CBSE Class 12 Physics 2020 Outside Delhi Set 1 Paper - Question ...

CBSE Class 12 Physics 2020 Outside Delhi Set 1 Paper

© examsiri.com

Question : 34 of 37

Marks: +1, -0

Two objects

P

and

Q

when placed at different positions in front of a concave mirror of focal length

20 {\cm}

, form real images of equal size. Size of object

P

is three times the size of object

Q

. If the distance of

P

is

50 {\cm}

from the mirror, find the distance of

Q

from the mirror.

Solution:

Mirror formula :

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

\;\text " Magnification "\;=m=\;{h^{'}}/{h}=\;{v}/{u}

For object

{\P}

:

u_{ {\p}}\;=-50 {\cm}

f\;=-20 {\cm}

\;{1}/{v_p}+\;{1}/{u_p}\;=\;{1}/{f}

\;\text " Or, "\; \;\; \;{1}/{v_p}+\;{1}/{-50}\;=\;{1}/{-20}

\; ∴ v_P=-\;{100}/{3} {\cm}

\;m_{ {\P}}=\;{h_P^{'}}/{h_P}=\;{v_P}/{u_P}=\;{-{100}/{3}}/{-50}=\;{2}/{3}

For object

Q

:

m_{ {\Q}}\;=\;{h_{ {\Q}}^{'}}/{h_{ {\Q}}}=\;{v_{ {\Q}}}/{u_{ {\Q}}}

Or,

\;{h_p^{'}}/{{1}/{3} h_{ {\P}}}\;=\;{v_{ {\Q}}}/{u_{ {\Q}}}

∴ {v_{ {\Q}}}={u_{ {\Q}}}

\;\; \;{1}/{v_{ {\Q}}}+\;{1}/{u_{ {\Q}}}\;=\;{1}/{f}

\;{1}/{2 u_{ {\Q}}}+\;{1}/{u_{ {\Q}}}\;=\;{1}/{-20}

∴

u_{ {\Q}}\;=-30 {\cm}

So, the distance of

Q

from mirror is

30 {\cm}

.

© examsiri.com

Go to Question:

Prev Question

Next Question