Which of the following statements are correct?

(a) For a lens the first principal focus is the position of the object whose image is at infinity

(b) For a lens the first principal focus is the position of the object whose image is formed at twice the focal length.

(c) For a lens the second principal focus is the position of the real image whose object is at infinity

(d) For a lens the second principal focus is the position of the object whose image is formed at twice the focal length

The correct answer is Both (a) and (d).

Key Points

• Lens: The transparent curved surface used to refract the light and make an image of any object placed in front of it is called a lens.
  • Convex lens: ​A lens having two spherical surfaces, bulging outwards is called a double convex lens (or simply convex lens).
  • It is thicker in the middle as compared to the edges.
  • Convex lenses converge light rays; convex lenses are also called converging lenses.

• The lens formula is given by:

\(\frac{1}{v} - \frac{1}{u} = \frac{1}{f} \)

Where V = Distance of image from the optical centre, U = Distance of object from the optical centre.

• Depending upon the situation convex lens produces real and virtual images.

• The minimum distance between the and the real image in a convex lens is 4f. Hence option 4 is the answer.

Additional Information

• Nature, position, and relative size of the image formed by a convex lens for various positions of the object:-