CBSE Class 12 Physics — Ray Optics and Optical Instruments

A concave lens of focal length 10 cm is cut into two identical plano-concave lenses. The focal length of each lens will be

(a) Draw the ray diagram to show the image formation by a refracting telescope and write the expression for angular magnification for the telescope in normal adjustment. (b) Give two reasons to explain why a reflecting telescope is preferred over a refracting telescope.

(a) State the two conditions under which total internal reflection occurs. (b) A transparent container contains layers of three immiscible transparent liquids A, B and C of refractive indices n, $(3/4)n$ and $(2/3)n$, respectively. A laser beam is incident at the interface between A and B at an angle θ as shown in figure. Prove that the beam does not enter region C at all for $\sin \theta \ge 2/3$.

(a) Using the relation for refraction at a curved spherical surface, derive the expression for lens maker's formula. (b) Three lenses $L_1$, $L_2$ and $L_3$, each of focal length 40 cm, are placed coaxially. The distance between $L_1$ and $L_2$ and between $L_2$ and $L_3$ are 120 cm and 20 cm respectively. An object is kept at a distance of 80 cm to the left of lens $L_1$. Find the distance of the final image formed from the object. OR (a) Draw a ray diagram to show the image formation by a concave mirror when the object is kept between its focus and the centre of curvature. Using this diagram, derive the mirror formula. (b) A concave mirror produces a two times magnified virtual image of an object kept 10 cm in front of it. Calculate the focal length of the mirror.