# Thermal Physics

1. Apr 6, 2014

### nicu15

Lens & Disk

The image of the sun is projected on the disk using a lens of radius r<<d and focal length f<<d (d is the distance between the sun and the disk.. The magnification of the lens is Mf/d, so that the image completely fills the disk R=MR. When the disk reaches thermal equilibrium, what is its temperature?

I don't really know where to start, so a hint would help a lot.

Thanks

2. Apr 6, 2014

### Simon Bridge

How would you do the problem if there was no lens?

3. Apr 7, 2014

### nicu15

Computed the flux in and compared it with the flux out of the disk. got a temperature of $T_{disk}=T\sqrt{\frac{R}{d}}$ where d is the distance from the sun to the disk and R is the radius of the sun.

4. Apr 7, 2014

### Simon Bridge

... good.

So what does the lens do in terms of the solar flux that gets to the disk?