In homogeneous, transparent and isotropic means, light travels in a straight line.

## Principle of Rectilinear Propagation

Observing the behavior of light, it is possible to find some principles for their behavior:

The Principle of Independence
It states that when the light beams intersect, each follows its path as if the other did not exist.
The Principle of Reversibility
It says that the path followed by a beam of light does not change if the direction of propagation is reversed.
The Principle of Fermat
It states that the path of a light beam, to travel from one point to another, is such that the travel time is minimal. This is true for homogeneous media. In this case, the time to go through the path is minimal and the length of the beams as well. In the case of light successively passing through many different means, its trajectory, in every mean, will be rectilinear, so that the path taken is the shorter one.

## Consequences of Rectilinear Propagation of Light

Visual Angle (apparent diameter)
It is the angle at which a viewer sees an object. It depends on the size of the object and its distance from the eye of the viewer.
Visual Acuity Limit
It is the smallest visual angle necessary for an observer to distinguish details of an object. Approximately, this value corresponds to $$0.0003\ rad$$ or one minute $$(1')$$.
Dark Camera
It is a box with opaque walls and having a small hole on one of its faces. The use of this device can serve, among other things, to determine the distance from a source to the box. See illustration below.

## The moon

Moon Eclipse
When the Earth comes between the Sun and the Moon, a lunar eclipse ocurrs. This eclipse can be total or partial.
Solar Eclipse
A solar eclipse is when the moon comes between the sun and Earth, preventing all or some of the sun's beams to reach certain regions of the Earth's surface. The eclipse can be total or partial annular.
Annular Eclipse
An annular eclipse happens when the shadow cone of the moon is not long enough to touch the Earth. This eclipse is seen by an observer, located on Earth, as a luminous ring generated by the sun around the moon.
Moon Phases
Due to the formation of the very shadow of the moon, making visible only part of its surface, we can distinguish what we call the moon phases. Every 27.3 days it completes a cycle formed by the following sequence of phases: new moon, first quarter, full moon and waning moon.