Class 10 Physics



Image Formation by Concave Lens

There are only two possibilities of position of object in the case of a concave lens:

  • Object is at infinity
  • Object is between optical centre, O and infinity

Object is at infinity:

A highly diminished point sized, virtual and erect image is formed when object is at infinity by a concave lens at principal focus F1.

object at infinity concave lens

Fig: Object at Infinity

Properties of Image: Image is point sized, highly diminished, virtual and erect.


Object is between optical centre, O and infinity:

A diminished, virtual and erect image is formed between principal focus F1 and optical centre, O; when object is placed between optical centre and infinity of a concave lens.

object between infinity and O concave lens

Fig: Object between Infinity and O

Properties of Image: Image is diminished, virtual and erect.


Sign convention for lens:

Sign convention for lens is similar to that of spherical mirror. Signs are taken left of the optical centre as negative, right of the optical centre as positive, above of the principal axis as positive and below of the principal axis as negative.

sign convention lens

Fig: Sign Convention

The new sign convention is known as New Cartesian Sign Convention. In this sign is taken negative towards left and taken as positive towards right at X-axis from origin.

The sign is taken as positive (+) above the origin point at Y-axis; and below the origin point as negative (-) at Y-axis.