What are different types of optical illusions?
There are three types of optical illusions: Physical, physiological and cognitive.

Physical optical illusion occurs when the light bends or changes direction before it reaches human eye. When light travels through different mediums such as air , water or glass, its speed changes and change in speed causes the light to bend. This phenomenon is called refraction. Rainbow is an example of physical optical illusion caused by refraction.   

Physiological optical illusion occurs when a pattern repeats over and over again and it is too much information for the brain. This causes the brain to think the images are moving. Kinetoscope uses images to create a video using physiological optical illusion. Flip books are another example of physiological optical illusion. 

Cognitive optical illusion occurs when multiple people view the same image but they interpret it differently. Interpretation of the image depends of people's past experience and personalities.