HardPhysicsHigh School (11–12)OpticsNumericalpython

A thin biconvex lens of glass with refractive index 1.50 has a focal length of 15 cm in air…

Try it on paper first. A hint, the key idea and the worked solution are here when you need them.

A thin biconvex lens of glass with refractive index $1.50$ has a focal length of $15\ \text{cm}$ in air. A teacher dips it, in turn, in water of refractive index $1.33$ and in a dense oil of refractive index $1.62$ . The lens is held with its axis along a laser beam that is wide and parallel. Find the focal length of the lens in the water and in the oil, and the power of the lens in the oil, in dioptre. Say whether the oil lens converges or diverges the beam, and say in which liquid of what refractive index the lens would have no effect at all.

Need a reminder? The key ideaShow

With the object and the screen $D$ apart, a lens that gives a sharp image at two positions $d$ apart has $f = \dfrac{D^2 - d^2}{4D}$ . The two magnifications are $m$ and $\dfrac{1}{m}$ , so $h = \sqrt{h_1h_2}$ . In a liquid, $\dfrac{1}{f_m} = \left(\dfrac{n_l}{n_m} - 1\right)\left(\dfrac{1}{R_1} - \dfrac{1}{R_2}\right)$ . With two different media, $\dfrac{n_3}{v} - \dfrac{n_1}{u} = P$ and $m = \dfrac{n_1v}{n_3u}$ .

The worked solution

Have you tried it?

Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.

← All Problems

← A student wants to know the height of a small bright slit and the focal length of a convex lens…A small tank is closed at one end by a thin biconvex lens of glass with refractive index 1.50… →

More onOptics

Discussion (0)

Loading…