Lens

Technical Terms

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 Power of a Lens

The power of a lens is defined as the reciprocal of the focal length in unit meter.

Unit: The unit of power is diopter (D)

Focal length: Shorter focal length (f) - Higher Power

Thickness: Thicker lens - Higher power

Power - Positive or Negative

Convex lens has positive power, hence it is also called a positive lens (or converging lens).

Concave lens has negative power, hence it is called a negative lens (or diverging lens).

Ray Diagram 1

A ray of light through the optical centre of the lens, P. This passes through the lens unbent.

Ray Diagram 2

A ray of light parallel to the principal axis. This passes through F when it leaves the lens.

Ray Diagram 3

A ray of light through F. This leaves the lens parallel to the principal axis. It is equivalent to ray diagram r 2 in reverse.

Ray diagram - Image Form by a Convex Lens

Ray diagram - Image Form by a Concave Lens

Properties of Image of Convex Lens

Figure above shows the summary of the properties of images formed by a convex lens when the object is placed at different posotions.

Properties of Image of Concave Lens

The image form by a concave lens is always:

• Virtual
• Upright
• Diminish (Smaller)

The Lens Equation

Linear Magnification