Question

a concave lens of focal length 15cm form on image 10cm from the lens. How far is the object placed from the lens?​

Answer 1

Given,

focal lenght,f= -15cm

Distance of image,v= -10cm

Distance of object, u=?

we know that,

1/v - 1/u=1/f

so,

1/-10 -1/u=1/-15

so,-1/10+1/15=1/u

(-3 + 2)/30=1/u

-1/30=1/u

Therefore u= -30 cm

Thus object is placed 30cm away from the concave lens.

Negative sign shows that object is at 30cm in front of the lens.

Answer 2

$\huge\sf\underline{GIVEN:-}$

$\large\sf{f = -15cm}$

$\large\sf{v= -10cm}$

$\large\sf{u = ?}$

$\huge\sf{A/C,}$

$\longrightarrow$$\large\sf{\frac{1}{v}-\frac{1}{u}=\frac{1}{f}}$

$\longrightarrow$$\large\sf{\frac{1}{u}=\frac{1}{v}-\frac{1}{f}}$

$\longrightarrow$$\large\sf{\frac{-1}{10}-\frac{1}{(-15)}}$

$\longrightarrow$$\large\sf{\frac{-1}{10}+\frac{1}{15}}$

$\longrightarrow$$\large\sf{\frac{-5}{150}}$

$\longrightarrow$$\large{\boxed{\sf{u= -30cm}}}$