CBSE Class 10 Science 2019 Delhi Set 1

Question : 19 of 28

Marks: +1, -0

An object is placed at a distance of

60 {\cm}

from a concave lens of focal length

30 {\cm}

.
(i) Use lens formula to find the distance of the image from the lens.
(ii) List four characteristics of the image (nature, position, size, erect/inverted) formed by the lens in this case.
(iii) Draw ray diagram to justify your answer of part (ii).

Solution:

(i)

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

[lens formula]

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

u\;=-60

f\;=-30

(In case of Concave lens)
Hence, putting the values in formula, we get

\;{1}/{-30}+\;{1}/{-60}\;=\;{1}/{v}

\;{-2-1}/{60}\;=\;{1}/{v}

\;{-3}/{60}\;=\;{1}/{v}

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

∴ \;\; v\;=-20 {\cm}

∵ \;\; m\;=\;{v}/{u}=\;{-20}/{-60}=0.33

(ii) So, the image formed will be virtual, erect, small in size and image will be formed

20 {\cm}

from lens on the same side as the object

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