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 and schlieren photography methods that perform a similar function. Shadowgraph is a type of flow visualisation. 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 light rays, so they can cast shadows. 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 who first scientifically demonstrated the sunlight shadowgraph and Jean-Paul Marat 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 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, explosions, 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 on the '' Three Blind Mouseketeers'' (1936) in the series of animated short films Silly Symphonies.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