Question

Lenses of power 3D and - 5D are combined to from a compound lens.An object is placed at a distance of 50 cm from this lens. The position of its image will be;-

• A. - 10 cm
• B. + 10 cm
• C. -25 cm
• D. + 25 cm

Answer 1

The correct option is D + 25 cm

Power $P_1=+3Dand{P}_2=-5D$

Position of object $u=-50cm$

Power gets added

$P=P_1+P_2$

$P=3-5=-2$

$f=\frac{1}{P}=\frac{1}{-2}=-50cm$

Apply lens formula

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$

$v=\frac{uf}{f+u}=\frac{(-50)\times (-50)}{(-50)+(-50)}=+25cm$

Position of image is $v=+25cm$