Image Formation by Concave Mirror at Different Positions of the Object
Introduction
A concave mirror is a spherical mirror whose reflecting surface curves inward. Because it converges parallel rays of light toward a common point, it is also known as a converging mirror.
Concave mirrors can produce both real and virtual images depending on the position of the object.
Basic Principles of Concave Mirrors
When parallel rays of light strike a concave mirror, they converge at a point called the focus after reflection.
The position of the image depends on the location of the object relative to the focus (F) and centre of curvature (C).
Important Terms
- Pole (P) – Centre of the mirror surface.
- Principal Axis – Imaginary line passing through the pole and centre of curvature.
- Focus (F) – Point where parallel rays converge after reflection.
- Centre of Curvature (C) – Centre of the sphere of which the mirror is a part.
Image Formation by Concave Mirror
Different image characteristics are observed when the object is placed at different positions along the principal axis.
Object at Infinity
When the object is extremely far away, parallel rays from the object converge at the focus.
Image Characteristics
- Image Position: At Focus (F)
- Nature: Real and Inverted
- Size: Highly Diminished (Point Sized)
Example
Observation of distant stars and celestial bodies through reflecting telescopes.
Object Beyond Centre of Curvature (C)
The image forms between the focus and the centre of curvature.
Image Characteristics
- Image Position: Between F and C
- Nature: Real and Inverted
- Size: Diminished
Object at Centre of Curvature (C)
The image is formed at the centre of curvature itself.
Image Characteristics
- Image Position: At C
- Nature: Real and Inverted
- Size: Same as Object
Object Between Centre of Curvature and Focus
The image is formed beyond the centre of curvature.
Image Characteristics
- Image Position: Beyond C
- Nature: Real and Inverted
- Size: Enlarged
Example
Projection of images onto a screen in laboratory demonstrations.
Object at Focus (F)
When the object is placed exactly at the focus, reflected rays become parallel.
Image Characteristics
- Image Position: At Infinity
- Nature: Real and Inverted
- Size: Highly Enlarged
Object Between Pole (P) and Focus (F)
The reflected rays diverge and appear to meet behind the mirror.
Image Characteristics
- Image Position: Behind the Mirror
- Nature: Virtual and Erect
- Size: Enlarged
Example
Makeup mirrors and shaving mirrors.
Summary Table of Image Formation by Concave Mirror
| Object Position | Image Position | Nature of Image |
|---|---|---|
| At Infinity | At Focus (F) | Point Sized, Real, Inverted |
| Beyond C | Between F and C | Diminished, Real, Inverted |
| At C | At C | Same Size, Real, Inverted |
| Between C and F | Beyond C | Enlarged, Real, Inverted |
| At F | At Infinity | Highly Enlarged, Real, Inverted |
| Between P and F | Behind Mirror | Enlarged, Virtual, Erect |
Applications of Concave Mirrors
- Reflecting telescopes
- Dental mirrors
- Shaving mirrors
- Makeup mirrors
- Solar furnaces
- Headlights and searchlights
- Scientific optical instruments
Frequently Asked Questions
What is a concave mirror?
A spherical mirror with a reflecting surface curved inward.
Why is a concave mirror called a converging mirror?
Because it converges parallel rays of light at a focus.
When is the image virtual in a concave mirror?
When the object lies between the pole and the focus.
Which image is formed when the object is at the centre of curvature?
A real, inverted image of the same size.
What are common uses of concave mirrors?
Telescopes, shaving mirrors, dental mirrors and solar concentrators.