Here's an optical effect that I've always found interesting. Consider the picture below. You see a central cross set against a blurry background pattern of pastel colours.
Focus on the cross. Do not let your eyes wander, however tempting it is to do so. It may take up to 30 seconds but then you will probably see the background dissolve and only the central cross stay visible.
If you let your eyes move, the background springs back into view.
If you don’t notice any fading, focus a bit longer on the cross, and also try closing one eye, or unfocussing, while still fixating on the cross. Varying the distance from the screen, or enlarging the image, may also help.