Blue light travelling in a vacuum has a wavelength of $450 \mathrm{nm}.$ It enters a medium whose refractive index is $1.5.$ What is its frequency in the medium?
(Speed of light in vacuum $=3\times {10}^8 \mathrm{m} {\mathrm{s}}^{-1}$)

The position of the final image formed by the given lens combination from the third lens will be at a distance of $\left(f_1=+10 \mathrm{cm}, f_2=-10 \mathrm{cm}\right.$ and$\left.f_3=+30 \mathrm{cm}\right)$

A converging mirror is placed on the right hand side of a converging lens as shown in the figure. The focal length of the mirror and the lens are $20 \mathrm{cm}$ and $15 \mathrm{cm}$ respectively. The separation between the lens and the mirror is $40 \mathrm{cm}$ and their principal axes coincide. A point source is placed on the principal axis at a distance $d$ to the left of the lens. If the final beam comes out parallel to the principal axis, then the value of $d$ is: