Test: Spherical Lens & its Power - UPSC MCQ

Net power =0
F=infinity 
The emergent ray in the rectangular glass slab is parallel to the incident ray so the focal length of the glass slab is infinite.

So  focal length = - 50 cm = - 0.50 meter
power = - 1/0.50 = -2 Dioptre or - 2 D

Power of a lens is always reciprocal to the focal length. So,  a convex lens with 50 cm focal length has a power of :- 1/0.5 
⇒  2D

Power of a Lens
The power of a lens is its ability to bend light – the greater the power the greater the refraction of light.

The power of a lens is also related to the focal length by the following formula:

Power of a lens is measured in Dioptres (D).

Focal length is measured in metres (m).

The correct option is C.
The simplest compound microscope is constructed from two convex lenses. The objective lens is a convex lens of short focal length (i.e., high power) with typical magnification from 5× to 100×. The eyepiece, also referred to as the ocular, is a convex lens of longer focal length.

The focal length of concave mirror and lens is always negative.

Aperture of a lens is the length of the surface of lens exposed for incident light. Focal length is the distance between focus of the lens and lens optical center. There is no term like efficiency of a lens.

Power one of lens = P1 = +3.5D (Given)
Power two of lens = P2 = -2.5D (Given)
Power combination = P1 + P2

Thus,
Power of combination = +3.5 D +(-2.5 D )
= 3.5 D -2.5 D
= 1 D

Power = 1/focal length(m)  
 1 = 1/f
 f = 1/1
 f = 1 metre
Thus, the  f of combination = 1×100 = 100 cm

Therefore, if two thin lenses of power +3.5D and -2.5D are placed in contact then the power and focal length of the lens combination will be 100cm.