# Planck's blackbody law

1. May 29, 2012

### robertjford80

I don't see how this follows.

2. May 29, 2012

### phyzguy

What don't you see? If you do the substitution, the h's cancel, one of the c's cancel, and one of the lambda's cancel, leaving the result. What are you missing?

3. May 29, 2012

### Staff: Mentor

If your problem is with the approximation in the second line, consider the power series for ex:

$$e^x = 1 + x + \frac{x^2}{2!} + \frac{x^3}{3!} + ...$$