Spherical mirrors and lenses have made a very important and integral part of life. Many of us might be visionless without these mirrors and lenses. Concave and convex mirrors are most commonly used spherical mirrors .Learn about concave mirror in this article.
Click here to learn about convex mirrors.
Define Concave Mirror
If a hollow sphere is cut into parts and the outer surface of the cut part is silvered, then it becomes a mirror with its inner side as the reflecting surface. This type of mirror is known as a concave mirror.
Sign Convention for Concave Mirrors
Directions taken are as follows and shown in figure below:
- Distances behind silvered face of mirror are positive.
- Distances in front of reflecting face are negative.
- Height above principal axis is positive.
- Height below principal axis is negative.
Define The Principal Focus of a Concave Mirror
Light rays that are parallel to the principal axis converge at a specific point on its principal axis after reflecting from the mirror. This point the principal focus of the concave mirror. Point F is principal focus in the image below.
Note that, focal length for concave mirrors are negative.
Radius of Curvature and Focal Length of Concave Mirror
Radius of the sphere of which mirror is a part is the radius of curvature. In simple concave mirrors (covered in this blog) focal length is half of radius of curvature.
Thus, f = c/2
Image formation by Concave Mirror
| S. No. | Object Position | Image Position | Image Characteristics |
|---|---|---|---|
| a | At infinity | At Focus | Highly Diminished Real Inverted |
| b | Beyond Centre of Curvature | Between F and C | Diminished Real Inverted |
| c | At Centre of Curvature | At Centre of Curvature | Same Size Real Inverted |
| d | Between C and F | Beyond C | Beyond C Real Inverted |
| e | At Focus | At Infinity | Highly Enlarged Real Inverted |
| f | Between F and P | Behind the Mirror | Magnified Virtual Erect |
Ray Diagrams
Characteristics of Concave Mirror
- Light converges at a point when it strikes and reflects back from the reflecting surface of the concave mirrors. Hence, the name converging mirror.
- When we place the concave mirror very close to the object, we obtain a magnified and virtual image.
- The image formed by the concave mirrors can be small or large or can be real or virtual.
Uses of Concave Mirror
Common Uses
Concave mirrors is used in
- Searchlights
- Shaving mirrors
- Satellite dishes
- Reflectors in torches and headlights
Uses of Concave Mirrors in Science.
- Used in microscopes as condensers.
- Reflecting telescopes have concave mirrors to collect light from distant sources.
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