Which object lets light through
Propagation of light¶
Electromagnetic waves that are within the sensitivity range of the human eye are referred to as light. The light visible to humans has a wavelength of (purple) to (red).
Objects that independently generate and emit light are called self-luminous objects or light sources.  Most objects (usually called “objects” in optics) do not generate any light themselves, but are still visible. They reflect part of the light falling on them. They are called reflective or illuminated objects.
If light comes into our eye from a luminous or illuminated object, it becomes visible. Light that does not fall into our eye is invisible (the universe therefore appears black to us).
Sun, stars, fire, incandescent and fluorescent lamps, candles, fireflies, LEDs, etc.
Earth and moon, planets, objects (satellites, reflectors, dust particles, etc.)
The light spreads from the light source - as long as it does not hit an obstacle - in a straight line and in all directions:
- Sunlight illuminates all celestial bodies, regardless of which direction from the sun they are.
- A candle in the middle of the room illuminates all the walls, as well as the ceiling and floor.
The speed of light¶
In a vacuum (and approximately also in air) light can move with about Spread kilometers per second ("speed of light"). In other translucent materials, the speed of light is lower; in water, for example, light can "only" move at about Spread kilometers per second. 
If we could move at the speed of light, we could see the earth (circumference at the equator: approx. ) in a second times circle. In the universe, however, light has to cover such great distances that path lengths are sometimes given in light seconds, light minutes, light hours, light days or even light years. In order to get from the sun to the earth, a ray of light needs to be round Light minutes.
Light and illuminance¶
In order to be able to compare the brightness of different light sources, the light intensity was introduced as a physical quantity. If a light source appears brighter than another from the same distance, it has a greater luminous intensity.
Definition and unit:
The light intensity is a basic quantity of the international system of units. Your unit is the candela .
While the light intensity is a property of the light source, gives the illuminance on what proportion of the light hits a certain surface.
Since the light spreads evenly in all directions, the light rays are distributed over ever larger surfaces. The illuminance decreases to the same extent as the surface of the (spherical) surface flooded with light increases.
Formula and unit:
The illuminance is greater, the greater the light intensity the light source is. At the same time, the illuminance increases with the square of the distance from the light source. The following applies in total:
The illuminance is in lux specified.
Depending on the activity, different illuminance levels are perceived as pleasant.
Sunlight can have an illuminance of up to in summer cause only about on a cloudy winter day to . The light of a full moon on a clear night only has an illuminance of around .
The light transmission¶
There are two types of translucent objects:
- Transparent objects (such as window glass) let light through so that objects behind them can be seen.
- Translucent objects (e.g. frosted glass) let light through in such a way that objects behind them cannot be seen.
The transparency of an object depends on the material and the layer thickness. Water, for example, allows light to pass through in thin layers, but not in thick layers; in great depths of the sea there is complete darkness.
Objects that are opaque to light (e.g. metal, cardboard, etc.) do not allow light to pass through; they are opaque - provided that they are of sufficient material thickness. For example, very thin layers of paper are translucent, even though paper is inherently opaque.
Fades and shadows¶
Objects that are impermeable to light can limit the spread of light.
An aperture is a small, usually round opening in an otherwise opaque object. Only part of the light, a “bundle of light”, passes through a screen. This light bundle can be represented graphically by the bundle axis and marginal rays.
Creation of light bundles through a diaphragm.
The drop shadow
If an opaque object is illuminated by a small (point-like) light source, a shadow is created on the side facing away from the light source.
Umbra and penumbra
If an opaque object is illuminated by two point light sources, two shadow zones are formed: a core shadow and a penumbra. An extended light source also creates an umbra and a penumbra behind an opaque object. 
With "diffuse" lighting, the light comes from many directions. The individual shadow areas cancel each other out; no shadow can be seen.
Since some people (especially small children) are scared of shadows, indirect lighting is often used in living rooms and lounges. This is achieved by means of diffuse reflection by illuminating a light, matt surface with a (concealed) lamp.
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