Illusions are experiences of perception in which one perceives stimuli incorrectly, not completely, or in distorted way. Some are caused by external factors (unrelated to the receiver's sensory organs and brain), as in the case of the “Doppler effect”, when one hears, say, the siren of an ambulance change its pitch as it passes by when, in reality, the pitch stays the same. The ears and brain of the receiver function normally, but this function is impeded by external natural factors like distance, interference of elements in the air, wind and its direction, etc. Illusions of this kind are called stimulus-distortion illusions. Other illusions are more receiver-related, and are referred to as perceiver-distortion illusions. These are caused by the way in which the human brain functions and, in particular, its tendency to group similar objects together as well as to “complete” unfinished objects. The latter function is what makes watching a movie possible, since it “blocks” perception of the intervals between projected stills and makes them look like a “moving” picture.

Researchers have designed various optical perceiver-distortion illusions. Some of the best known are the “figure and ground” illusion, occurring when looking at a white illustration on a solid black background; the Müller-Lyer illusion, where two equally long straight lines feature inward and outward arrow-shaped ends which make them look unequal; the Zöllner illusion, in which long parallel lines look as if they tend to intersect by means of other, shorter lines which do intersect them, and the Ponzo illusion, where two equal circles are placed inside an acute angle, resulting in the circle located closer to the angle's top giving the impression of being larger.

Much more interesting than these optical party tricks are illusions brought about by intersensory effects. Normally, the senses cooperate and enhance the experience of perception. The experience of a pleasant moment at the seaside, for instance, comprises visual experience, like the brightness of the sky and the color of the sea, auditory experience, like the sound of the waves lapping on the shore and the voices of other people, perhaps even experience of smell, perceiving the fresh, salty aroma of the sea. All senses combine to contribute to the enjoyment of the experience. There is a condition, however, called synaesthesia, where this intersensory cooperation becomes confused. In such cases, sounds and smells are often reported to carry the visual sensation of color, or certain words to produce musical tones. Synaesthesia is not to be confused with the manner in which a certain experience might strongly remind us of another. Rather, people who have it actually experience an additional sense when they perceive certain stimuli. It is often a symptom of various psychiatric disorders and, in extreme cases, one is unable to distinguish between sight and sound.

These are caused by the way in which the human brain functions and, in particular, its tendency to group similar objects together as well as to “complete” unfinished objects. The latter function is what makes watching a movie possible, since it “blocks” perception of the intervals between projected stills and makes them look like a “moving” picture.

Because of its porous quality, an opal absorbs liquids and therefore a stone which is nearly opaque when dry may be practically transparent when soaked in water. This may aid in deception, as light-colored opals may be dyed darker to resemble rarer varieties of the stone.

Opals are milky white or colorless gemstones that reflect other colors. They are composed of silica, or silicon dioxide, a compound of silicon which is also found in sand, quartz and flint.

Although opals are basically colorless, they are rarely found to be so, as various impurities which spread throughout the gem give it dull tinges of color, from yellow or red, when iron oxide is present, to black, where there are manganese oxides and organic carbon. If an opal is milky white or gray, there will be many tiny gas-filled cavities inside it.

The black opal, which is an intense gray or blue or near-black, is seldom found and, for this reason, particularly valuable. The lighter white opal or the yellow, orange or red fire opal are far more frequently mined. Transparent opals reflect changing colors as the stone is turned. This is caused by the play of light on tiny cracks and other imperfections inside the stone. Because of its porous quality, an opal absorbs liquids and therefore a stone which is nearly opaque when dry may be practically transparent when soaked in water. This may aid in deception, as light-colored opals may be dyed darker to resemble rarer varieties of the stone.

Australia has the finest gem opals, most of them mined in Queensland and New South Wales. In particular, the Lightning Ridge field is renowned for its black stones. High quality opals are also mined in Mexico, India and Slovakia.

Highly prized by the Romans, who considered it to be inferior only to the emerald, and regarded as lucky in the Middle Ages, the opal is nowadays sometimes considered to bring bad luck. Its presence in early Northern European mythology, where it was used by sorcerers to weave black magic and cast evil spells, may be associated with this conviction.

Opals are milky white or colorless gemstones that reflect other colors. They are composed of silica, or silicon dioxide, a compound of silicon which is also found in sand, quartz and flint.

Although opals are basically colorless, they are rarely found to be so, as various impurities which spread throughout the gem give it dull tinges of color, from yellow or red, when iron oxide is present, to black, where there are manganese oxides and organic carbon. If an opal is milky white or gray, there will be many tiny gas-filled cavities inside it.

The black opal, which is an intense gray or blue or near-black, is seldom found and, for this reason, particularly valuable. The lighter white opal or the yellow, orange or red fire opal are far more frequently mined. Transparent opals reflect changing colors as the stone is turned. This is caused by the play of light on tiny cracks and other imperfections inside the stone. Because of its porous quality, an opal absorbs liquids and therefore a stone which is nearly opaque when dry may be practically transparent when soaked in water. This may aid in deception, as light-colored opals may be dyed darker to resemble rarer varieties of the stone.

Australia has the finest gem opals, most of them mined in Queensland and New South Wales. In particular, the Lightning Ridge field is renowned for its black stones. High quality opals are also mined in Mexico, India and Slovakia.

Highly prized by the Romans, who considered it to be inferior only to the emerald, and regarded as lucky in the Middle Ages, the opal is nowadays sometimes considered to bring bad luck. Its presence in early Northern European mythology, where it was used by sorcerers to weave black magic and cast evil spells, may be associated with this conviction.

Opals are milky white or colorless gemstones that reflect other colors. They are composed of silica, or silicon dioxide, a compound of silicon which is also found in sand, quartz and flint.

Although opals are basically colorless, they are rarely found to be so, as various impurities which spread throughout the gem give it dull tinges of color, from yellow or red, when iron oxide is present, to black, where there are manganese oxides and organic carbon. If an opal is milky white or gray, there will be many tiny gas-filled cavities inside it.

The black opal, which is an intense gray or blue or near-black, is seldom found and, for this reason, particularly valuable. The lighter white opal or the yellow, orange or red fire opal are far more frequently mined. Transparent opals reflect changing colors as the stone is turned. This is caused by the play of light on tiny cracks and other imperfections inside the stone. Because of its porous quality, an opal absorbs liquids and therefore a stone which is nearly opaque when dry may be practically transparent when soaked in water. This may aid in deception, as light-colored opals may be dyed darker to resemble rarer varieties of the stone.

Australia has the finest gem opals, most of them mined in Queensland and New South Wales. In particular, the Lightning Ridge field is renowned for its black stones. High quality opals are also mined in Mexico, India and Slovakia.

Highly prized by the Romans, who considered it to be inferior only to the emerald, and regarded as lucky in the Middle Ages, the opal is nowadays sometimes considered to bring bad luck. Its presence in early Northern European mythology, where it was used by sorcerers to weave black magic and cast evil spells, may be associated with this conviction.

Because of its porous quality, an opal absorbs liquids and therefore a stone which is nearly opaque when dry may be practically transparent when soaked in water. This may aid in deception, as light-colored opals may be dyed darker to resemble rarer varieties of the stone.