Light, Optics, and The Visual system

Light:

Light, as kinetic energy, is one of the essential energy types in the world. The primary light sources in nature are the sun, electric lights, and fire. However, different theories in physics explain light. In classical physics, light is considered a wave phenomenon, while in quantum mechanics it is regarded as a particle. The quantum theory, which describes energy transferring by bundles of energy (or photons), and the theory of electromagnetic radiation, which represents the shape and form of this transfer, are two well-known theories proposed by Albert Einstein in 1905 and James Maxwell in 1865, respectively.

Visible light carries energy between 2.69x10^(-19) J to 5.23x10^(-19) J and covers a wavelength in the range of 400–700 nanometers. Ultraviolet, X-, and gamma-rays have a wavelength below 400 nm and energy greater than 5.23x10^(-19) J. Infrared radiation, microwaves, and radio waves have a wavelength above 700 nm and energy less than 2.69x10^(-19) J. The higher frequency, the more energy carried by EMR.

 

The visibility of EMR is dependent on three parameters that are directly related to each other: energy, frequency, and wavelength. Energy is the leading property of electromagnetic waves, influencing frequency and wavelength. Therefore, from a biological point of view, it is more reliable to consider energy as a crucial factor in optico-visual studies than the wavelength.

The mass of energy in EMR has two aspects or dimensions. The first one, the magnetic field, determines wavelength, frequency, and, in our vision, color. The second one specifies the wave’s amplitude, which implies the magnitude of the electric field. Eyes perceive a wave's amplitude as brightness (subjective), while photometric measurements detect it as luminance (objective). The combination of these two fields forms the electromagnetic radiation that our eyes can detect, allowing us to appreciate the variety of color and brightness in our environment.

Frequency, S^-1 or 1/s:   7.1x10^14     6.4x10^10     5.7x10^14     5.2x10^14     4.8x10^14     4.2x10^14   

Color:                                  Violet             Blue          Green          Yellow          Orange          Red

Optics:

Reflection: every object our eyes can see reflects light from their surfaces or borders, except objects that produce or scatter light. We can see the light directly and indirectly (reflected). 
The reflection and absorption of electromagnetic waves depend on the arrangement of atoms and molecules on the surfaces of different materials. The types of reflections that take place on the surfaces of different materials can be divided into two categories: specular reflection (off of a smooth surface) and diffuse reflection (off of a rough surface). The amount of energy reflected from the surface of an object creates a wave with a specific frequency and wavelength.  However, there is a significant difference between material surfaces and the retina, which is a biological surface. Therefore, the intraocular reflection deteriorates the vision.

Absorption: different tissues and materials could absorb the energy from electromagnetic waves. This principle of absorption was applied to research in fields such as materials science, engineering, and medicine. In this case, some energy from the waves stays in the materials. This energy absorption is often referred to as "dielectric loss" and can be seen in materials such as metal. All visible light spectrums are absorbed by black and reflected by white. It is difficult to determine whether reflected or absorbed waves are important and primary factors in determining the color of an object's surfaces. The choroid plexus's dark-colored pigment (melanin) absorbs the light and prevents its reflection.

Transmission: the "transmission" of light is the moving of electromagnetic waves through a material or a vacuum. Light energy, both electrical and magnetic, decreases as it passes through different materials. This can cause the light to bend, refract, or even change color, depending on the material. In the eyes, this energy leads the waves to transmit through intraocular tissues and stimulates retinal rod and cone receptor molecules. Nevertheless, the retina only reacts to EMR with adequate energy levels. 
Intraocular tissues decrease the wave’s energy level and lengthen EM-wavelengths, shifting the wavelength to the red (right side of the visible light spectrum). It implies that the human visual system sees the world as sharper, more reddish, and less bright. According to this, we live in a brighter and more purple (higher amplitude and shorter wavelength) world than our eyes perceive. These differences in the perception of colors can have profound effects on the way we interact with our environment.

Diffraction: diffraction is the bending of light and its deviation from its original direction. The bending occurs when the light passes through a small split or around the object's edge. Small pupil size results in diffraction and, to some extent, a decline in visual acuity.

Dispersion: the separation of light into different wavelengths refers to dispersion occurring when light passes the glass prisms. The phenomenon of dispersion is known as the "rainbow effect." As a result, white light is separated into its constituent colors. Dispersion is related to the electromagnetic waves' velocity and materials' refractive indices. 
For example, a red light bends more than a violet light because of its higher speed. However, in lenses, dispersion causes an undesired phenomenon called chromatic aberration that may degrade images in the eyes.

Transformation: three typical examples of light transformation are converting to thermal, 
electrical (photoelectric effect of the sun or photovoltaic cells), and chemical energy (photosynthesis). In addition, in the optico-visual system, EM-waves are transformed into neurophysiological pulse signals.

Polarization: the electrical field vector in light waves moves in different directions. Polarizers act as filters that permit light waves traveling in the same direction to pass.

Scatter: small particles (e.g., ice crystals, dust, gases, and atmospheric particles) can scatter light. In this manner, the light waves are refracted and reflected in various directions. This phenomenon explains the color of the sky. The shortwave blue light is more easily scattered by the small particles in the atmosphere, so more of it reaches our eyes and gives us the illusion of a blue sky.  

