- The image formed by a concave lens always has the same characteristics, namely
- virtual
- upright
- diminish
- Figure below shows the ray diagram for the formation of image of a concave lens.
Characteristics of the Image: Virtual, uprigh, magnified
Position of image: at the same side of the object
Lens
|
Power of the Lens
|
Converging (Convex) | Positive |
Diverging (Concave) | Negative |
Thick, with short focal length. | High |
Thin, with long focal length. | Low |
Thicker – Higher Power – Shorter Focal Length
Example:
The power of a lens is labeled as +5D. What is the focal length of the lens (in cm)? Is this a concave lens or a convex lens?
Answer:
Optical centre, P | Light passing through the central block emerges in the same direction as it arrives because the faces of this block are parallel. P marks the optical centre of the lens. |
Principle Axis | The principle axis of a lens is the line joining the centres of of curvature of its surfaces. |
Principle focus, F | The principle focus of a lens is the point on the priciple axis to which all rays originally parallel and close to the axis converge, or from which they diverge, after passing through the lens. |
Focal length, f | The focal length of a lens is the distance between the optical centre an the principle focus. |
The critical angle can be calculated by using the following equation:
or