Introduction

When radiation hits an object, different phenomena show how the incident radiation can interact with matter. This includes the following interactions:

• absorption
• transmission
• reflection

In order to demonstrate the interaction of radiation with matter, a laser beam is directed onto a white and a black surface, onto a glass pane and onto a mirror.

Absorption

While the laser spot on the white surface appears bright, the light spot on the black surface is less intense. Some of the radiant energy is obviously absorbed by the dark surface and is not reflected back afterwards. This phenomenon is therefore called absorption.

Absorption refers to the taking up of radiant energy by an irradiated object!

How strongly an object absorbs the incident radiant energy depends to a large extent on the color of the irradiated surface. Surfaces in dark colors absorb visible light more strongly than bright surfaces. This is also the reason why the light spot of the laser beam is less intense on the black surface compared to the white surface. The light is therefore absorbed more strongly by the black surface and is therefore no longer reflected. The white surface, on the other hand, absorbs less light and therefore reflects more. The light spot therefore appears larger and more intense.

As a result of the absorbed energy, the temperature of the irradiated object increases, since the absorbed energy results in increased particle motion. Since dark surfaces absorb more energy, the temperature of a black object increases more than that of a white surface.

Dark surfaces absorb light more than bright surfaces and therefore heat up more!

For this reason, materials for solar heating systems are kept in dark colors. The picture below shows a solar heater mat for a swimming pool. Water is pumped through the black plastic mat. The strongly absorbing mat heats up in the summer with sunshine on temperatures over 60 °C and warms up the water streaming through it.

Another example that uses the strong absorption of dark surfaces is the solar cooker shown below. The sun’s rays, which are bundled by reflection, fall on a black cooking pot, which heats up strongly as a result. Within a few minutes, the water inside starts to boil.

Conversely, white surfaces absorb much less of the light than black objects. The temperature increase is correspondingly lower. This is also the reason why in hot regions the house walls are kept in white.

Transmission

A further interaction of radiation with matter occurs when a laser beam hits a pane of glass. While hardly any light spot can be seen on the glass pane, it is clearly visible on a wall behind it. Obviously, hardly any radiant energy is absorbed when passing through the glass, but is almost completely transmitted through it. This phenomenon is therefore called transmission.

Transmission refers to the penetration of an object with radiation!

One could now think that there is no “interaction” between radiation and matter during transmission in the literal sense of the word, since the radiation apparently passes through the glass pane unaffected. However, the fact that the light beam is indeed influenced by the glass pane only becomes apparent when the beam is directed with an angle onto a thick glass pane.

When the laser beam enters the glass pane, there is a change of direction, i.e. the light beam is refracted. Such refraction is caused by the fact that the propagation speed of the light changes when it enters the glass pane. In glass, the light propagates at around 30% less speed than in air. This slower propagation speed results in a change of the propagation direction. The more the speed of propagation changes, the more the beam is refracted. The article Refraction explains the cause of this light deflection in more detail.

Refraction refers to the deflection of a light beam at the transition from one medium to another due to different propagation speeds!

The greatest speed of propagation of electromagnetic radiation is when radiation is not influenced by matter, i.e. in a vacuum. This is also referred to as the vacuum speed of light (the term “light” refers to any kind of electromagnetic radiation, not just visible radiation!). In matter, however, the speed of propagation will always be lower. The factor by which the speed of light in matter is lower compared to vacuum is also called the refractive index. The greater the refractive index, the stronger the change in the speed of propagation and the stronger the refraction.

Since air contains relatively few particles compared to liquid or solid materials, electromagnetic radiation is hardly influenced by air. Therefore, the speed of propagation of radiation in air can be equated in a very good approximation with that of vacuum. The refractive index of air is therefore 1 (to be precise: 1.0003 at 1 atm and 0 °C).

However, glass has a relatively high refractive index of 1.45. Therefore, an incident light beam from air is refracted relatively strongly. If, for example, water with a lower refractive index of 1.33 is used instead of glass, a slightly lower refraction is also shown.

The magnitude of the refraction is not only influenced by the different propagation speeds, but also by the angle at which the beam hits the boundary layer. The flatter the beam incident on the medium, the stronger the refraction. Conversely, this means that a light beam that hits a translucent medium perpendicularly is not refracted.

