Lesson 7 of 11 · 10 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.
Simulations in this lesson
Each on its own page