Question

A convex and a convex lens have the same focal length of $20cm$ and put in contact to from a lens combination. The combination is used to view an object of $5cm$ length kept at $20cm$ from the lens combination. As compared to the object, the image will be

• A. magnified and ijnverted
• B. redused and erect
• C. of the same size as the object and would be erect
• D. of the same size as the object and would be inverted

Answer 1

The correct option is D of the same size as the object and would be inverted

The focal length of the convex lens

is positive and that of concave is negative,

hence, combination

$\begin{array}{cc}\frac{1}{f}& =\frac{1}{f_1}+\frac{1}{f_2}\\ \text{Given,}{f}_1=& f_2\\ \Rightarrow \frac{1}{f}& =\frac{1}{f_1}\frac{1}{f_1}=0\end{array}$

$\Rightarrow f=\infty$

The cambin ation has infinite

focal length. Hence, image of

object is of same size and errect

$\begin{array}{cc}\text{because,}& \\ \frac{1}{v}=\frac{1}{u}+\frac{1}{f}& [\frac{1}{f}=0]\\ \Rightarrow v=u& \\ \&\frac{v}{u}=m=1& \\ \therefore \text{option}c\text{is correct}& \end{array}$