light are far away then they won't reach the focus. Isn't their irony in this sentence?
(1 vote)