Note that the laser beam considered above, which hits the glass pane, changes its propagation speed twice. On the one hand when entering the glass pane and on the other hand when leaving the glass. Since the propagation speed increases again to the original value at the exit, a deflection in the original direction also occurs (principle of the reversibility of the light path). The light beam is shifted parallel by the glass. Such a parallel shift can be seen, for example, when looking at objects through a thick pane of glass. With an inclined viewing angle, the objects appear displaced behind the glass pane.

Reflection

If a laser beam is directed onto a mirror, there is no spot of light there, but on the opposite wall. The light beam is deflected (almost) without loss of energy. This phenomenon is called reflection.

Reflection refers to the throwing back of radiation by a “mirroring” object!

In the case of reflection, a distinction can also be made between specular reflection and diffuse reflection.

Specular (mirror-like) reflection

With specular reflection, the individual rays of a light beam are reflected in a common direction. The so-called law of reflection applies to each ray, which states that the angle at which a light ray hits the reflecting surface is also the angle at which the light ray leaves the surface.

A reflection is called “specular” (mirror-like) when the incident light rays of a light beam are reflected in a common direction!

Specular reflection occurs on smooth, reflective surfaces, e.g. a polished metal surface. Typical examples are mirrors, which ultimately consist of a smooth, reflective metal layer (usually silver or aluminium). To protect against mechanical damage, a glass pane is attached to the front and a lacquer layer to protect against corrosion is attached to the back.

The specular reflection on a mirror is also the reason why you can see objects with a mirror without “distortions”. The individual incident light rays maintain their relative position to each other even after reflection and do not cross each other. Thus, all light rays will fall on the retina of the eye exactly as if one would observe the object directly (apart from the mirror image perception).

Diffuse reflection (scattering)

However, if the metal surface of a mirror is scratched or a corroded metal plate is used, the individual incident light rays are reflected in different directions by the irregularities. The light rays originally incident in the same direction are scattered. Such diffuse reflection is therefore also referred to as scattering.

A reflection is called diffuse if the incident light rays of a light beam are reflected in different directions!

Since the diffuse reflection of a corroded metal plate reflects the light rays in different directions, they hit the retina of the eye at different points (represented by the white sheet of paper in the animation below). This results in a distorted or blurred image of the reflected object.

However, the fact that the object can still be seen in a blurred form on the metal plate is due to the fact that in reality there is usually no completely diffuse reflection. In most cases it is a mixed form, as the scattered light rays often still have a certain preferred direction.

Interactions in reality

Reality shows that absorption, transmission and reflection (both specular and diffuse) are generally not separate but always occur in combination! This can be seen, for example, very clearly with a tinted glass pane.

The darkening effect is due to the strong absorption of the light energy by the glass pane. However, the fact that objects can still be seen through the pane shows that light is still transmitted. In addition, reflections can be seen on the glass pane which suggest a certain reflection. Due to the surface roughness, the reflection will not only be specular, but also be diffuse.

Dependence of the interactions on the wavelength

The interactions explained above, such as absorption, transmission and reflection, were illustrated using visible light. However, visible light in a wavelength range from approx. 380 nm to 780 nm actually forms only a small part of the entire electromagnetic spectrum. In principle, the interactions described above apply to all types of electromagnetic radiation. The figure below shows the classification of electromagnetic radiation according to wavelength. The transitions between the individual types of radiation are always smooth and overlap partially.

Note: In the narrower sense, the term “light” usually refers only to the part of the electromagnetic spectrum visible to the human eye in the wavelength intervall from 380 nm to 780 nm. In some literature the term “light” in the broadest sense also stands for any kind of electromagnetic radiation.

The ability of an object to absorb, reflect or transmit depends not only on the material, but also on the type of radiation, i.e. the wavelength of the radiation. A pane of glass almost completely transmits visible radiation, i.e. it has a very high transmittance in the visible wavelength range. Shorter wavelengths such as ultraviolet radiation (UV) below 320 nm, on the other hand, cannot penetrate glass! For this type of UV radiation, glass shows a very high absorption capacity! This is also the reason why one cannot tan behind a glass pane, since the tanning UV-B rays do not pass through the glass pane. But you don’t get sunburn for that either.

How strongly an object transmits, reflects or absorbs depends decisively on the wavelength of the radiation!

Another example, which illustrates the dependence of interactions on wavelength, can be seen with human tissue. Visible radiation practically does not penetrate human tissue such as the skin. Therefore no bones can be seen through the skin. Radiation with shorter wavelengths such as X-rays, however, is able to penetrate human skin.

