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An optical illusion is a type of illusion characterized by visually perceived images that are deceptive or misleading.
Information gathered by the eye is interpreted by the brain to give the perception that something is present when it is not.
There are physiological illusions, that occur naturally, and cognitive illusions, that can be demonstrated by specific visual tricks that show particular assumptions in the human perceptual system.
Physiological illusions, such as the afterimages following bright lights or adapting stimuli of excessively longer alternating patterns are the effects on the eyes or brain of excessive stimulation of a specific type - brightness, tilt, colour, movement, and so on. The theory is that stimuli have individual dedicated neural paths in the visual outer wall of an organism for the early stages of visual processing; repetitive stimulation of only a few channels misleads the visual system.

Cognitive illusions are often better known.
Instead of demonstrating a physiological base they interact with different levels of perceptual processing, in-built assumptions or 'knowledge' are misdirected. Cognitive illusions are commonly divided into ambiguous illusions, distorting illusions, paradox illusions, or fiction illusions. They often exploit the predictive hypotheses of early visual processing.

A mirage is an illusion that results from a natural optical phenomenon. The variation in the apparent size of the Moon is another natural illusion; it is not an optical phenomenon, but rather a cognitive or perceptual illusion.

Developed illusions include phenomena such as the Necker cube and the Scintillating/Hermann grid. They could also be called discovered illusions. Understanding these phenomena is useful in order to understand the limitations of the human visual system.

Ambiguous illusions are pictures or objects that offer significant changes in appearance. Perception will 'switch' between the alternates as they are considered in turn as available data does not confirm a single view.

Distorting illusions are the most common, these illusions offer distortions of size, length, or curvature. They were simple to discover and are easily repeatable. Many are physiological illusions, such as the Café wall illusion which exploits the early visual system encoding for edges. Other distortions, such as converging line illusions, are more difficult to place as physiological or cognitive as the depth-cue challenges they offer are not easily placed. All pictures that have perspective cues are in effect illusions. Visual judgements as to size are controlled by perspective or other depth-cues and can easily be wrongly set.

From Wikipedia

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This is the Swiss Hotel Stamford, the hotel where i was. Is part of the Raffles City complex, and for many years it was the highest hotel of the world with 73 floors for 226 meters.