JEE Advanced 2026 — Paper 1 — PHY-I solution

JEE Advanced2026Paper 1PHY-I

A double convex lens made of glass of refractive index $1.5$ and radii of curvature of the curved surfaces $20$ cm each is immersed in a liquid of refractive index $n_{L}$ . The correct plot showing the variation of the power, in the units of diopter (D), as a function of $n_{L}$ is :

Solution

Lensmaker's formula in a medium. $\frac{1}{f} = (\frac{n _{glass}}{n _{L}} - 1) (\frac{1}{R _{1}} - \frac{1}{R _{2}})$ .

With $R_{1} = + 20$ cm $= 0.20$ m and $R_{2} = - 20$ cm $= - 0.20$ m:

$\frac{1}{f} = (\frac{1.5}{n _{L}} - 1) (\frac{1}{0.20} - \frac{1}{- 0.20}) = (\frac{1.5}{n _{L}} - 1) \cdot 10 m^{- 1}$ .

$P = n_{L} / f$ (used to define power inside a medium): $P = (1.5 - n_{L}) \cdot 10$ , a straight line of slope $- 10$ passing through zero at $n_{L} = 1.5$ , giving $+ 5$ D at $n_{L} = 1.0$ and $- 5$ D at $n_{L} = 2.0$ . This matches option (B).

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