In hospitals, X-rays are therefore used to make bones (which in turn are impermeable to X-rays) visible with the aid of X-ray images. However, X-rays are not only transmitted by human tissue but are also absorbed to a certain degree, which ultimately makes X-rays dangerous for the human organism, as cells are destroyed by the large amount of radiant energy absorbed. As protection against unwanted irradiation, lead mats are therefore used, which in turn are impermeable to X-rays.

Introduction

When light hits an object, this radiation exerts a certain force on that object and, with respect to the surface, also a certain pressure. This pressure is referred to as radiation pressure.

In the particle model, it is relatively easy to understand how the pressure of radiation comes about. According to quantum mechanics, any propagating wave can also be imagined as a beam of particles (wave–particle duality). The individual light particles are then also called photons.

In quantum theory, photons are light particles that make up electromagnetic radiation!

Each photon carries both energy and a momentum. If the photons from the radiation hit an object with a certain energy, they exert an collision force on impact, analogous to tennis balls thrown against a wall.

The radiation pressure is also the reason why the tail of comets is always directed away from the sun. Under atmospheric conditions, the detached gas and dust particles of a comet would normally be slowed down by air resistance. The tail formed by the detached particles would therefore always form against the heading. In the vacuum of space, however, there is no air resistance. Only the radiation pressure of the sun acts (not to be confused with the solar wind). The photons of solar radiation “blow” the tail away from the sun.

In theory, radiation pressure can also be used as an alternative propulsion technology for space travels. With so-called solar sails, the power of radiation is used for propulsion in the same way as wind power for wind sails. The force of the radiation is relatively low, but due to the lack of air friction in space this can lead to an enormous speed over a longer period of time. This technology is therefore particularly suitable for small probes and satellites that travel very long distances between planets. Such a solar-sail propulsion is currently still the subject of research.

Derivation

Energy of photons

The force that the photons exert on an impact with a surface area \(\Delta A\) depends on the momentum of the photons. The momentum can be calculated from the energy of the photons. The energy of a photon \(W_p\) in turn results from the frequency of the radiation \(f\) or its wavelength \(\lambda\):

\begin{align}
\label{w}
&W_p = h \cdot f = h \cdot \frac{\lambda}{c} \\[5px]
\end{align}

\(h\) refers to Planck’ constant and \(c\) to the speed at which electromagnetic radiation propagates (speed of light).

Momentum of photons

By the definition of the momentum as the product of mass and velocity (\(p=m v\)) and the famous mass-energy equivalence equation of Einstein (\(E=m c^2\)), a relationship between momentum and energy for photons can be derived.

The energy \(E\) in Einstein’s equation denotes the energy \(W_p\) of a photon and the velocity \(v\) in the formula for the momentum, denotes the speed of light of the photon \(c\) (\(m\) in this case stands for the relativistic mass of the photon).

\begin{align}
p &=m \cdot c &&~~~~~\text{momentum of photons}\\[5px]
W_p &=m \cdot c^2 &&~~~~~\text{mass-energy equivalence for photons}\\[5px]
\end{align}

If one divides the upper equations, then the following relationship results between momentum and energy of a photon:

\begin{align}
&\frac{p}{W_p}=\frac{m \cdot c}{m \cdot c^2} = \frac{1}{c} \\[5px]
& p = \frac{W_p}{c} \\[5px]
\end{align}

The same equation can also be derived using the energy-momentum relation from the theory of relativity:

\begin{align}
&W_p = \sqrt{(p \cdot c)^2+ (m_0 \cdot c)^2} ~~~~~\text{energy-momentum relation}\\[5px]
\end{align}

Since photons move at the speed of light and thus obviously cannot have a rest mass (\(m_0\)= 0), there is a relatively simple relationship between energy and momentum for photons:

\begin{align}
&W_p = p \cdot c \\[5px]
& p = \frac{W_p}{c} \\[5px]
\end{align}

Radiation pressure with complete absorption

In the following, the formula for calculating the radiation pressure is derived. It is assumed that the object completely absorbs the incident radiation.

