A $5\text{ cm}$ long pencil is placed along the principal axis of a concave mirror of focal length $20\text{ cm}$ such that its nearest end is at a distance of $25\text{ cm}$ from the mirror. Calculate the length of the image of the pencil. OR In a Young's double-slit experiment, a beam of light consisting of two wavelengths $500\text{ nm}$ and $600\text{ nm}$ is used. The interference fringes are observed at a screen placed $1.8\text{ m}$ away from the plane of slits (slit separation $0.3\text{ mm}$). Calculate the least distance from the central maximum where the bright fringes due to both the wavelengths coincide.