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$Example 9.1: The distance between the slits in Youngs double slit experiment is 0.25 \mathrm{~cm} . Interference fringes are formed on a screen placed at a distance of 100 \mathrm{~cm} from the slits. The distance of the third dark fringe from the central bright fringe is 0.059 \mathrm{~cm} . Find the wavelength of the incident light.$

$9.1 Under what conditions two or more sources of light behave as coherent sources?$

$9.6 A light is incident normally on a grating which has 2500 lines per centimetre. Compute the wavelength of a spectral line for which the deviation in second order is 15.0^{\circ} .$

$15. When crest of one wave falls over the trough of the other wave this phenomenon is known asa) Polarizationb) Constructive interferencec) Destructive interferenced) Diffraction$

$77. When a soap film (or oil film) on water is observed in day light it exhibits beautiful colours due to:a) Dispersionb) Refractionc) Reflectiond) Interference$

$22. The blue to the sky is due toa) Diffractionb) Reflectionc) Polarizationd) Scattering$

$52. Which of the following demonstrates that light waves are transverse waves?a) Interferenceb) Polarizationc) Diffractiond) Reflection$