Propagation of light¶

Light is the term used to describe electromagnetic waves that are within the range of sensitivity of the human eye. Light visible to humans has a wavelength of" /> (purple) to" /> (red).

Light sources¶

Objects that independently generate and emit light are called self-luminous objects or light sources.[1] Most objects (usually called "objects" in optics) do not produce light themselves, but are visible nonetheless. They reflect back a part of the light falling on them. They are called reflective or illuminated objects.

When light from a luminous or illuminated object reaches our eye, the object becomes visible. Light which does not fall into our eye is invisible (therefore the universe appears black to us).

Solar rays as light source of our universe.

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 meet any obstacle – in a straight line and in all directions:

  • sunlight illuminates all celestial bodies, no matter in which direction from the sun they are located.
  • A candle in the middle of the room illuminates all the walls as well as the ceiling and floor.

The speed of light¶

In vacuum (and approximately in air), light can propagate at about kilometers per second ("speed of light"). In other translucent materials, the speed of light is lower; in water, for example, light can "only" propagate at about kilometers per second.[2]

Values of the speed of light in different substances ¶

Medium Speed of light in>" />

If we could travel at the speed of light, we could reach the earth (circumference at the equator: approx. " />) in one second times. In the universe, however, light has to travel such great distances that path lengths are sometimes also given in light seconds, light minutes, light hours, light days or even light years. To get from the sun to the earth, a ray of light needs about minutes of light.

A beam of light emitted by the earth reaches the moon about one second later.

Light and illuminance¶

In order to be able to compare the brightness of different light sources, the luminous intensity was introduced as a physical quantity. If a light source appears brighter than another at the same distance, it has a greater luminous intensity.

Definition and unit:

The luminous intensity>" /> is a basic quantity of the international system of units. Its unit is the candela )" />.

  • A candle flame has a luminous intensity of about" />.
  • Incandescent lamps have a luminous intensity (depending on the type) of" /> to" />.
  • Car headlights have a luminous intensity of around" />.

While luminous intensity>" /> is a property of the light source, gives the illuminance>" /> indicates which part of the light hits a certain area.

The illuminance as a function of the distance from the light source.

Since the light spreads evenly in all directions, the light rays are distributed over larger and larger areas. The illuminance decreases at the same rate as the surface area of the (spherical) surface flooded with light increases.

Formula and unit:

The illuminance¶ /> is greater the greater the luminous intensity" /> of the light source is. At the same time, the illuminance decreases quadratically with the distance from the light source. Overall:

= \frac>" />

Illuminance is measured in lux )" /> specified.

Depending on the activity, different illuminance levels are perceived as pleasant.

  • A candle flame has in" /> distance an illuminance of about" /> have.
  • Traffic routes should have an illuminance of at least" /> show.
  • The recommended illumination intensity for living rooms is" /> to" />; in offices are also illuminance of" /> not uncommon.
  • The recommended illuminance for reading and writing is " /> /> to" />.

Sunlight can have an illuminance of up to" in summer /> on a dull winter day only about" /> to" />. The light of a full moon on a clear night has an illuminance of only about" />.

The light transmittance¶

Light-transmissive objects exist in two ways:

  • Transparent objects (e.g. window glass) allow light to pass through them in such a way that objects behind them can be seen.
  • Translucent objects (such as frosted glass) allow light to pass through them in such a way that objects behind them cannot be seen.

A candle behind a transparent or translucent pane.

The light transmission of an object depends on the material and the layer thickness. Water, for example, is light-transmissive in thin layers but not in thick ones; at great ocean depths, there is complete darkness.

Light-impermeable objects (e.g. metal, cardboard, etc.) are.) do not let any light through, they are opaque – provided that they have a sufficient material thickness. For example, very thin layers of paper are translucent, even though paper itself is opaque.

Apertures and shadows¶

Light-impermeable objects can be used to limit the spread of light.

An aperture is a small, usually round opening in an otherwise opaque object. Only a part of the light, a "light beam", passes through an aperture. This light bundle can be represented graphically by the bundle axis and edge rays.

Creation of light beams through an aperture.

If an opaque object is illuminated by a small (point) light source, a shadow is created on the side facing away from the light source.

Formation of a cast shadow.

Core and penumbra

When 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 a core shadow and a penumbra behind a light-impermeable object.[3]

Formation of core and penumbra.

In "diffuse" lighting, light comes from many directions. The individual shadow areas cancel each other out in the process; no shadow is visible.

Since some people (especially small children) are skittish about shadows, indirect lighting is often used in living rooms and lounges. This is achieved by means of diffuse reflection by illuminating a bright, matte surface from a (concealed) lamp.

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