Answer: 1/R = 1/r1 + 1/r2 + 1/r3
Explanation:
Just did the test myself. Hope this helps!
Answer:
E = h f = h c / λ energy of single photon
n E = 4.00E-11 W/m^2 energy required for visibility
n h c / λ = 4.00E-11 number of photons required for visibility (per square meter)
n = 4.00E-11 * 5.00E-7 / (6.63E-34 * 3.00E8)
n = 20.0E-18 / 19.9 E-26 = 1.00E8
100 million photons would be required on 1 m^2 to create visibility
a / A = π * (3.6E-3)^2 / 1 = 4.07E-5 fraction of area available
4.07E-5 * 1.00E8 = 4.07E3 photons required
