Lesson 7 of 11 · 7 min
Power of a lens and combinations of thin lenses
NCERT § "Power of a Lens"
Kabir's lens is written up on an optician's slip as "+10 D". The same slip could turn it into a weaker lens with just one more piece of glass.
The lesson in notes
In short
The power of a lens is the reciprocal of its focal length in metres, P = 1/f. Its SI unit is the dioptre (D), 1 D = 1 m⁻¹.
A converging lens has positive power; a diverging lens has negative power.
Two thin lenses in contact behave as one lens with 1/f = 1/f₁ + 1/f₂, so powers add: P = P₁ + P₂ + ….
The total magnification of a combination is the product of the individual magnifications: m = m₁m₂m₃….
Lenses are combined in instruments to raise magnification and to reduce certain aberrations; a combination also lets designers make lenses of the required power.
For lenses separated by a distance, find the image formed by the first lens and use it as the object (real or virtual) for the next lens, measuring each distance from that lens.