You are standing at x90km Lightning bolt 1 strikes at x0km a

You are standing at x=9.0km. Lightning bolt 1 strikes at x=0km and lightning bolt 2 strikes at x=14.0km. Both flashes reach your eye at the same time. Your assistant is standing at x=2.0km.

What is the time difference between the two flashes as observed by your assistant?

Solution

Bolt 1 is 9.0 km away, so it takes

t₁=9000/(3*10⁸)=30 us   for the light to reach you

Bolt 2 is 14-9=5 km away from you, so it takes

t₂=5000//(3*10⁸)=16.67 us to reach you.

The time difference between the two flashes as observed by your assistant

t=30-16.67=13.33 us