# Homework Help: Angle of deviation general relativity, trigo

1. May 5, 2013

### Unicorn.

1. The problem statement, all variables and given/known data
Hello, I have a little problem with this figure, this is not an assignment or something else I just don't understand how we can find that d∅=gocos3(α)/c² *dz

2. Relevant equations
go=G*M/R² with R the sun's radius

3. The attempt at a solution
I wrote all what I can deduce
ge=gr*cos(α)
r=R/cos(α)
dt=dz/c
tg(∅)≈∅= R/dz
tg(α)=z/R

Please I really need to understand this

#### Attached Files:

• ###### RG.png
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Last edited: May 5, 2013
2. May 5, 2013

### voko

What is this all about, exactly?

3. May 5, 2013

### Unicorn.

In the course the teacher advised us to do it and just gave us the expression, so I'm trying to, my exam is Tuesday and I want to understand how we find dphi because it's from this equation, by integration that we find that phi=2*G*M/(c²*R)
Thanks

4. May 5, 2013

### voko

To do "it" - what is "it"? Describe what the drawing represents and what you are supposed to find.

5. May 5, 2013

### Unicorn.

"It" is to prove that d∅=go*cos3(α)/c² *dz
It's the angle of deviation of a light beam in the gravitational field of the sun.
What I'm trying to find is this expression d∅=go*cos3(α)/c² *dz from the drawing
Thanks

6. May 5, 2013

### voko

Are you going to feed us one spoon at a time? Unless you explain what all those symbols mean - except $R$ and $g_0$ which you did explain - you won't get much help here.

7. May 5, 2013

### Unicorn.

dz is the width of the reference frame
r is the distance between the reference frame and the center of sun
gr gravitational interaction
ge is the effective gravitational acceleration

Last edited: May 5, 2013
8. May 5, 2013

### voko

None of this is making sense. There cannot be a distance to a "reference frame" because a reference frame fills up the entire space, and there is no width of it, for the same reason.

In the context of curving a light beam by the Sun, I would expect a distance source (star) and a receiver (observer) on the Earth. I cannot see any of that on your diagram.

9. May 5, 2013

### Unicorn.

In the example in the class the teacher explained that the little squares were local inertial reference frame (as elevators) and as the photon of a star were crossing them they fall in the direction of the sun.
I'm sorry for the lack of informations, I'm trying to remember because my notes are not complete..