Wherever we go, a shadow follows us everywhere. In the morning and in the evening, it becomes long, much more than human growth, and by noon it shortens, turning into a scanty blot underfoot.
If you enter a room illuminated by a gas-discharge lamp, it disappears altogether. So what is a shadow in terms of physics?
Shadow from the point of view of physics
Since physics deals exclusively with material objects, the shadow for this scientific discipline is an optical phenomenon, inextricably linked with the light source. If you take a lamp, candle or other source of light radiation, and place any opaque object in its rays, then a dark silhouette, called a shadow, will appear on the surface of objects behind this object. Shadow does not exist by itself, it always appears when the physical body prevents light from reaching other physical bodies.
The contours of the shadow always repeat to a certain extent the contours of the object that forms it, but with a certain distortion. Theoretically, you can get a shadow, the contours of which will be completely identical to the object, if you position the object and the plane on which the shadow is formed strictly perpendicular to the light rays. In this case, the size of the light source should repeat the size of the object so that all rays of light fall on it at right angles.
In practice, the shadow always distorts the contours of the body that casts it, since the light rays fall on it and on the plane where the shadow is formed at different angles. Distortion depends on:
– the angle of incidence of light on the object and on the surface of the shadow formation;
– light intensity (the brighter it is, the visually thicker the shadow is);
– transparency of the medium for the passage of rays (part of the rays of light can be scattered if the medium is inhomogeneous in transparency – sea water, frosted glass, etc.);
– remoteness of the light source from the object and the surface;
– the distance between the object and the surface on which the shadow is formed;
– surface textures – smooth, rough, wavy, etc.
By changing these factors, you can change the outlines of the shadow, making them clear or blurred, increase and decrease the length of the shadow, its intensity from pale gray to deeply dark, as well as some other parameters of the shadow cast by the object.
The shadow is not an independent object, therefore any interactions between the shadows of objects are possible only when these objects come into real contact. Even if you position the light source at the right angle, causing the shadows of the objects to touch, it will not in any way affect their current location.
The same subject can cast multiple shadows if it is illuminated by multiple light sources. The brighter the source, the thicker its shadow will be. If you place an object between two light sources, the shadow will appear on only one side, opposite to the more powerful light source.
When illuminating an object with several identical light sources from different sides, its shadows will visually disappear. At the same time, the illumination of the surface around the object will be different in different areas, i.e. where the shadow from one of the light sources falls, it will be less than in the neighboring area, where there are no shadows.
In a number of industries – medicine, instrumentation, watchmaking, etc. – it is extremely important that the field of activity is always well lit. In these cases, shadowless lamps are used, the light emission of which does not lead to the formation of shadows.
Uniform illumination of all surfaces is achieved by distributing light sources throughout the room, as well as using special light-scattering and reflective fittings. Shades of frosted glass, mesh and curved reflectors are used as fittings, which ensure the scattering of light rays in different directions. Gas-discharge, halogen and LED lamps are used as light-emitting lamps.