Two types of illusions: optical and auditory

Misrepresentation of a "real" sensory stimulus—that is, an interpretation that differs from objective "reality" as defined by consensus. A youngster, for example, may be considered to be having an illusion if he or she sees tree branches as goblins at night. An illusion differs from a hallucination, which is an experience that appears to occur without external stimuli. Neither experience is inherently a symptom of a psychiatric disorder, and both are reported by almost everyone on a frequent basis.

Types of illusory experiences

Stimulus-distortion illusions
When the environment modifies or warps the stimulus energy on its journey to the human, who sees it in its distorted pattern, this form of false sense-perception occurs.

Auditory anomalies

The aural impression that a blowing automobile horn changes pitch as it passes an observer on a roadway is a regular phenomenon. The Doppler effect is named after Christian Doppler, an Austrian scientist who discovered in 1842 that the pitch of a passing railroad train's bell or whistle drops while the train and the perceiver are moving away from each other and rise when they are approaching each other. Factors such as wind moving toward or away from the individual might also impact the sound heard.
When two sound sources in close proximity release sound waves with slightly different frequencies (i.e., vibrations per second), there will be times when the waves from both sources arrive at the ear in-phase (simultaneously) and produce the sensation of a combined, stronger sound. "Beats," or periodic alternations of sound intensity, will be heard as these intervals of mixed sound. When auditory beats occur too quickly to be distinguished, a harsh, continuous noise known as interference might develop. Another example of interference occurs when two tones are played simultaneously and a third tone is perceived. It's called a different tone when the third tone is lower in pitch than the other two; its frequency is the difference between the other two tones' frequencies.

Some examples-

• The Shepard tone looks to rising and fall in pitch continuously yet does not.
• Diana Deutsch discovered the Tritone Paradox illusion. Some people hear rising sounds, while others hear declining notes.
• Diana Deutsch identified the Scale Illusion, in which listeners hear different scales in each ear as a result of the brain grouping sounds together.
• Phantom Words is another optical illusion in which nonsensical sound is repeatedly layered on top of itself. Listeners may begin to hear words or phrases that apply to what is now relevant in their lives despite the lack of actual words. Diana Deutsch of the University of California was the first to show this auditory illusion.
• The MgGurk effect is an auditory/visual illusion that tricks the brain into believing a man is saying "BA" over and over again. The majority of people hear "DA," yet his lips appear to be pronouncing "GA." 

Optical effects

The refraction (bending) of light when it goes through one substance to another in which the speed of light is significantly different causes a variety of optical illusions. As it emerges from one transparent medium (air) into another (water), a ray of light is twisted. As a result, a pencil standing in water appears to be broken at the point where air and water contact, just as a half-submerged log in swamp water appears to be bent.
Refraction can also produce rainbows. When the sun's rays pass through raindrops, the white light is separated (refracted) into its component colours. When rays of white light from any source pass through a prism, they are refracted into a spectrum of colours, much like a summer morning's rainbow. A mirage is another optical illusion that is dependent on atmospheric conditions, such as the image of a pool of water formed by light travelling through layers of air above a heated highway surface.

Some examples of optical illusions:

• The artwork of M.C. Escher is designed to aesthetically challenge the audience.
• A magician can cut in half a box with a person inside without the person being cut.
• When one stares at an image for a long time, it appears to be nothing but numerous shapes and colours that vary.
• When vision is adjusted, a picture of an elderly woman looking downward appears to be a young woman looking sideways.
• The popular image of a goblet appears to be two faces looking at each other when vision is adjusted.
• A picture is made out of various coloured and sized ovals that form larger circles. The circles appear to revolve when observed, even though they are not moving.

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