If a total of \(\Delta N\) photons from the radiation hit a surface area \(\Delta A\) within a certain time \(\Delta t\), then the following total momentum \(p_{tot}\) results:

\begin{align}
&p_{tot} = p \cdot \Delta N = \frac{W_p}{c} \cdot \Delta N = W_p \cdot \Delta N \cdot \frac{1}{c} \\[5px]
\end{align}

At this point it can be used that the product of photon energy \(W_p\) and number of photons \(\Delta N\) corresponds to the amount of energy \(\Delta W\) that hits the area \(\Delta A\) within the time \(\Delta t\):

\begin{align}
&p_{tot} = \overbrace{W_p \cdot \Delta N }^{=\Delta W} \cdot \frac{1}{c} =\Delta W \cdot \frac{1}{c} \\[5px]
\end{align}

Due to the law of conservation of momentum, this total momentum remains completely conserved even after the impact on the initially stationary object. If it is assumed that all photons are absorbed on impact, then this total momentum must be transferred completely to the object. From the change in momentum of the object \(\Delta p\) (impuls), which therefore corresponds to the total momentum \(p_{tot}\) of the particles, the exerted force \(F_{tot}\) on the object can be determined as momentum change per unit time:

\begin{align}
&F_{tot} = \frac{\Delta p}{\Delta t} = \frac{p_{tot}}{\Delta t} = \frac{\Delta W \cdot \frac{1}{c}}{\Delta t} = \frac{\Delta W}{\Delta t} \cdot \frac{1}{c} \\[5px]
\end{align}

In this equation, the quotient of energy and time can be interpreted as the radiant power \(\Phi_{beam}\) of the beam, i.e. as the amount of energy that hits the surface \(\Delta A\) per unit time:

\begin{align}
&F_{tot} = \overbrace{\frac{\Delta W}{\Delta t}}^{\Phi_{beam}} \cdot \frac{1}{c} = \Phi_{beam} \cdot \frac{1}{c} \\[5px]
\end{align}

Since the force \(F_{tot}\) acts on the surface \(\Delta A\), it is now possible to calculate the pressure caused by the incident particle beam with the power \(\Phi_{beam}\). This pressure is also called radiation pressure \(p_{beam}\) (not to be confused with the photon momentum \(p\)!):

\begin{align}
\label{ww}
&p_{beam} = \frac{F_{tot}}{\Delta A} =\frac{\Phi_{beam}}{\Delta A} \cdot \frac{1}{c} \\[5px]
\end{align}

The quotient of the incident radiant power \(\Phi_{beam}\) and the respective surface area \(\Delta A\) is also called intensity or irradiance or radiant flux density, since this quantity is a measure of how strongly an area is irradiated with energy (surface power density). There is therefore a relatively simple relationship between intensity \(I\) and radiation pressure:

\begin{align}
&\boxed{p_{beam} =\frac{I}{c}} ~~~~~\text{only valid for complete absorption} \\[5px]
\end{align}

Intensity or irradiance is the term used to describe the area-related radiant power (surface power density)!

Radiation pressure with complete reflection

Note that the above relationship between radiation pressure and intensity only applies if the radiation is completely absorbed when it hits the object!

If, on the other hand, the radiation is completely reflected, then the photons are returned with the same magnitude of momentum, but in the reflected direction. In order for the total momentum to be conserved, the object had to be subjected to an impulse change twice as large. Accordingly, a radiation pressure twice as high acts on the reflecting object at the same intensity!

\begin{align}
&\boxed{p_{beam} =2 \frac{I}{c}} ~~~~~\text{only valid for complete reflection} \\[5px]
\end{align}

The radiation pressure on an ideally reflecting surface is twice as large as on an ideally absorbing surface!

The fact that the radiation pressure with reflection is twice as high as with absorption can also be clearly understood. An analogous example is a person standing on a frictionless skateboard. If a heavy medicine ball is thrown at this person, the momentum is transferred to the person including the skateboard when the ball is caught. This catching of the ball corresponds in the figurative sense to the complete absorption of the photons.

But not only by throwing and catching the ball can the person be set in motion, but also by throwing it away. When throwing the ball away, the person pushes himself away from the medicine ball and also gets an impulse (“rocket principle”).

So if the person gets a medicine ball thrown at him and then throws it away again, then he has used it in two ways. Not only does he get an impulse by catching it, but he also generates an impulse by throwing it away. If the ball is thrown off again with the same (relative) speed as it was caught, then the person has been able to transfer the double impulse to himself (double force) compared to the pure “absorbing” of the ball.

If one makes the catching and throwing of the ball faster and faster in thought, then it is kinematically regarded as a pure rebound of the ball at the person, i.e. a reflection! A reflection thus produces an impulse twice as large as an absorption.

