There are many ways to create a light effect in Photoshop. One way is to use a lightening effect. To do this, first create a new file and fill it with a white background. Next, use the Ellipse tool to create a small circle on the top left corner of the background.
Next, use the Rectangle tool to create a rectangle that is the same size as the circle. Finally, use the Lighting Effect plug-in to apply the lightening effect to the rectangle.