Physics - Geometrical Optics Question with Solution | TestHub

A homemade illuminator is a cylindrical tube of radius $\mathrm{R}=10\mathrm{mm}$ with a black inner surface. A point source of light is located on the tube axis at a distance $\mathrm{d}=20\mathrm{cm}$ from the lens. The screen is placed in front of illuminator at such a distance that a sharp image of the point source of light is produced on it. Focal length of the lens is $\mathrm{F}=15\mathrm{cm}$. What will be the inner radius of circle of light (in cm ) directly from the source on the screen, if you remove the tube and just keep the remaining arrangement as it is?