In a single slit diffraction, the slit is illuminated with light of wavelength $6000 \mathop {\rm{A}}\limits^{\rm{o}}$. If the slit were illuminated by a light of wavelength $\lambda$ the angular width of the central maximum decreases by $30 \%$. Then $\lambda$ is

A point source of light is placed at the focus of a concave mirror. Consider only paraxial rays. The shapes of the wavefronts of incident and reflected lights respectively are

Two slits separated by a distance of 1 mm are illuminated with light of wavelength $6.5 \times 10^{-7} \mathrm{~m}$. The interference fringes are observed on a screen placed at 1 m from the slits. The distance between the third dark fringe and the fifth bright fringe is equal to

Two slits are made one millimetre apart and the screen is placed one metre away from the slits. The fringe width when light of wavelength 500 nm is used is