Find the focal length (in cm) of a convex lens if object is place

Find the focal length (in cm) of a convex lens if object is place
| Find the focal length (in cm) of a convex lens if object is placed at 10 cm from the lens and image is obtained at 50 cm from the lens on the same side?

A. 12.5

B. 25

C. 10

D. 7.5

Please scroll down to see the correct answer and solution guide.

Right Answer is: A

SOLUTION

CONCEPT:

  • Converging lens: A lens in which light rays enter into it parallel to its axis and converge at a single point on the opposite side.

 

  • Focal length: When light passes through a lens, the measure of how strongly the system converges or diverges the light.
    • focal length is the inverse of the system's optical power.
  • Formula: The following formula is used to form a relation between image, object and focal length:

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

where f is focal length v is the distance of the image from the lens, and u is the distance of the object from the lens.

CALCULATION:

Given that u = -10 cm and v = -50 cm.

\(\frac{1}{f}=\frac{1}{-50}-\frac{1}{-10}\)

\(\frac{1}{f}=\frac{4}{50}\)

f = 12.5 cm

So the correct answer is option 1.

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