CH 49: Your Eyes do not Tell the Whole Story
Optical Illusions are generally when your brain processes some visual information a certain way, but the actual objective truth of that image is different. There are three main types of optical illusions; literal illusions that make your brain create a different image then actually exists, physiological illusions that over stimulate your brain and cognitive illusions where your brain makes inferences that are incorrect as it pertains to the visual information that is actually present. In many cases, illusions can be a manifestation of the different properties of light and how these principles can fool our optical systems, understanding physics can help clarify how illusions actually trick us. In the meantime, consider these same sized lines:
Ok, now this might be an Optical Illusion