The rays of different colours fail to converge at a point after passing through a thick converging lens. This defect is called

Two coherent sources of wavelength '$$\lambda$$' produce steady interference pattern. The path difference corresponding to 10$$^{th}$$ order maximum will be

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