1. Create a new file and fill background with black using the Paint Bucket¹ tool. Set foreground color R:0, G:119, B:158. Select the Paintbrush¹ tool and set its Opacity to 100%. Select one of the smooth brushes the size of your laser beam. Create a new layer and draw the laser beam (to draw straight lines, hold the Shift key while drawing).




2. Duplicate layer² with the laser beam. Set Blending Mode of the copy to Color Dodge. Duplicate it again and set Blending Mode to Color Dodge. Now you will have two layers with Blending Mode set to Color Dodge on top of one Normal layer.




3. To make the beam appear more smooth, apply Gaussian Blur filter (Filter > Blur > Gaussian Blur) to the very first layer (which doesn't have Blending Mode set to Color Dodge) with Radius value set to 5 or any other value you find appropriate.




4. This step is optional. If you want to make you laser beam to be "cartoon"-like, apply Ripple filter (Filter > Distort > Ripple) to one of the layers which has its Blending Mode set to Color Dodge.




And there you have it, your laser. Hope this tutorial came in handy.