Image formation by Concave Lens

(i) When the object is at infinity

→ Image is formed at focus F1
→ Image is highly diminished or point sized
→ Image is virtual and erect

 (ii) When the object between infinity and optical centre O of lens

→ Image is formed between focus F1 and optical centre O
→ Image is diminished
→ Image is virtual and erect

Uses of Spherical lenses
In photographic cameras, magnifying glass, microscope, telescope, the human eye.

Sign convention for Spherical lenses

  • All distances, object distance (u), image distance (v) and focal length f are measured from the optical centre.
  • The distances measured in the direction of incident ray are taken as positive and distances measured against the direction of incident ray are taken as negative.
  • All distances (heights) of objects and images above principal axis are taken as positive and those below the principal axis are taken as negative.

 For the two lenses, the sign conventions take the form

  • u is- ve, if the object is in front of the lens. (Real object)
  • u is +ve, if the object is virtual.
  • v is – ve, if the image is on the same side as that of the object. (Virtual image )
  • v is +ve, if the image is real.
  • Focal length of a concave lens is taken as – ve.
  • Focal length of a convex lens is taken as +ve.

Lens formula  

Magnification

Power of a lens: The degree of convergence or divergence of light rays is expressed in terms of power. It is given as: P = 1/f 
→ The SI unit of power is dioptre (D).
→ Power of convex lens is positive.
→ Power of concave lens is negative.