CBSE Class 10 Science 2015 Term II Delhi Set 1

Question : 23 of 36

Marks: +1, -0

"A convex lens can form a magnified erect as well as magnified inverted image of an object placed in front of it." Draw ray diagram to justify this statement stating the position of the object with respect to the lens in each case.
An object of height

4 {\cm}

is placed at a distance of

20 {\cm}

from a concave lens of focal length

10 {\cm}

. Use lens formula to determine the position of the image formed.

Solution:

(i) Image formed is magnified and erect when-object is placed between optical centre and focus of a convex lens.

(ii) Image formed is magnified and inverted when the object is placed between

F

and 2F of a convex lens.

Given :

u=-20 {\cm}, f=-10 {\cm}, v=

?
We know that,
or

\;{1}/{f}=\;{1}/{v}-\;{1}/{u}

\;{1}/{v}=\;{1}/{f}+\;{1}/{u}

\;{1}/{v}\;=-\;{1}/{10}-\;{1}/{20}

\;{1}/{v}\;=\;{-2-1}/{20}=\;{-3}/{20}

∴ \;\; v\;=\;{-20}/{3} {\cm} .

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