Relationship between radiation pressure and energy density

Directed radiation (photon beam)

At this point equation (\ref{w}) shall be interpreted in another respect. If the propagation speed of the wave \(c\) (speed of light) is expressed by the distance travelled (\Delta s) within the time \(\Delta t\), then we obtain:

\begin{align}
&p_{beam} = \frac{\Phi_{beam}}{\Delta A} \cdot \frac{1}{c} = \frac{\Phi_{beam}}{\Delta A} \cdot \frac{1}{\frac{\Delta s}{\Delta t}} = \frac{\overbrace{\Phi_{beam}\cdot \Delta t}^{\Delta W}}{\underbrace{\Delta A \cdot \Delta s}_{\Delta V}} =\frac{\Delta W}{\Delta V} = w \\[5px]
&\boxed{p_{beam} = w} ~~~~~\text{only valid with complete absorption}
\end{align}

In the equation above, it was used that the product of radiant power and time corresponds to the energy of the radiation \(\Delta W\) contained in the volume \(\Delta V\) and which then exerts the radiation pressure \(p_{beam}\). The quotient of radiant energy and volume is also called energy density \(w\) and represents the radiant energy per unit volume.

As this equation states, the radiation pressure can also be understood as energy density! This direct correlation between volumetric energy density and radiation pressure is only valid if the radiation is completely absorbed when it hits the object!

In this case, the radiation must also hit the absorbing surface perpendicularly. For example, if the surface is parallel to the radiation, then the photons obviously do not collide with the surface. Consequently, no radiation pressure is exerted, although the radiation of course still has an energy and thus an energy density (the energy does not hit the surface, so to speak).

The energy density of a directed beam of radiation corresponds with complete absorption to the radiation pressure!

In contrast to the case of complete absorption, with complete reflection of the radiation the same energy density leads to a radiation pressure twice as high, since the impulse is twice as high:

\begin{align}
&\boxed{p_{beam} = 2w} ~~~~~\text{only valid with complete reflection}
\end{align}

Non-directed radiation (photon gas)

Note that the derived correlations between radiation pressure and energy density are only valid for directional radiation. In these cases, all photons move in the same direction. Thus also all photons take part in the exercise of the pressure.

The situation is different when it is no longer a matter of directional radiation but of a completely statistical distribution of photon motion. This is the case, for example, when radiation is observed in a mirrored cavity.

The photons are then permanently reflected on the walls and over time a completely random photon movement will form. From the originally directed motion, in which all particles moved towards the wall, on average only one sixth will now be moving in the direction of the wall. The remaining particles will move to the right, to the left, upwards, downwards or away from the wall. At the same energy density (photon density), the radiation pressure at such a random photon motion is only one sixth compared to a directed photon beam:

\begin{align}
&\boxed{p= \frac{1}{6} \cdot p_{beam} = \frac{1}{6}\cdot 2w = \frac{1}{3}w} ~~~~~\text{applies only to non-directed radiation}
\end{align}

The chaotic motion of photons is similar to the motion of particles in gases. For this reason, this type of disordered radiation is also called photon gas. The Stefan-Boltzmann law for radiating bodies, for example, can be derived by looking at radiation as a photon gas and the relationship between energy density and radiation pressure.

Perception of colors (cones)

The human eye can perceive different colors. This is typically seen when looking at a rainbow that contains all colors visible to the human eye: red, orange, yellow, green, blue, indigo and violet. These clearly distinguishable colors are also known as spectral colors. In addition, the human eye can perceive color nuances between the spectral colors and differentiate between light and dark. This results in millions of color impressions that the human eye can theoretically perceive.

Whether a color is perceived as red or blue depends on the wavelength of the light that hits the retina of the eye. In the wavelength interval from about 380 nm to 450 nm, the light appears violet to us. In the range from about 630 nm to 700 nm, however, the light is perceived as reddish.

The color vision of the human eye is caused by different wavelengths of light!

Responsible for color vision are receptors on the retina of the eye, the so-called cones. There are three types of cones, each of which reacts differently to certain wavelengths:

• short wavelength receptor (S-cone: “blue receptor”)
• medium wavelength receptor (M-cone: “green receptor”)
• long wavelength receptor (L-cone “red receptor”)

The color perception depends on how strongly the different cones react to the incident light. The sensitivities of the different types of cones are not sharply limited to certain wavelength ranges, but merge smoothly into one another. Red light mainly addresses the L-cones and blue light mainly addresses the S-cones. With violet light, on the other hand, both of these receptor types react equally to the light stimulus. With yellow light, the L-cones and M-cones are mainly addressed.

