An object is kept at rest on the principal axis of a lens. Initially, the object is at a distance three times the focal length $f$ of the lens. The lens runs towards the object at a constant speed $u$, until the distance between the object and its real image becomes $4f$. If the image always forms on a moving screen then express the velocity of the screen as a function of time.

A thin equiconvex lens of a glass of refractive index $\mu =\frac{3}{2} \&$ of focal length $0.3 \mathrm{m}$in air is sealed into an opening at one end of a tank filled with water $(\mu =\frac{4}{3})$. On the opposite side of the lens, a mirror is placed inside the tank on the tank wall perpendicular to the lens axis, as shown in the figure. The separation between the lens and the mirror is $0.8 \mathrm{m}$. A small object is placed outside the tank in front of the lens at a distance of $0.9 \mathrm{m}$ from the lens along its axis. Find the position (relative to the lens) of the image of the object formed by the system.

A prism of refractive index $n_1$ and another prism of refractive index $n_2$ are stuck together without a gap as shown in the figure. The angles of the prisms are as shown. $n_1$ and $n_2$ depend on $\mathrm{\lambda }$, the wavelength of light according to $n_1=1.20+\frac{10.8\times {10}^4}{{\mathrm{\lambda }}^2}$ and $n_2=1.45+\frac{1.80\times {10}^4}{{\mathrm{\lambda }}^2}$ where $\mathrm{\lambda }$ is in $\mathrm{nm}$.

(i) Calculate the wavelength ${\mathrm{\lambda }}_0$ for which rays incident at any angle on the interface $BC\mathit{ }$pass through without bending at that interface.

(ii) For light of wavelength ${\mathrm{\lambda }}_0$, find the angle of incidence $i$ on the face $AC$ such that the deviation produced by the combination of prisms is minimum.

A fly $F$ is sitting on a glass slab $S, 45\mathrm{cm}$ thick and of refractive index $3/2$. The slab covers the top of a container $C$ containing water $(\mathrm{R}.\mathrm{I}.= 4/3)$upto a height of $20 \mathrm{cm}$. The bottom of the container is closed by a concave mirror $M$ of radius of curvature $40 \mathrm{cm}$. Locate the final image formed by all refractions and reflection assuming paraxial rays.

The figure below depicts a concave mirror with centre of curvature $C$ focus $F$, and a horizontally drawn $OFC$ as the optic axis. The radius of curvature is $R(OC=R)$ and $OF=\frac{R}{2}$. A ray of light $QP$, parallel to the optical axis and at a perpendicular distance $w(w\le \frac{R}{2})$ from it, is incident on the mirror at $P$. It is reflected to the point $B$ on the optical axis, such that $BF=k$. Here $k$ is a measure of lateral aberration.

(a) Express $k$ in terms of $\left\{w,R\right\}$.

(b) Sketch $k \mathrm{vs} w \text{for }w\in [0, \frac{R}{2}]$

(c) Consider points $P_1 , P_2 ......P_n$ on the concave mirror which are increasingly further away from the optic centre $O$ and approximately equidistant from each other(see figure below). Rays parallel to the optic axis are incident at $P_1 , P_2 ,.......P_n$ and reflected to points on the optic axis. Consider the points where these rays reflected from $P_n , P_{n–1} , .....P_2$ intersect the rays reflected from $P_{n–1} , P_{n–2} , ..... P_1,$ respectively. Qualitatively sketch the locus of these points in figure below for a mirror (shown with solid line) with radius of curvature $2 \mathrm{cm}$.

A symmetrical converging convex lens of focal length $10 \mathrm{cm}$ & diverging concave symmetrical lens of focal length $- 20 \mathrm{cm}$ are cut from the middle and perpendicularly and symmetrically to their principal axis. The parts thus obtained are arranged as shown in the figure. Find the focal length $(\mathrm{in} \mathrm{cm})$ of this arrangement.