LENSES

Lenses are of two types, namely: converging lenses and diverging lenses. These two are easily distinguished by their shapes. Converging lens is thickest at the center while diverging lens is thinnest at the center. A converging lens brings light rays together. It is sometimes called as convex lens. A diverging lens, also called as concave lens spreads light rays apart.


IMAGES in CONVERGING LENSES

The following three rays are drawn to locate and determine the image formed in a converging lens:

  1. Ray 1 – A ray parallel to the principal axis is drawn towards the lens and is refracted passing through the focal point.
  2. Ray 2 – A ray is drawn passing through a focal point in front of the lens and extend the ray straight passing through the lens, parallel to the principal axis.
  3. Ray 3 – A ray passing straight through the center of the lens, never bends.

The properties of the images formed in a converging lens depends on the objects position.

  1. When the object is inside the focal point.
    1. Place the object between F and the lens.
    2. Draw rays 1, 2 and 3.
    3. Locate the image where the refracted rays intersect at a common point.


  1. When the object is at the focal point.
    1. Place the object at F.
    2. Draw rays 1, 2 and 3.
    3. Locate the image where the refracted rays intersect at a common point.


  1. When the object is outside the focal point.
    1. Place the object before F.
    2. Draw rays 1, 2 and 3.
    3. Locate the image where the refracted rays intersect at a common point.

  • The principal focus in the converging lens is real.

IMAGES in DIVERGING LENSES

The following three rays are drawn to locate and determine the image formed in a diverging lens:

  1. Ray 1 – A ray parallel to the principal axis is drawn towards the lens and is refracted passing through the focal point.
  2. Ray 2 – A ray is drawn passing through a focal point in front of the lens and extend the ray straight passing through the lens, parallel to the principal axis.
  3. Ray 3 – A ray passing straight through the center of the lens, never bends.

ADD IMAGE

  • The principal focus in the diverging lens is virtual.
  • For all position of the object, the image formed in a diverging lens is always virtual, erect and small and is always located between the principal focus and the optical center.