Different receptors in our eyes, so-called cones, react differently to the wavelengths contained in the light and thus create the color vision!

The figure below shows which colors are perceived according to the additive color mixing when the different cone types perceive a light stimulus.

Perception of light and dark (rods)

Experience shows that our eyes can perceive not only colors (so-called chromatic colors), but also achromatic colors without hue or saturation such as white and black (as well as grey). An object is perceived as black when there is no sensory stimulus for the receptors. Thus, such a black object does not emit any visible radiation. Therefore we “see” at night, far away from any light source, everything in black or more exactly said nothing, because the eye does not perceive any radiation!

If there is no sensory stimulus for the human eye, an object appears black!

In In most cases, even a black object will always emit or reflect a certain amount of radiation, no matter how small. The object will therefore never be exactly black but will appear in grey. In fact, even supposedly colored objects appear gray in poor lighting conditions. This is because the cones in our eye need a relatively high light intensity to trigger a stimulus. The color vision of objects therefore disappears in low light conditions because the cones are no longer stimulated.

The fact that we can still see these objects without (but without hue oder saturation), however, is due to another type of receptor, the so-called rods. In contrast to the cones, the rods react at much lower intensities. However, the rods cannot perceive any differences in the wavelengths of the light and thus do not give any color vision. Rods are therefore only responsible for perceiving light and dark. In fact, the human eye has about 20 times as many rods as cones, namely more than 100 million!

While cones are responsible for color vision, rods are responsible for the perception of light and dark!

Perception of white

In contrast to the perception of a black color, which lacks any sensory stimulus, the perception of white, on the other hand, addresses all three types of cones in our eye equally (see figure “Additive color mixing”). This suggests the conclusion that the achromatic color white contains all visible wavelengths equally.

And in fact, this can be demonstrated relatively easily with a prism. If the white sunlight hits such a prism, the rays are refracted. Since the strength of the refraction depends on the wavelength (dispersion), the different colored light rays are refracted to different degrees. For example, the bluish rays are more strongly refracted than the reddish ones. In this way, the spectral colors contained in white light are separated and can be perceived individually.

If one would bundle these individual spectral colors again to a single light beam, then this light would appear white again! The individual spectral colors of the rainbow that emerge from the white sunlight are also produced according to the principle of such a prism (raindrops serve as “prisms”).

White light contains the entire visible wavelength spectrum equally!

Colour appearance of objects

The appearance of the color of an object is ultimately based on the principle of absorption and reflection of light when it hits the object and then the eye. If, for example, sunlight hits an opaque object with all its wavelengths, certain wavelengths are absorbed depending on the properties of the surface; the rest is reflected. Depending on which wavelengths are present in the reflected spectrum that hits our eyes, the object appears to us with the respective color.

From the reflected part of the incident light on an object, the color appearance of the object is created!

For example, a leaf on a tree absorbs almost all visible wavelengths except those between 500 nm and 550 nm (“green”). This non-absorbed wavelength interval is therefore reflected. It is the green wavelength spectrum. This is why a leaf appears green in summer.

If, on the other hand, an object absorbs only the color green from the incident white light and reflects all other wavelengths, then the S-cones (“blue receptors”) and L-cones (“red receptors”) are mainly triggered in the eye. As already mentioned, in this case the object appears violet.

If an object does not absorb any visible wavelengths and thus reflects all wavelengths equally, all receptor types are addressed to the same extent in the eye. In this case, the object appears white.

If, on the other hand, an object absorbs all visible wavelengths to a particular degree, then (almost) no visible radiation is reflected. The receptors in the eye are then also not stimulated and the object appears black.

Complementary colors

Two colors are always complementary to each other when they are mixed to produce the color white.

Complementary colors are two colors that are mixed to produce the achromatic color white!

One can imagine the formation of the complementary colors as follows. First, white light is broken down into its spectral colors with a prism. Now one filters out a color in one’s mind by, for example, placing a small absorbing object in the appropriate place in the separated color beam. The remaining rays are now refocused with a lens. The resulting color of the bundled light is now the complementary color to the filtered color.

Complementary colors are often represented in a color wheel. Two opposing colors are complementary to each other. Very often such color wheels are found in computer programs for color selection.