Question

A convex lens forms a virtual image when an object is placed at a distance of 18cm from it. The focal length must be :
a) greater than 36cm
b) greater than 18cm
c) less than 36cm
d) less than 18cm​

Answer 1

Answer:

A convex lens forms a virtual image when an object is placed at a distance of 18 cm from it. The focal length must be. ... greater than 18 cm

Answer 2

The focal length must be greater than 18cm. (option b)

To properly understand this this question, we have to know more about the focal length of the convex length.

Focal length :

• Focal length of a convex length is basically the distance between the optical centre of the lens and the the focus of the lens.
• The focal length is important while determining the position of the object w.r.t the lens, in order to get the required image. (properties of the image changes with the different positions of the object)

Furthermore,

• In most cases the convex lens produces a real image. But, in only one case, when the object is placed at distance less than the focal length, a convex lens produces a virtual (erect and magnified) image.
• So, the distance at which the object is placed to generate a virtual image by a convex lens, is certainty less the the focal length of that convex lens.
• Which implies, in the given case 18cm (object's distance from lens) is less than the focal length as the said position is producing virtual image. So, we can say that the focal length of the lens is greater than 18cm.

Hence, the focal length is greater than 18cm and the reason behind the such focal length is elaborated in the above mentioned discussion.