A plano convex lens is made of material having refractive index 1.8. If the radius of curvature of the curved surface is 80 cm, then focal length of the lens is:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → 100 cm
The focal length of a thin lens follows from the lens maker's formula.
\( \displaystyle \frac{1}{f} = (n - 1)\left(\frac{1}{R_1} - \frac{1}{R_2}\right) \)
Taking light incident on the curved surface first: \( R_1 = +80 \) cm, and the plane surface has \( R_2 = \infty \), so \( 1/R_2 = 0 \).
\( \displaystyle \frac{1}{f} = (1.8 - 1)\times\frac{1}{80\,\mathrm{cm}} = \frac{0.8}{80\,\mathrm{cm}} \)
\( f = 100\,\mathrm{cm} \)
For a plano-convex lens this is \( f = R/(n-1) \), whichever face meets the light first.