# Homework Help: Astrophysics class help

1. Nov 2, 2004

### stuffy

Here is the problem...

Consider a row of isotropically emitting streetlamps 15m apart. On a foggy light, an observer standing next to the 1st light notes the radiation flux from the 2nd light is 5.0 times that of the 3rd. The last streetlamp definitely seen is the 12th.
If the air+fog density is 1.20x10^-3 g/cm^3, what are the volume and mass absorption coeff?

This is my work so far...

F2 = 5F[SIZE]3[/SIZE]

Using the equation dIv/ds = -kvIv
I set Iv=Fv (Intensity = flux)
therefore dFv/Fv = -kvds
Integrating the left side from F2 to F3, the right hand side from 0 to s.
I got ln(F3/F2) = -ks, where F2 = 5F[SIZE]3[/SIZE]
so k = -ln(1/5) / s, where s = 15m

here i got that k = .001073 (1/cm) for volume absorption coeff
and 0.89 cm^2/g for the mass absorption coeff...

They dont seem right, can anyone give a better direction?

2. Nov 3, 2004

### stuffy

No takers?

According to my calculations, the 2nd streetlap is about 9 million times "brighter" than the 12th. Optical depth of the 2nd lamp is around 1.7, 12th about 18.

Binary system of stars ($\alpha$ - centauri) Mar 10, 2018