Question

If a convex lens of power, +4D and a concave lens of power. -10D are placed in
contact with each other calculate their resultant of focal length​

Answer 1

$\red{\bigstar}\underline{\underline{\textsf{\textbf{ Given :- }}}}$

• A convex lens of power +4D and a concave lens of power -10D are placed in contact to each other.

$\red{\bigstar}\underline{\underline{\textsf{\textbf{ To Find :- }}}}$

• The resultant focal length .

$\red{\bigstar}\underline{\underline{\textsf{\textbf{ Solution :- }}}}$

The power of the two lens are +4 Dioptre and -10 Dioptre . We can find its resultant power by adding them as ,

$\sf\longrightarrow Power_{(net)}= 4D - 10D \\\\\\\sf\longrightarrow \red{ Power_{(net)}= -6\ Dioptre }$

We know that power of a lens is reciprocal of its focal length . Therefore ,

$\sf\longrightarrow Power =\dfrac{1}{Focal\ length } \\\\\\\sf\longrightarrow f =\dfrac{ 1 }{Power} \\\\\\\sf\longrightarrow f =\dfrac{1}{-6D} </p><p>\\\\\\\sf\longrightarrow \underset{\blue{\sf Required\ Answer }}{\underbrace{\boxed{\pink{\frak{ Focal\ length (f)= -0.166 \ m }}}}}$

Answer 2

Answer:

A

0.5 m

Power of the combination of lenses P

c

=P

1

+P

2

∴ P

c

=4−2=2D

Thus focal length of the combination F=

P

c

1

∴ F=

2

1

=+0.5m

Explanation:

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