In Young's double slit experiment, the two slits are 'd' distance apart. Interference pattern is observed on a screen at a distance 'D' from the slits. A dark fringe is observed on a screen directly opposite to one of the slits. The wavelength of light is

A parallel beam of monochromatic light falls normally on a single narrow slit. The angular width of the central maximum in the resulting diffraction pattern

Light waves from two coherent sources arrive at two points on a screen with path difference of zero and $$\frac{\lambda^{\prime}}{2}$$. The ratio of intensities at the points is $$\left(\cos 0^{\circ}=1, \cos \pi=-1\right)$$

A person is observing a bacteria through a compound microscope. For better observation and to improve its resolving power he should