The speed of sound at [tex]20^{\circ}C[/tex] is approximately v=343 m/s. The distance covered by the sound wave is
[tex]s=1 cm=0.01 m[/tex]
And the time it takes is
[tex]t= \frac{S}{v}= \frac{0.01 m}{343 m/s}=2.9 \cdot 10^{-5} s [/tex]
Now we want to find how far the light travels during this time. Light travels at speed [tex]c=3 \cdot 10^8 m/s[/tex], therefore the distance it covers during this time is
[tex]S=ct = (3 \cdot 10^8 m/s)(2.9 \cdot 10^{-5} s)=8700 m[/tex]
