# Thread: Confused about E!

1. So, let's say we are talking about a photon. We can find E by the equation E=pc, which only works for light. But, when I take p=h/λ, then I fill in λ with v_p/f since λ=v_p/f, so then we get p=hf/v_p, then since E=hf we get p=E/v_p, then to p(v_p)=E, and E=pc for light so we get E=pc=p(v_p), which implies c=v_p, which in a vacuum is true for light but quite frequently isn't. I am confused! What am I doing wrong!
v_p is phase velocity

2.

3. Originally Posted by Muon321
So, let's say we are talking about a photon. We can find E by the equation E=pc, which only works for light. But, when I take p=h/λ, then I fill in λ with v_p/f since λ=v_p/f, so then we get p=hf/v_p, then since E=hf we get p=E/v_p, then to p(v_p)=E, and E=pc for light so we get E=pc=p(v_p), which implies c=v_p, which in a vacuum is true for light but quite frequently isn't. I am confused! What am I doing wrong!
v_p is phase velocity
You have already opened two identical threads here and your question has been answered. Why do you feel compelled to spam multiple forums with the same nonsense?

4. And here.

5. Muon321. If you are still confused about this topic, having provoked discussion elsewhere and received advice and what looks like satisfactory answers to me, you need to restate - in a new thread - exactly where your confusion now lies.

This thread is closed.

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