n=1.458, c=3.00×108ms−1 ,λo = 589nm; f=? Speed of light in a medium (v)=cn where n is the refractive index of the medium ⇒λn=5891.458 = 404nm v=fλ ⇒f=vλ =2.06×108404×10−9 1nano=10−9 ∴f=5.09×1014 Hz
