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A student measures the thickness of a human hair by looking at it through a microscope of magnification \[100\]. He makes \[20\] observations and finds that the average width of the hair in the field of view of the microscope is \[3.5mm\] . What is the estimate on the thickness of hair? 
A) $0.035mm$
B) $0.007mm$
C) \[3.5mm\]
D) None of the Above

Answer
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Hint: Start by describing a microscope and the magnification it produces. Then move on to describe the relation between width of actual hair, observed width and the magnification of a microscope. Use this concept to reach the solution.

Complete Step By Step Answer:
A microscope is an optical instrument, specifically designed to view objects that are too small to be seen by the naked eye. A microscope magnifies an object and to achieve this microscopes use a combination of lenses (convex or concave) to form a highly enlarged image.
A point to note in the given problem is that the students made \[20\] observations and calculated the average observed width of the hair. This is a good example of how to make a scientific observation. We have to take multiple observations and then calculate an average.
Let the original width of hair be $W$.
In this problem, it is given that the microscope has a magnification$(M)$ power of $100$ and the observed width of hair is $3.5mm$.
We know that, Width of magnified hair $ = W \times M$
$ \Rightarrow 3.5 = W \times 100$
$ \Rightarrow W = \dfrac{{3.5}}{{100}}$
$ \Rightarrow W = 0.035mm$
Hence, option (A) is the correct choice.

Note: We have multiple types of microscopes, most common are the light microscope and electron microscope. As the name suggests light microscope uses light to form the image and electron microscope uses a beam of light. The light microscope has a general magnification of 40x-100x and an electron microscope can achieve a magnification of up to 10,000,000x.
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A student measures the thickness of a human hair by looking at it through a microscope of magnification \[100\]. He makes \[20\] observations and finds that the average width of the hair in the field of view of the microscope is \[3.5mm\] . What is the estimate on the thickness of hair? 
A) $0.035mm$
B) $0.007mm$
C) \[3.5mm\]
D) None of the Above

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NCERT EXERCISE 1.7 | NCERT Solution for Class 11 Physics Chapter 1 | Units and Measurement NCERT
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