In Young's experiment, fringes are obtained on a screen placed at a distance $$75 \mathrm{~cm}$$ from the slits. When the separation between two narrow slits is doubled, then the fringe width is decreased. In order to obtain the initial fringe width, the screen should be moved through.

Two coherent sources 'P' and 'Q' produce interference at point 'A' on the screen, where there is a dark band which is formed between 4^th and 5^th bright band. Wavelength of light used is 6000 $$\mathop A\limits^o $$. The path difference PA and QA is

In diffraction experiment, from a single slit, the angular width of central maximum does NOT depend upon

In biprism experiment, 21 fringes are observed in a given region using light of wavelength 4800 $$\mathop A\limits^o $$. If light of wavelength 5600 $$\mathop A\limits^o $$ is used, the number of fringes in the same region will be