Question
Define power of lens, obtain its equation and write its unit.
✓
Answer
→Power of a lens is a measure of the convergence or divergence, which a lens introduces in the light falling on it.
→Clearly, a lense of shorter focal length bend the incident light more, while converging it in case of a convex lens and diverging it in case of a concave lens.
→"The tangent of the angle by which it converges or diverges a beam of light parallel to the principal axis falling at unit distance from the optical centre is defined as the power P of a lens."
→From figure, $\tan \delta=\frac{h}{f}$;
if $h=1$ then $\tan \delta=\frac{1}{f}$ or $\delta=\frac{1}{f}$
for small value of $\delta$. Thus
power $P =\frac{1}{f}$
→The SI unit for power of a lens is dioptre (D). $\therefore \quad 1 D =1 m^{-1}$.
→Power of a lens is positive for a convex lens and negative for a concave lens.
→When an optician prescribes a corrective lens of power +2.5 D , the required lens is a convex lens of focal length +40 cm . A lens of power of -4.0 D means a concave lens of focal length -25 cm .
→Clearly, a lense of shorter focal length bend the incident light more, while converging it in case of a convex lens and diverging it in case of a concave lens.
→"The tangent of the angle by which it converges or diverges a beam of light parallel to the principal axis falling at unit distance from the optical centre is defined as the power P of a lens."
→From figure, $\tan \delta=\frac{h}{f}$;
if $h=1$ then $\tan \delta=\frac{1}{f}$ or $\delta=\frac{1}{f}$
for small value of $\delta$. Thus
power $P =\frac{1}{f}$
→The SI unit for power of a lens is dioptre (D). $\therefore \quad 1 D =1 m^{-1}$.
→Power of a lens is positive for a convex lens and negative for a concave lens.
→When an optician prescribes a corrective lens of power +2.5 D , the required lens is a convex lens of focal length +40 cm . A lens of power of -4.0 D means a concave lens of focal length -25 cm .
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