Question 156: Ray Optics

Question

The graph shows how the magnification m produced by a thin lens varies with image distance v. What is the focal length of the lens used?
![](https://imagex.cdn.examgoal.net/3rsa0w2w9jwzlsxj3/7c26ab84-8b64-4969-85ad-0bd7769383da/a9ccf4f0-9091-11e9-86c1-73f7f991b766/file-3rsa0w2w9jwzlsxj4.png)

Accepted Answer

As the graph between magnification (m) and
image distance (v) varies linearly, then
m = k
1
v + k
2
$ \Rightarrow {v \over u} = {k_1}v + {k_2}$
$ \Rightarrow {1 \over u} = {k_1} + {{{k_2}} \over v}$
$ \Rightarrow {{{k_2}} \over v} - {1 \over u} = {k_1}$
Clearly, ${k_1} = {1 \over f}$ and  k
2
= 1 here
$ 	herefore f = {1 \over {slope\,of\,m - y\,graph}} = {b \over c}$

Answer

${{{b^2}} \over {ac}}$

Answer

${{{b^2}c} \over a}$

Answer

${a \over c}$

Answer

${b \over c}$

Ray Optics

The graph shows how the magnification m produced by a thin lens varies with image distance v. What is the focal length of the lens used?

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