Q: 56 (NDA-I/2017)
question_subject:
Maths
question_exam:
NDA-I
stats:
0,4,5,4,2,2,1
keywords:
{'double convex lens': [0, 0, 0, 1], 'refractive index': [0, 1, 1, 5], 'lens': [0, 0, 0, 1], 'glass': [0, 0, 1, 4], 'dioptre': [0, 0, 0, 5], 'cui': [0, 0, 0, 1], 'power': [24, 3, 21, 61], 'units': [1, 2, 4, 7], 'radii': [0, 0, 0, 2], 'cm': [2, 0, 7, 20]}
The power of a lens is defined as the reciprocal of its focal length (in meters), and it is measured in dioptres (D). The formula for calculating the power of a lens is:
Power = (refractive index - 1) * (1 / radius of curvature 1 - 1 / radius of curvature 2)
In this case, the refractive index is given as 1.5 and the radii of curvature of the faces of the lens are 10 cm and 20 cm.
First, we need to convert the radii of curvature from centimeters to meters. 10 cm is equal to 0.1 meters, and 20 cm is equal to 0.2 meters.
Using the formula, we can calculate the power:
Power = (1.5 - 1) * (1 / 0.1 - 1 / 0.2)
= 0.5 * (10 - 5)
= 0.5 * 5
= 2.5 D
So the correct answer is option 3: +2.5 D.