Image Formation by Lenses

A lens bends light at its two curved surfaces. By following how a few special rays travel through the lens, I can locate the exact position and nature of the image. For most cases, drawing just two rays is enough:

• A ray parallel to the principal axis.
• A ray passing through (or appearing to pass through) the focus.
• A ray passing through the optical centre (goes undeviated).

The point where the refracted rays meet (or appear to meet) gives the image location.

A convex lens can form real or virtual images depending on where the object is placed. It can also magnify or reduce the object. The behaviour changes smoothly as the object moves along the axis.

A concave lens always spreads out rays. Because of this, the refracted rays never meet on their own — only their backward extensions meet. So the image is always virtual.

Lens diagrams become easy if I stick to a few standard rays:

• Ray 1: Parallel to principal axis → passes through F (convex) or appears from F (concave).
• Ray 2: Passing through the optical centre → travels straight.
• Ray 3: Passing through F → emerges parallel.

Using any two of these rays gives the correct image position.

Suppose an object is placed between F and 2F of a convex lens:

• The refracted rays meet beyond 2F.
• The image is real, inverted and magnified.

If the same object is placed in front of a concave lens at any distance:

• The rays diverge.
• The extended rays meet between O and F.
• The image is always virtual, upright and smaller.

Image formation by lenses is the basis of:

• Magnifying glasses
• Eyeglasses
• Cameras
• Microscopes
• Telescopes

Each device works by positioning objects and images strategically with convex or concave lenses.