Optics is an area of ​​physics that seeks to understand a large number of light  related phenomena . In view of this, it can be understood as a particular case of the wave , which studies the behavior of waves in the entire electromagnetic spectrum and not only in visible light.

Within classical physics, optics is divided into two subfields — geometric optics and physical optics. However, taking into account the knowledge of modern physics, there is also quantum optics — an area that studies the quantum and corpuscular behavior of light and other radiation, as well as their interaction with matter.

Geometric optics interprets light as straight lines, called light rays. Light rays are used to illustrate the direction and direction of propagation of light . A large part of the luminous phenomena that we observe in our daily lives can be explained solely with the contributions of geometric optics, such as shadows , eclipses and light reflection .

Geometric optics makes use of a relatively simple conception of light, so through it we can easily explain how the formation of images occurs in reflective optical systems , such as plane and spherical mirrors , but also in refracting optical systems , such as thin lenses , prisms and others.

Understanding luminous phenomena, according to geometric optics, involves some principles, so we will understand each of them in the next topic. If you want to learn more about this sub-area of ​​optics, read our text: Geometric Optics .

Principles of geometric optics

These principles explain how light rays behave in different situations. They are valid under specific conditions involving homogeneous ( constant refractive index ) and isotropic (having the same properties, regardless of direction) optical media . Learn about each of these principles:

  • Principle of Rectilinear Light Propagation: Light rays travel in a straight line.
  • Principle of independence of light rays: when crossing each other, two light rays cross each other as if they did not exist mutually.
  • Principle of reversibility of light rays: the direction of propagation of light rays is reversible.

wave optics

It is the division of optics that interprets light as an electromagnetic wave , with a well-defined frequency and wavelength . Wave optics allows the understanding of phenomena that cannot be explained by geometric optics, such as interference, diffraction , polarization , etc.

light sources

We call any body that emanates light a source of light . There are basically two types of light sources: primary and secondary:

  • Primary sources : are the bodies that produce light, also called luminous bodies. Light can be produced by different processes, such as thermoluminescence and luminescence, which involve different phenomena of light emission at low temperatures. Examples of primary sources are: the Sun and other stars, the flame of a candle, a light bulb, the resistance of an electric barbecue on, etc.
  • Secondary sources : these are bodies that only reflect the light that falls on them and, therefore, are known as illuminated bodies. Examples of secondary sources are : the Moon, humans, vegetables, etc.

In addition to the classifications related to the way light emerges from bodies, light sources can be divided between point sources and extensive sources, check:

  • Point sources : are those that have negligible dimensions, that is, they are very small in relation to the observer. Examples : the stars , a television pixel, a flashlight lit from several kilometers away, etc.
  • Extensive sources: these are light sources whose size cannot be disregarded, as their dimensions are comparable to those of the scene that is illuminated. Examples : Sun and Moon.


When white light falls on an object , part of it is absorbed by it. This light that has been absorbed can be transmitted directly to atoms, exciting them and providing them with thermal energy , for example. However, part of the incident light will be reflected back , and it is this part that defines the color of the illuminated bodies , so when we look at a red ball, we only see it that way because its atoms are not able to absorb red light.

Colors are also how the brain interprets visual stimuli. The human eye is capable of detecting a range of frequencies of electromagnetic waves known as visible radiation, which extends between infrared and ultraviolet radiation .

The human eye has different types of cells sensitive to three frequency peaks, which correspond to the colors green , red and blue. It is based on the combination of these three stimuli that the human brain “creates” our perception of colors.

monochromatic light

It is one that ideally presents a single frequency , that is, a single color . White light, for example, is polychromatic, that is, it is composed of different frequencies of light.

optical media

They can be transparent, translucent or opaque, let’s check the characteristics of each one:

  • Transparent media: are those in which light can be transmitted with little or no loss of intensity, in addition, it is possible to see clearly through them. Examples are : vacuum , air, glass, etc.
  • Translucent media: allow partial transmission of light, however, it is not possible to see through these media clearly. Examples are : mist, tracing paper, frosted glass, etc.
  • Opaque media : interrupt the passage of light, reflecting or absorbing it. Examples are walls, bones, metals, etc. The opacity of an optical medium depends on many factors, such as the density and distance traveled by light, but it also depends on its frequency . Some media are opaque only for certain frequencies, that is, they block the passage of certain colors.

optical systems

They are optical media arranged in different shapes and sizes, used to manipulate the direction of propagation of light . There are reflector and refractor optical systems.

  • Reflective optical systems: polished surfaces, plane mirrors, spherical mirrors, etc.
  • Optical refractor systems: flat diopters, concave and convex spherical lenses, etc.

shadow and twilight

Shadows are produced when some opaque medium intercepts light rays. When this happens, a region of space is formed where there is no direct incidence of light rays, this region is called a shadow.

The penumbra , in turn, is partially illuminated by light rays and is located in a transition region between shadow and luminosity. Penumbras are produced when opaque objects are illuminated by extensive sources of light. If you want to delve more into the formation of these effects, read: Shadow and Penunbra .

optical phenomena

They are events that can be observed and that occur by the interaction of light with matter. Check out the properties of the main phenomena of this type:


Occurs when light strikes a reflecting surface and returns to its original propagation medium . There are two types of reflection: regular and diffuse. In regular reflection , the angles of incidence and reflection are equal, and the incident and reflected rays are in the same plane, enabling the formation of reflected images. In diffuse reflection , it is not possible to see reflected images.


It is characterized by the passage of light through two media of different refractive indices . When light passes through media with different refringences, its propagation speed changes, causing lateral deviations in its trajectory. Want to know more about this type of optical phenomenon, read: Refraction of light .


It is the phenomenon in which a part or even all of the light incident on a body is absorbed . Bodies capable of absorbing all light incident on them are known as blackbodies . Most bodies, however, are not black, that is, they absorb only a part of the incident light. The color of secondary light sources is determined by the absorption spectrum of that body, that is, by its ability to absorb certain frequencies of visible light.


It is a process in which light passes through translucent or transparent optical medium. When light is transmitted through these types of media, its speed can change, as well as its propagation direction, which characterizes refraction.


It occurs when light passes through a slit of dimensions similar to its wavelength . When this happens, the slit starts to produce circular wavefronts Furthermore, the difference between the spaces traversed by the spherical wavefronts produces an interference pattern , which produces regions of high luminous intensity, followed by regions of low luminous intensity. To delve deeper into this property of waves, read: Diffraction .


It is a phenomenon in which the phase difference between two or more waves produces regions of high or low light intensity. We call constructive interference the interaction between waves that produces waves of greater amplitude; and destructive interference , the production of waves of lower or even zero amplitude at some points in space.


It is the name given to the process that selects the direction of oscillation of the electric field of an electromagnetic wave . For this to happen, the wave must pass through a polarizer , which will eliminate all components of the electric field that do not oscillate in the desired direction.

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