Join 60,000+ competitive exam aspirants
or with email
Your Cart
SubtotalтВ╣0
TotalтВ╣0
Examoogle тАв User тАв info@examoogle.com тАв EE-2024-8821
Chapter 1 of 12 тАв Page 1 of 248ЁЯФТ Protected PDF тАв Watermarked
Join 60,000+ competitive exam aspirants
A convex lens has a focal length of 20 cm. What is its power in dioptres?
+5 D
5 D
+0.2 D
0.2 D
+5 D
The power of a lens is defined as the reciprocal of its focal length when expressed in meters. Since the focal length of the convex lens is 20 cm, or 0.2 m, the power is calculated as P=f(m)1тАЛ=0.21тАЛ=+5 D.
The power of a lens is defined as the reciprocal of its focal length when expressed in meters. Since the focal length of the convex lens is 20 cm, or 0.2 m, the power is calculated as P=f(m)1тАЛ=0.21тАЛ=+5 D.
Think of the focal length as the 'reach' and power as the 'strength'; a lens with a shorter focal length bends light more sharply, thus possessing higher power, similar to how a tighter muscle can pull an object more effectively.
P = 1/f (m) - 'Power is the inverse of the focus when meters are in focus.'
P=f(m)1тАЛ тАФ Power calculation in Dioptres
f(m)=100f(cm)тАЛ тАФ Unit conversion from centimeters to meters
The power of a lens is a measure of its ability to converge or diverge light rays. According to the Cartesian sign convention, convex lenses have a positive focal length because the principal focus lies on the side opposite to the light source, resulting in a positive optical power. Dioptre (D) is the SI unit of power, where 1D=1mтИТ1.
The power of a convex lens is always positive (+).
The power of a concave lens is always negative (-).
Dioptre is defined for focal length measured strictly in meters.
Provides a simple, scalar value to describe lens performance.
Allows for easy addition of power when multiple lenses are placed in contact: PtotalтАЛ=P1тАЛ+P2тАЛ+...
Does not account for lens aberrations or thickness.
Only valid for paraxial rays near the optical axis.
Spectacle prescription design
Optical instrument calibration
The focal length 20 cm equals 0.2 meters.
Option B is incorrect because it ignores the sign convention required for convex lenses.
Options C and D are mathematically incorrect based on the reciprocal calculation.
A is correct тАФ The power of a convex lens is given by the reciprocal of its focal length in meters, resulting in +5 D.
Always convert cm to meters before calculating power, as the unit 'Dioptre' is defined specifically for meters; failing to do so is the most common error in RRB physics problems.
Get instant step-by-step AI explanations, full shift Previous Year Papers (SSC GD, RRB, SSC JE), and regional rank analysis on the official Examoogle App.
Free Download тАв 100K+ Aspirants