Young's double slit experiment is performed with monochromatic light of wavelength $6000 \mathring{A}$. If the intensity of light at a point on the screen, where path difference of $2000 \mathring{A}$ is $I_1$ and the intensity of light at a I point on the screen, where the path difference is $1000 \mathring{A}$ is $I_2$, then $I_1: I_2=$

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