Refraction: the refraction of waves and the declining sharpness of our environment are dependent on the passage of light through different surfaces. The oil, water, and mucous layer covering the cornea are the first media to refract the light in the optico-visual system. Subsequently, the cornea, aqueous humor (anterior and posterior chambers), lens, and vitreous body refract the light in turn. Due to the various layers of these materials, light is scattered and lost, causing a blurred image. The eye globes' primary function is to accommodate EM waves and attract them to the retina's macula (with the fovea in the center, which is specialized for high acuity vision). This property is determined by the shape of the eye globes, cornea, and lenses and, on the other hand, by the consistency and refractive index of different intraocular tissues. Together, these components work to ensure that light is refracted correctly to reach the macula of the retina. For example, a person with astigmatism distorts the focal point of light in front of and/or behind the retina. In the absence of eye globes, the high energy level of EM waves could damage the sensitive photoreceptors in the retina, leading to blindness after a few minutes. Eye globes with extraocular muscles have a directive and modifying role in the visual system. The extraocular muscles control the position of the eye in the orbit, and this is important for allowing a wide field of view. Other directive factors such as neck and face muscles, eyelids, and pupils help us accommodate EM-waves to the macula.

The visual system:

The visuality begins with an EM-wave with a wavelength of 360–400 nm and gradually decreases to reach EM-waves with a wavelength of 700–750 nm. Visual ability in human beings starts its establishment with red and is gradually reduced by orange, yellow, green, blue, and violet. This means that red has the highest visible wavelength and violet has the lowest. Outside of this range, visual acuity is significantly diminished. Human eyes see red better than violet. The activation of rod and cone cells in the retina establishes the visual process. When the eyelids are open, these cells are constantly exposed to energetic waves. The appropriate energy level stimulating the retina’s receptors is around 2.69x10^(-19) J, which matches red. Unfortunately, the retina’s photoreceptor cells could not detect the energy below this level. Therefore, it is evident that the visual ability in the eyes does not follow the normal distribution.

The cone and rod cells are photoreceptor cells responsible for the vision of daytime (photopic) and nighttime (scotopic). The three kinds of cone cells respond to different levels of light energy (amplitude); furthermore, they react to long, medium, and short wavelengths and detect colors. EMR with too low or high electrical energy levels does not stimulate the cone cells appropriately; therefore, color determination is absent in these situations. This determination is very limited in the case of bright light or darkness. This explains why colors are perceived differently under various lighting conditions and why people experience difficulty when trying to differentiate colors in extreme light levels. The reaction of the visual system to the magnetic field is impaired in patients with color blindness. On the contrary, patients with night blindness (nyctalopia) have a reduced ability to respond to the electrical field.

The network of synapses in the optic nerve at the back of the orbit transforms point images into patchy images and finally into panorama images. This transformation happens within milliseconds and requires millions of neurons to work together to create a representation of the world that our brains can interpret. The visual system is a result of projecting a three-dimensional world onto a two-dimensional surface. This system is structured for three-dimensional and panoramic images, which are missing from mobile phones, tablets, and computers. Repeated switching from three-dimensional to two-dimensional vision and vice versa can result in dizziness, blurred vision, headaches, or vertigo. These symptoms have been more common in today’s children and adolescents. As a result, it is important to limit the amount of time that people spend in front of screens.

The electromagnetic waves are changed into neurophysiologic pulse signals, leading to the lateral geniculate nucleus via optic nerves, optic chiasma, and optic tracts. The pulse signals then travel through the thalamus to the primary visual cortex, where they are processed and analyzed before being sent to other parts of the brain for interpretation, integration, and memorization. The retinal ganglion cells' axons have a relay station in the lateral geniculate body, as a thalamic nucleus. This process is known as "retinogeniculate transmission.” However, it needs to be clarified if electromagnetic waves move through the optico-visual system with the same feature, i.e., with their electric and magnetic fields. To answer this question, a more in-depth study of the physiology and psychology of vision is needed. Indeed, it is evident that these waves transmit through optic nerves containing water, soluble and insoluble proteins, and lipids.

- Additive (light) vs. subtractive (paint or pigment) colors

- Visual acuity: clarity or sharpness of vision

- Visual perception: color, perspective, and contrast determination (static and dynamic)

- Visuospatial function: detection of distance, depth, or speed

- Visual attention: visual awareness of specific locations, objects, or attributes

- Visuoauditory integration: head orientation, threat detection

- Visuomotor integration: eye-hand or eye-leg coordination. Painters have good eye- hand coordination and football players have excellent eye-leg coordination.

- Visuoautonom function: images or photos can lead to different autonomic reactions such as violence, love, fear, or hate.

- Visual memory: recognition (dorsal and ventral stream pathway)

Summary:

Optico-visual system is a complex system where physics, medicine, and art (painting and photographing) meet. By studying the optico-visual system, scientists can gain a deeper understanding of how humans process visual information and perceive reality. This system has unique anatomical properties with different operative functions, such as:

Function:                                                              Site of action:

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- Reflection                                                          cornea

- Diffraction                                                          pupil

- Refraction                                                          cornea, lens

- Transmission                                                     lens, vitreous body

- Polarization                                                       cornea?

- Reaction                                                            cone and rod cells

- Transformation                                                 retina, optic nerve

- Perception                                                        lateral geniculate body? visual cortex

- Interpretation                                                   visual cortex Integration visual cortex Memorization dorsal and ventral stream pathway ————————————————————————————————————————

In the last decades, diverse scientific methods have been established to increase our knowledge about light, optics, and the visual system. In particular, the integration of molecular and physiological techniques with detailed anatomical studies of the brain has provided new insights into the complex pathways through which light is translated into meaningful information. Colorimetry, spectrophotometry, densitometry, color temperature measurement, and other techniques in this field try to determine the correlation between objective and subjective values. This system perceives the world as sharper, darker, and more reddish with its diverse functions. Color is the interpretation (not hallucination) of specific electromagnetic wavelengths or frequencies by the visual cortex.

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By: AmirHossein Mahdavian MD
Pediatric neurologist
Januari 2023
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