Question

The focal length of the convex lenses are 10cm and 20 cm .If we placed there two in contact with each other .The combined effect of the focal length is

Answer 1

we know that the focal length of a convex lens is positive so combined focal length would be

= 20cm+ 10cm  = 30cm

its should be converted into meters as power is inversely proportional yo the 1 meter of the focal length

= 0.3m

= 1/0.3

= 3.3D = POWER

Answer 2

Given,

Focal length (f₁) = 10 cm

Focal length (f₂) = 20 cm

To find,  

The combined effect of the focal length

Solution,  

We can simply solve this numerical problem by using the subsequent mechanism

Now,  

Focal length (f₁) = 10 cm

Focal length (f₂) = 20 cm

Combined focal length (f) =?

As we know that,

The Combined focal length is given by

⇒ 1/f = 1/f₁ + 1/f₂

⇒ 1/f = 1/10cm + 1/20cm

⇒ 1/f = 1/20 cm

⇒ f= 20 cm

Hence, The combined focal length is given by 20 cm