Question

2 lenses of power +8D and -3D are placed in contact with each other, the focal length of the lens combination is:

Answer 1

Given :

➳ Two lenses of power +8D and -3D are placed in contact with each other.

To Find :

➢ Focal length of the lens combination.

SoluTion :

✴ Eq. power of the combination :

⇒ P = P¹ + P²

⇒ P = (+8) + (-3)

⇒ P = +5D

✴ Focal length of the combination :

➨ f = 1/P

➨ f = 1/5

➨ f = 0.2m = 20cm

Learn More :

↗ The equivalent focal length of thin lenses in contact is given by

• 1/f = 1/f¹ + 1/f² + 1/f³ ...

where, f¹, f², f³ are focal lengths of individual lenses.

Answer 2

$\blue{\bold{\underline{\underline{Given}}}}$

• 2 lenses of power +8D and -3D are placed in contact with each other.

$\blue{\bold{\underline{\underline{To \: Find}}}}$

• Focal length of the lens combination.

$\blue{\bold{\underline{\underline{Solution}}}}$

★ Power of the combination :

We know that,

P = P¹ + P²

=> P = (+8) + (-3)

=> P = +5D.

★ Focal length of the combination :

We know that,

f = 1/P

=> f = 1/5

=> f = 0.2m = 20cm.