On this example I will also use a soft brush, with black foreground color, to paint away some of the light rays in the top left corner.
I only want the light rays to be coming out of the mist. If the light rays are too bright, you can lower the opacity of the light rays layer.
Or if you want them brighter, duplicate the layer. You can also vary by setting the duplicated layer to for instance Soft Light.
And that’s how you create light rays in an easy way in Photoshop.