# Deflection of light by the Sun

Can you give details of a recent experiment of deflection of light by the Sun?

What is the distance from the surface of the Sun and what is the exact value of the angle of deflection?

## 1 Answer

Have a look at the Wikipedia article on Tests of General Relativity. The most recent measurement quoted there was by a group from the University of Texas. You can find a copy of the paper here.

They measured a deflection of $1.66$ arcseconds $\pm 10\%$, compared to the prediction from General Relativity of $1.75$ arcseconds.

Respond to comment:

The angular deflection of the light at a distance $r$ from an object of mass $M$ is approximately given by:

$$\theta = \frac{4GM}{c^2r}$$

Put in the mass and radius of the Sun and you'll find $\theta = 8.48 \times 10^{-6}$ radians. Convert this to degrees and multiply by $3,600$ to convert to arcseconds and you'll recover the figure of $1.75$ arcseconds I quoted above.

• Is there specified the distance from the Sun? I couldn't find it. Should I assume it was just grazing the surface , at 7c from the centre? – bobie Aug 3 '14 at 12:23
• @bobie: yes, that's the angular deflection of a ray that just grazes the surface. – John Rennie Aug 3 '14 at 14:10
• Aren't there any data relative to rays passing at a certain distance from the Sun? Do we know how the angle of deflection varies with distance? – bobie Aug 4 '14 at 4:19
• @bobie: I've updated my answer to respond to your comment – John Rennie Aug 4 '14 at 5:06
• @bobie: the equation I gave is an approximate equation that works when $\theta$ is small. At these small angles the difference between $\theta$ and $\tan\theta$ is negligable. – John Rennie Aug 5 '14 at 11:12