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Visible Light and the Eye

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Visible light is responsible for our vision. The effect of visible light on the eye's cellular and neural pathways produces the object's image.

Medically reviewed by

Dr. Shikha Gupta

Published At March 28, 2023
Reviewed AtMarch 28, 2023

Introduction

Light is one of the most important parts of the vision. Light is reflected from the surface of the object and hits the eye. From there, it passes the cornea and lens, and ultimately it hits the focal point. In this process, vitreous and aqueous humor and their refractive index play a vital role. As they help in focusing the incoming light. Following this, rods, cones cells, and neurons present in the visual path analyze the incoming signal and process it. But the human eye can not process all types of light; the human eye can detect only light within a certain wavelength.

What Is Visible Light?

Visible light is the spectrum of light that is visible to the naked eye. The wavelength ranges from 380 to 750 nanometers. The wavelength of light below 380 nanometers is known as the ultraviolet spectrum of light. The wavelength of light ultraviolet A is 315 to 400 nanometers, the wavelength of ultraviolet B is 280 to 315 nanometers, and 100 to 280 nanometers is the wavelength of ultraviolet C light. On the other hand, the infrared spectrum's wavelength ranges between 700 to 1000 nanometers. Needless to say, in the naked eye, only light within the spectrum of 380 to 750 nanometers is visible. The wavelengths of the different visible spectra of light are:

  • Violet: 380 to 450 nanometers.

  • Blue: 450 to 485 nanometers.

  • Cyan: 485 to 500 nanometers.

  • Green: 500 to 565 nanometers.

  • Yellow: 565 to 590 nanometers.

  • Orange: 590 to 625 nanometers.

  • Red: 625 to 750 nanometers.

What Is the Effect of Light?

The effect of light on the eyes can be divided into three parts.

1. The Visual Effect - The visual effect of the light is based on two events one is a cellular event and another one is a neural event. Together these two lead to the visualization of the object.

2. Cellular Event - Rhodopsin is a photopigment present in the eye. it is derived from the vitamin A. Chromophore present on the rhodopsin act as a photoactive molecule. This molecule helps to absorb the energy of the photon. After absorbing the photon, these chromophore molecules change their molecular form and become more stable. As a result, the rhodopsin's chemical composition also changes, and the active site of this molecule is exposed. This activated form is known as metarhodopsin II. After this, metarhodopsin II activates many copies of the G protein transducin (a protein molecule in the eye). The activated form of transducin complexes activates cyclic nucleotide phosphodiesterase (PDE), which can hydrolyze molecules of cGMP to 5'-Guanosine monophosphate. In the presence of light, cGMP-gated channels in the plasma membrane of these rods (or cones) remain closed. This prevents sodium ion influx and reduces glutamate efflux in the cells, As a result, cells remain hyperpolarized. The enzyme rhodopsin kinase binds metarhodopsin II and stops its activity. The unstable molecules split and form opsin and free trans-retinal. After being transported to pigmented epithelial cells, trans-retinal molecules are changed into 11-cis-retinal. These compounds are combined with opsin within cones/rods to reform rhodopsin. This event also causes changes in membrane permeability. The potential difference formed in the photoreceptor cells generates action potentials in the ganglion cells through the bipolar cells.

3. Neural Event - The nerve impulse from ganglion cells travels to the optic nerve and reaches the optic chiasm (optic nerves from both the eyes cross each other and from the optic tract). From here, the visual impulse crosses the side and reaches the opposite occipital cortex. Then the visual information travels through different structs such as the pretectal nuclei (involved in visuomotor behavior) and the superior colliculus in the brainstem (to generate visual reflexes to focus on certain objects) or to the suprachiasmatic nuclei (known as the circadian clock of the brain) of the hypothalamus. Ultimately the information reaches lateral geniculate nuclei of the thalamus and then projects into the visual cortex. Six layers of the neural network are present in the lateral geniculate nucleus. Layers II, III, and V receive information from the same visual field, and layers I, IV, and VI receive information from the opposite side of the visual field. Layers I and II are made of magnocellular neurons (large neuroendocrine cells), and layers III, IV, V, and VI are parvocellular neurons. The same types of neurons are present in the retina. Black and white contrast of the object and rapid change in the position of the object is processed by magnocellular type of ganglion cells. Parvocellular-type of neurons processes information about the object's color.

What Is the Role of Rods and Cones?

As previously mentioned, the rods and cones and responsible for vision. Rods are sensitive to the intensity of light. And the cons are the color-sensing cells of the retina. There are three types of cones: red, green, and blue. This denotes their respective sensitivity towards each spectrum of light.

What Is the Effect of Visible Light on the Eyes?

It is thought that visible light forms an image of the object and aid in vision. It should always remember that visible light is not always healthy for the eye.

  • The Blue Light - Blue light is known as high-energy visible light. The wavelength of this light is 380 to 500 nanometers. The wavelength of the blue-violet portion of this light is near to the wavelength of ultraviolet- Light. As a result, this light is not absorbed by the anterior segment eye and reaches the posterior segment. This causes retina damage and macular degeneration.

  • Too Much Light - Excess luminous causes discomfort in the eye. This is of two types. One is disability glare which measurably reduces the visual information. Another one is discomfort glare, which causes discomfort in the eyes.

Conclusion

Visual light is a key component of vision. Reflected visual from an object is transformed into nerve impulses by rods and cones. The hypothalamus processes these signals. The whole process helps the human being to contact the surrounding surfaces. Also, the effect of light depends on the frequency and intensity of the light.

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Dr. Shikha Gupta
Dr. Shikha Gupta

Ophthalmology (Eye Care)

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