We learned about Concave and Convex mirrors in the last post

But:

Did you know that Concave and Convex Mirrors are a part of a sphere?

Mirrors as a Part of Sphere - Teachoo.jpg

For these mirrors, we have some important terms like

  • Center of Curvature
  • Radius of curvature
  • Pole of the Mirror
  • Principal Axis
  • Focus of the mirror

 

Let's look at them one by one

 

Center of Curvature (C)

We can say that the mirror is a part of the hollow sphere

The center of this sphere is the Center of Curvature

It is represented by the letter C

Note: - Center of curvature of Concave mirror is on the front, and of convex mirror is on the back

Radius of Curvature

The radius of the hollow sphere which the mirror is a part of is the Radius of Curvature

It is represented by the letter R

Note : -  that R = PC

Pole of a Mirror(P)

Center of the spherical mirror is called Pole

It is represented by the letter P

 

Principal Axis

A straight horizontal line passing through pole and center of curvature

 

Principal focus

For principal focus, let's look at Concave and Convex Mirrors separately

 

Principal Focus (F) of concave mirror

The point at which

rays parallel to principal axis

coming from infinity converge

after being reflected from concave mirror

is called the principal focus of concave mirror.

Concave Mirror - Object at Infinity - Teachoo.jpg

Principal Focus (F) of convex mirror

The point at which

rays parallel to principal axis

coming from infinity appear to converge

after being reflected from convex mirror

is called the principal focus of convex mirror.

Convex Mirror - Object at Infinity - Teachoo.jpg

Note: - Focus of convex mirror is behind the mirror, so it is a virtual focus

Focal Length

It is distance between Pole (P) and Principal Focus (F)

It is represented by the letter f (small letter f)

 

Relationship between Radius of Curvature and Focal Length Of Spherical Mirror

The focal length is equal to half of the radius of curvature, for any spherical mirror.

Hence

Focal length = Radius of curvature / 2

f = R/2

 

Questions 

Question 1 Page 168 - Define the principal focus of a concave mirror.

View Answer

 

Question 2 Page 168 - The radius of curvature of a spherical mirror is 20 cm. What is its focal length?

View Answer

 

 

  1. Class 10
  2. Chapter 10 Class 10 - Light - Reflection and Refraction

About the Author

Davneet Singh's photo - Teacher, Computer Engineer, Marketer
Davneet Singh
Davneet Singh is a graduate from Indian Institute of Technology, Kanpur. He has been teaching from the past 9 years. He provides courses for Maths and Science at Teachoo.