# Homework Help: Magnitude of Sirius?

1. Jun 6, 2017

### Jussi Lundahl

1. The problem statement, all variables and given/known data
Stars are observed through a triple window.
Each surface reflects away 15% of the incident light.
a) What is the magnitude of Sirius (m = -1.5) seen
through the window?

And I know that the solution is -0,97.

2. Relevant equations
m1-m2=-2,5*log10*(F1/F2)

3. The attempt at a solution
I have no idea what to do...

2. Jun 6, 2017

### Bandersnatch

Ok, start by telling us what are the F1 and F2 in the equation.

3. Jun 6, 2017

### Voltageisntreal

If 15% is deflected each time, then sirius's intensity, which let's say is 1 becomes 0.614 since (0.85)^3=0.614

Then, log(1/0.614)*2.5=0.5295

0.5295 is hence the difference between the apparnt magnitudes of the star after it has passed through the three windows. -1.5+0.5295=-0.9704

4. Jun 6, 2017

### Jussi Lundahl

I think those are called flux density in English. $$F=\frac{L}{4 \pi r^2}$$, where the L is luminosity and r is radius. $$L= \omega r^2$$ where $$\omega$$ is solid angle?? and r is radius.

5. Jun 6, 2017

### Jussi Lundahl

Thank you! You saved my day :)

6. Jun 6, 2017

### Bandersnatch

Please, do not provide full answers to questions posted in the homework forum. Spoon-feeding is against the rules.

7. Jun 6, 2017

### Voltageisntreal

• Member warned not to use text-speak in posts.
;o, mb didn't kno rip

8. Jun 6, 2017

### Staff: Mentor

Text speak is also not permitted!