For an aperture of $5 \times 10^{-3} \mathrm{~m}$ and a monochromatic light of wavelength $\lambda$, the distance for which ray optics becomes a good approximation is 50 m , then $\lambda=$

In Young's double slit experiment with light of wavelength $\lambda$, the intensity of light at a point on the screen where the path difference becomes $\frac{\lambda}{3}$ is ( $I$ is intensity of the central bright fringe)