Shadowgraph is an optical method that reveals non-uniformities in transparent media like air, water, or glass. It is related to, but simpler than, the

schlieren Schlieren ( ; , ) are optical inhomogeneities in transparent media that are not necessarily visible to the human eye. Schlieren physics developed out of the need to produce high-quality lenses devoid of such inhomogeneities. These inhomogeneit ...

and

schlieren photography Schlieren photography is a process for photographing fluid flow. Invented by the Germans, German physicist August Toepler in 1864 to study supersonic motion, it is widely used in aeronautical engineering to photograph the airflow, flow of air ar ...

methods that perform a similar function. Shadowgraph is a type of

flow visualisation Flow visualization or flow visualisation in fluid dynamics is used to make the flow patterns visible, in order to get qualitative or quantitative information on them. Overview Flow visualization is the art of making flow patterns visible. ...

. In principle, a difference in temperature, a different gas, or a

shock wave In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a me ...

in the transparent air cannot be seen by the human eye or cameras. However, all these disturbances

refract In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenome ...

light rays, so they can cast

shadows A shadow is a dark area on a surface where light from a light source is blocked by an object. In contrast, shade occupies the three-dimensional volume behind an object with light in front of it. The cross-section of a shadow is a two-dimensiona ...

. The plume of hot air rising from a

fire Fire is the rapid oxidation of a fuel in the exothermic chemical process of combustion, releasing heat, light, and various reaction Product (chemistry), products. Flames, the most visible portion of the fire, are produced in the combustion re ...

, for example, can be seen by way of its shadow cast upon a nearby surface by the uniform

sunlight Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible spectrum, visible light perceptible to the human eye as well as invisible infrare ...

Sunlight shadowgraph

Some aquatic predators detect their transparent prey by way of their shadows cast upon the ocean floor. It was

Robert Hooke Robert Hooke (; 18 July 16353 March 1703) was an English polymath who was active as a physicist ("natural philosopher"), astronomer, geologist, meteorologist, and architect. He is credited as one of the first scientists to investigate living ...

who first scientifically demonstrated the sunlight shadowgraph and

Jean-Paul Marat Jean-Paul Marat (, , ; born Jean-Paul Mara; 24 May 1743 – 13 July 1793) was a French political theorist, physician, and scientist. A journalist and politician during the French Revolution, he was a vigorous defender of the ''sans-culottes ...

who first used it to study fire. A modern account of shadowgraphy is given by Gary S. Settles.

Applications

Applications of shadowgraphy in science and technology are very broad. It is used in

aeronautical engineering Aerospace engineering is the primary field of engineering concerned with the development of aircraft and spacecraft. It has two major and overlapping branches: aeronautical engineering and astronautical engineering. Avionics engineering is s ...

to see the flow about high-speed aircraft and missiles, as well as in

combustion Combustion, or burning, is a high-temperature exothermic redox chemical reaction between a fuel (the reductant) and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke. Combustion ...

research,

ballistics Ballistics is the field of mechanics concerned with the launching, flight behaviour and impact effects of projectiles, especially weapon munitions such as bullets, unguided bombs, rockets and the like; the science or art of designing and acceler ...

explosions An explosion is a rapid expansion in volume of a given amount of matter associated with an extreme outward release of energy, usually with the generation of high temperatures and release of high-pressure gases. Explosions may also be generate ...

, and in the testing of

glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...

. Ideal for identification of flow patterns.

Shadowgram (Shadowgraph)

According to F. J. Weinberg, the result of applying the shadowgraph technique should be known as a ''shadowgram''. A shadowgram is not a focused image, rather it is a mere shadow. In the shadowgram, the differences in light intensity are proportional to the second spatial derivative (

Laplacian In mathematics, the Laplace operator or Laplacian is a differential operator given by the divergence of the gradient of a scalar function on Euclidean space. It is usually denoted by the symbols \nabla\cdot\nabla, \nabla^2 (where \nabla is th ...

) of the

refractive index In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...

field in the transparent medium under study. Once the distance from the transparent disturbance to the cast shadow becomes too large, then the shadow no longer constitutes a useful representation of the disturbance that caused it.

Cartoons

The shadowgraph and shadowgram have been used in animation, where they reinforce the cartoon's realism. One first use was made by

Disney Studios The Walt Disney Studios is a major division of the Disney Entertainment business segment of the Walt Disney Company best known for housing its multifaceted film studio divisions. Founded on October 16, 1923, and based mainly at the namesake s ...

on the ''

Three Blind Mouseketeers ''Three Blind Mouseketeers'' is a ''Silly Symphonies'' animated short film based on the nursery rhyme ''Three Blind Mice'' and the 1844 novel ''The Three Musketeers'' by Alexandre Dumas. Directed by Dave Hand and Jack Cutting, it stars Billy Bl ...

'' (1936) in the series of animated short films

Silly Symphonies ''Silly Symphony'' (also known as ''Silly Symphonies'') is an American animation, animated series of 75 musical short films produced by Walt Disney Animation Studios, Walt Disney Productions from 1929 to 1939. As the series name implies, the ''Si ...

.Russel Merritt & JB Kaufman, ''Walt Disney's Silly Symphonies : A Companion to the classic cartoons series'', Italy:La Cinecita del Friuli, 2006, p 46

Postcards

Additionally the term ''Shadowgraph'' was used by English postcard publishers E.T.W. Dennis & Sons Ltd. of London and Scarborough for a series of 'Hold up to the Light' postcards in the 1950s. In these a saucy image can be seen through what seems an innocent picture when a light is shone through the card.

Gallery

References

{{Commons category, Shadowgraph Imaging