Question

A simple microscope has
A) 2 convex lens
B) 2 concave lens
C) 1 convex lens
D) 1 concave lens

Answer 1

Hint: A simple microscope is used to see the image of an object in a magnified form. A convex lens is called a magnifying glass. Human eye and a simple microscope can be related because both use the same type of lens.

Complete answer:
We know that a human eye consists of a convex lens. A convex lens produces a virtual, erect and enlarged image of an object when the object is placed within the focal length of the lens. It is due to this property of convex lens that it gives it the name magnifying glass.
A simple microscope is similar to a human eye. It uses a single converging lens to get an enlarged image of an object. It is mainly used in the field of microbiology to see enlarged images of tiny organisms like fungus. Magnification of a simple microscope can be defined as the ratio of the height of the image to that of the object. After providing suitable substitutions for the heights of image and object, magnification of a simple microscope is mathematically derived as:
$M=1+\dfrac{D}{f}$
where
$M$ is the magnification caused by a simple microscope
$D$ is the least distance of distinct vision
$f$ is the focal length of the convex lens used in a simple microscope
Least distance of distinct vision is nothing but the minimum distance at which an object can be distinctly seen without strain. For a human eye, the least distance of distinct vision $(D)$ is $25cm$.
A simple microscope uses the property of a convex lens to produce virtual, erect and enlarged images when an object is placed within the focal length of the lens. So, it is important that the object and the convex lens are very close to each other.
The ray diagram of a convex lens when an object is placed within the focal length is shown below.

The ray diagram shows the image formation by a convex lens of focal length(f), when the object distance(do) is within the focal length.
It can be seen that the height of the image(hi) is greater than the height of the object(ho).
Note that the magnification(M) is also provided in the diagram.
It is clear from the diagram that the image of the object is enlarged, erect(upright), virtual and forms on the other side of the lens, when viewed through the eyepiece of the simple microscope.
To conclude, a simple microscope uses a single convex lens.

So option C gives the correct answer for the question given above.

Additional information:
While a simple microscope uses a single convex lens to produce enlarged images, a compound microscope uses two or more convex lenses to produce enlarged images. A simple microscope enlarges an object by 300 times whereas a compound microscope enlarges an image by 2000 times. Compound microscopes are used in the field of advanced scientific research.

Note:
Students need not get confused with the different names given to a convex lens. A convex lens is sometimes called a double convex lens because it has two convex surfaces. It is also called a biconvex lens due to the same reason. Other names of convex lens include converging lens, magnifying glass and reading glass.