Question

Magnifying power of simple microscope is 6, when image is formed at distance of distinct vision, the focal length of simple microscope is:
A. 15 cm
C. 25 cm
C. 5 cm
D. 4.16 cm

Answer 1

Hint: Use a magnifying power equation to find the focal length of a simple microscope. For a human eye $25cm$ is the least distance of distinct vision. So, this will be helpful to choose the least distance of distinct vision in the magnifying power equation.

Complete step by step answer:
We can write the given details below,
Magnifying power of simple microscope,
$M=6$
The formula for magnifying power is given below.
$M=1+\dfrac{D}{f}$, where $D$ is the least distance of the distinct vision and $f$is the focal length of the lens.
Simple microscopes are constructed with convex lenses. Here we can take the least distance of distinct vision as $25cm$ since the least distance of distinct vision for a normal human being is $25cm$.
So, we can assign the given values into the equation of magnifying power.
$6=1+\dfrac{25}{f}$
Then focal length will be,
$f=\dfrac{25}{6-1}$
$f=5cm$

Thus, the correct option is C.

Additional information:
Simple microscope is commonly used to study small samples by making its magnified view. Enlarged image of this specimen made by a biconvex lens. If the distance between the sample and lens is less, better magnification can attain. Contrast of the image can be controlled by making slight changes to the light that we are using. It can be used in various fields including microbiology applications, study of soil, skin cells etc.

Note: Here we are taking the least distance of distinct vision as $25cm$. It is the minimum distance between the human naked eye and visible object, required for the clear vision. Convex lens can provide magnification. That’s why we are using convex lenses for microscope applications. Concave lenses can’t provide magnification and their images will be upright, virtual, size reduced and located at the same place where the object is situated.