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Create a new layer (Shift>Cmd/Ctrl+N) and use the Polygonal Lasso tool (L) to create a thin line. This will be the centre of the saber. Fill with white using the Paint Bucket tool (G) and deselect. Now choose a vibrant green colour using the Color palette (F6) and cut (Cmd/Ctrl+C) the code in the box. The bright green shown is swatch code #07f907. Create a new layer style (Layer>Layer Style>Outer Glow) and click the small colour square.
The colour palette box will appear, so paste (Cmd/Ctrl+V) your code into the box. Hit OK and make sure you select the neighbouring gradient box instead. Play around with the Spread and Size sliders until you have the perfect light saber.