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## Main Question or Discussion Point

I read somewhere that Heisenberg described his uncertainty principle by saying that you can't measure position more accurately than the wavelength of light (which makes sense), so Δx > λ.

This is what I don't get. He then says that p=h/λ, so Δp > h/λ

ΔxΔp > h

Why does the initial momenta of the photon, p=h/λ, determine the uncertainty of the momentum in the object scattered by light? And what if you knew the momentum of the photon exactly, then Δp=0?

This is what I don't get. He then says that p=h/λ, so Δp > h/λ

^{2}Δλ. He the multiplies and sets Δλ ≈ λ to get:ΔxΔp > h

Why does the initial momenta of the photon, p=h/λ, determine the uncertainty of the momentum in the object scattered by light? And what if you knew the momentum of the photon exactly, then Δp=0?