A simple experiment you can re-produce, that proves that light propagates along a rectilinear path.

Light can be represented as a transverse electromagnetic wave made up of mutually perpendicular, fluctuating electric and magnetic fields.
Here we have considered only the electric field vector but keep in mind that the magnetic field component behaves similarly. Light whose electric field oscillates in a single plane is said to be polarized.
Therefore light can be linearly polarized or circularly polarized. A combination of those two components results in the elliptical polarization, meaning that the electric field spirals with an elliptical cross-section.

Wave-particle duality is a central idea of modern physics. We now know that light exhibits properties of both waves and particles. Wave theory is the only way to explain interference patterns and diffraction of light but it failed to predict the photoelectric effect discovered in the early 20th century. This was the time when Einstein and Planck introduced the idea of Quanta of light, or Photons. The wave and particle properties of light are inseparable and complementary. One speaks of wave-particle duality, which idea marked the beginning of Quantum mechanics.

Starting with an elliptically polarized beam of light, the polarizer transforms this incident beam into a linearly polarized one.

Luminous objects are objects that generate their own light.
Illuminated or non-luminous objects are objects that only reflect light.

In order to view an object, you must sight along a line at that object. But in fact, you have to know that light will come from that object to your eye along this line of sight.
The object will only be seen if light reflected from that object penetrates the eye.

To see, enough light must go into the eye.
In general, light propagates from the source to the object and then from the object to the eye. The objects that are seen in this way are secondary sources. In contrast, the light bulb that illuminates the scene is a primary source.

Since light  moves  in straight lines, and because it is not visible, we can draw a line with an arrowhead to represent how the light behaves. Doing so illustrates the ray model of light. The “rays” are represented as line segments leaving primary or secondary sources, and travelling without interruption until they encounter an obstacle.  Because it is not possible to draw an infinite number of lines, we sometime draw a beam of light.

An animation to create day and night!
This photomontage allows you to turn on or off various light sources. We distinguish primary sources (the sun, the stars, or a lamp) and the secondary sources (the Moon or any reflecting object).
We also illustrate that shadows only appear in contrast to the supplementary light created by a source.