Answer:
8.3 × 10 ^ -20 Hz
[tex]energy \: of \: a \: photon \: = \: \frac{h}{f} \\ f \: = \: \frac{h}{energy \: of \: a \: photon} \\ f \: = \: \frac{6.63 \times 10 ^{ - 34} }{8.0 { \times 10}^{ - 15} } \\ f \: = \: 8.29 \times 10 ^{ - 20} [/tex]
hopefully it makes sense :)
