Question

2) Consider light of free-space wavelength 1300nm traveling in pure silica medium. Calculate the phase velocity and group velocity of light in this medium. Is the group velocity ever greater than the phase velocity? (Use figure 1.8) 149 48 47 .46 45 In 500 700 900 1100 1300 1500 1700 1900 Wavelength (nm) FIGURE 1.8 Refractive index n and the group index Ng of pure SiO (silica) glass as a function of wavelength

Answer 1

Soludi00る. Given, wave ang)hef the Lighe 1300nm uwhicn won't change when ight free-space to silica mectiun but velocity and Iresuenu ot the lignt uove changs occoiding to posses fon the gen efrac ve trdesof Medium Refactive irdex chod is qiven Rox siu ca medium 14A 1 Ns .46 soo 20 o tloo 3oo Isoo 00 Group )-M=1.46S @ าะเ300 nm (phase ) → velocity in medium, velouty n free space fs

Ng t4 LS Group velouhM . 445 hase velocty tbus that the gyoup velodty uch media (catled dispes sive) is less thoen the pnase veloityt fs quite possible fo the prae veloury 群perfectly monochroma o·wave of Light, assuming Such a thin exists , to exceed the volue o e', because conwes no i nfurnotion of Light m Su)

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