# Light and energy

1. Jan 4, 2010

### ssope

I am wondering if there is a significant loss in energy (greater than 2%) in light that is refracted through media. I also am wondering the same about energy loss in light that is focused via a parabolic lens.

2. Jan 5, 2010

### Bob S

Normally incident light on a refracting media with index of refraction n from air with n=1, or from the same media to air, has a power loss of

Power loss = (n-1)2/(n+1)2= 0.25/6.25 = 4% for glass with n=1.5.

So for going through a glass lens without anti-reflection coating, the loss is about 8%. The loss in a front-surface parabolic mirror is near zero.

Bob S

3. Jan 5, 2010

### tiny-tim

Hi Bob S!

And (I' m guessing ) is it (n1-n2)2/(n1+n2)2 for general media?

4. Jan 5, 2010

### ssope

Is there energy loss in light that is focused via a parabolic lens?

5. Jan 5, 2010

### tiny-tim

uhh? Bob S just said so … about 8%.

6. Jan 5, 2010

### Bob S

I said 0% using a front-surface-silvered parabolic mirror, 8% through a refracting glass lens.
Bob S

7. Jan 6, 2010

### ssope

I'm not sure that 0% is correct. I know that even the best silvered dielectric mirros offer 99% reflection