- Home page |
- Learning field nature& Technique |
- Light& Shadows |
- Factual information for the teacher
Factual information for the teacher
1. Hints for creating the sharpest possible shadows
If a light bulb is used as a light source, the filament should be as small as possible so that the light source is as point-like as possible. Then sharp shadows are created. Halogen table lamps, for example, meet this condition. However, the halogen bulbs get very hot, so if possible only use lamps with protective glass or protective rod.
Ordinary frosted 230-volt incandescent lamps provide a relatively extended source of light that provides soft, blurred shadows when the distance between the object and the wall increases.
Far better results are achieved with a 40-watt incandescent lamp with a clear glass bulb, if you orient the filament perpendicular to the wall. The projection of the filament on the wall is then very small. (See picture.)
Overhead projectors also provide sharp shadows. But they have the disadvantage that the light source is not as clearly visible to the students as it is with an incandescent lamp.
2. Shadow creation
In order to be able to sufficiently explain the formation of shadows, it is first necessary to talk about the straight-line propagation of light, in order to then be able to understand the process of the formation of shadows.
Even small children draw the sun as a round disk that emits straight rays of light. Exactly this childish idea already corresponds to a large extent to the physical truth. If you hold an opaque object between your eye and a light source for illustration, you can no longer see the light source. This means the light does not flow in an arc around the object, but is stopped by it. From this it follows that light must propagate in a straight line. This also becomes clear in experiment V1 in the teaching unit "How is shadow created??"Here a dusty blackboard cloth is placed over a ray of light or. tightly bundled light channel shaken out. The straightness of light propagation is clearly visible through the tiny particles that enter the light beam.
Accordingly, shadows are caused by the fact that light does not pass through an object unless it is translucent. The space behind the object (seen from the light source) can therefore not be illuminated. This dark area is clearly separated from the brightly illuminated surroundings and we see a shadow. Simplified, the shadow is always in the extension of the connecting lines between light source and object.