The power of a lens is taken to mean the converging or diverging ability of the lens to refract light. The greater the power, the stronger the lens’s ability to bend light rays.
Power of a lens P = 1
___ f
P = power (m−1), f = focal length of lens (m) By convention:
• converging (convex) lenses have a positive power rating • diverging (concave) lenses have a negative power rating.
Combining lenses
To calculate the total converging or diverging power of a set of lenses, we add the separate powers. The formula to calculate power in lenses in contact (combining lenses) is:
Power of lenses in contact Ptotal = P1 + P2
Ptotal = total power of combination (m−1), P1 = power of first lens (m−1), P2 = power of second lens (m−1)
The power of a lens is its ability to converge or diverge light rays. It is measured in m−1.
To find the focal length of the combined lens system, use this formula:
Focal length of combined lenses 1
_______ ftotal
= 1
___ f1
+ 1
___ f2
ftotal = focal length of the combined system (m), f1, f2 = focal lengths of lens 1 and lens 2 (m) The sign of the answer indicates whether the set of lenses converges or diverges light:
• if the result is positive, the lenses act as a single convex (converging) lens • if the result is negative, the lenses act as a single concave (diverging) lens.
