# Plot lines keep disappearing

I'm trying to plot the Compton Shift which I've defined as:

E2[ϕ_] := E1/(1 + (E1/(m*c^2))*(1 - Cos[ϕ Degree]))


I've initiated the constants and when I evaluate the function for various degrees, it gives me the correct answer.

The issue I'm getting is that when I try and plot this function (with m = 1, c = 299792458 and E1 = 180), my plot shows a line until the value of ϕ is increased to above ~30-40. I'll get a flat line for small values of Phi but for larger values, the line disappears and I get a blank plot.

What is the reason for this? Am I doing something wrong?

• What you have is essentially $\frac{180}{1+\mathcal{O}(10^{-16})}$, which is approx $180$. I see a line for large values of phi — i.stack.imgur.com/b8loH.png Try plotting E1 - E2[phi] if you want to see the change in E2. – rm -rf Oct 22 '13 at 15:26

As rm -rf pointed out in a comment to the question, the difference between E2 and the constant E1 is extremely small, so a plot of E2 is essentially a flat line at 180. If the difference between E2 and E1 is plotted, something useful is obtained.
With[{m = 1, c = 299792458, E1 = 180},