Question

what is the effective focal length of the convexlenses of focal length 10cm and 25cm respectively are. joined together
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Answer 1

$\huge\underline\blue{\sf Answer:}$

$\large\red{\boxed{\sf f_{net}=+7.14\:cm}}$

$\huge\underline\blue{\sf Solution:}$

$\large\underline\pink{\sf Given:}$

• Lenses are Convex lens

• Focal length of one convex lens ($\sf{f_1}$)=10 cm

• Focal length of other convex lens ($\sf{f_2}$)=25 cm

$\large\underline\pink{\sf To\:Find:}$

• Effective focal length ($\sf{f_{net}}$)=?

________________________________

We know that ,

$\large{♡}$$\large{\boxed{\sf f_{net}={\frac{f_1f_2}{f_1+f_2}}}}$

On putting value :

$\large\implies{\sf {\frac{10 \times 25}{10+25}}}$

$\large\implies{\sf {\frac{250}{35}}}$

$\large\implies{\sf f_{net}=7.14\:cm}$

$\huge\red{♡}$$\large\red{\boxed{\sf f_{net}=+7.14\:cm}}$

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Extra info.

• If there is convex lens then take focal length as = positive (+)

• If there is concave lens then take focal length as = negative (-)