In Young's double slit experiment, the wavelength of monochromatic light is increased by $20 \%$ and the distance between the two slits is decreased by $25 \%$. If the initial fringe width is 0.3 mm , then the final fringe width is

An unpolarised beam of light incidents on a group of three polarising sheets arranged such that the angle between the axes of any two adjascent sheets is $30^{\circ}$. The ratio of the intensities of polarised light emerging from the second and third sheets is

In Young's double slit experiment, the wavelengths of red and blue lights used are $7.5 \times 10^{-5} \mathrm{~cm}$ and $5 \times 10^{-5} \mathrm{~cm}$ respectively. If $n$th bright fringe of red color coincides with $(n+1)$ th bright fringe of blue colour, then the value of ' $n$ ' is