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2 wave superposition (with phase & group velocity)
以下是兩個波的疊加，紅色(ω1，k1)與藍色(ω2，k2)疊加形成黑色的波，而紫色的波是angular frequency = (ω2- ω1)/2，wave number = (k2-k1)/2 形成的包絡線，用來描述波包行進。
E點與F點分別以群速度與相速度前進，可看出群速度即為波包移動的速度