Refraction of Light

When an object is placed in front of a lens, light rays coming from the object fall on the lens and get refracted. The refracted rays produce an image at a point where they intersect or appear to intersect each other. The formation of images by lenses is usually shown by a ray diagram. To construct a ray diagram we need at least two rays whose paths after refraction through the lens is known. Any two of the following rays are usually considered while constructing ray diagrams.

• A ray of light passing through the optical centre of the lens travels straight without suffering any deviation. This holds good only in the case of a thin lens.

  

• An incident ray parallel to the principal axis after refraction passes through the focus.

  

• An incident ray passing through the focus of a lens emerges parallel to the principal axis after refraction.

  

  The nature of images formed by a convex lens depends upon the distance of the object from the optical centre of the lens. Let us now see how the image is formed by a convex lens for various positions of the object.

  Formation of Image by a Convex Lens

  In each ray diagram two rays are drawn from the top A of an object AB and the image A' B' is formed at the point of intersection of these two rays.

  When the Object is Placed Between F₁ and O

  

   The image is -

• Formed behind the object
• virtual
• erect
• magnified

When the Object is placed at 2F₁

The image is -

• formed at 2F₂
• real
• inverted
• same size as the object

When the Object is Placed Between F₁ and 2F₁

The image is

• formed beyond 2F₂
• real
• inverted
• magnified

When the Object is Placed at F₁

The image is -

• formed at infinity
• real
• inverted
• magnified

When the Object is Placed Beyond 2F₁

The image is -

• formed between F₂ and 2F₂
• real
• inverted
• diminished

When the Object is placed at Infinity

When the object is at infinity, the rays coming from it are parallel to each other.

The image is -

• formed at F₂
• inverted
• real
• highly diminished

The table gives at a glance the position, size and nature of the image formed by a convex lens corresponding to the different positions of the object and also its application.

Position of the object	Position of the image	Nature of the image	Size of the image	Application
Between O and F1	On the same side of the lens	Erect and virtual	Magnified	Magnifying lens (simple microscope), eye piece of many instruments
At 2F1	At 2F2	Inverted and real	Same size	Photocopying camera
Between F and 2F1	Beyond 2F2	Inverted and real	Magnified	Photographic camera, terrestrial telescopes,  Photocopier used for replication
At F1	At infinity	Inverted and real	Magnified	Search lights
Beyond 2F1	Between F2 and 2F2	Inverted and real	Diminished	objective lens of a telescope
At infinity	At F2	Inverted and real	Diminished	photographic camera

Convex lens is also used in spectacles the correct the eye defect hypermetropia.