• # question_answer Two monochromatic light waves of amplitudes A and 2A interfering at a point, have a phase difference of $60{}^\circ$. The intensity at that point will be proportional to- A) $3{{A}^{2}}$                        B) $5{{A}^{2}}$C) $7{{A}^{2}}~$                     D) $9{{A}^{2}}$

${{A}^{R}}=\sqrt{a_{1}^{2}\,+\,a_{2}^{2}\,+\,2{{a}_{1}}{{a}_{2}}\,\cos \phi }\,\,=\,\,5{{A}^{2}}$