The Human Eye and The Colourful World Formula

The Human Eye is the most valuable, sensitive and significant sense organ owing to the fact that it is the only one that allows humans to perceive objects surrounding them in colour. 

Keyterms: Human eye, vision, camera, retina, eyeball, refraction of light, Light, thin membranous, pupil

Read Also: Light-Reflection and Refraction Revision Notes

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Structure of the Human Eye

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Its many components facilitate vision, and it is designed roughly like a camera. 

The lens system allows it to form an image on the retina, which is a light-sensitive screen. The lens’ crystalline nature allows it to adjust its focal length so as to focus on different objects present at varying distances on the retina. The lens is responsible for forming a real, inverted image of an object on the retina.

The cornea is the thin membranous, transparent bulge through which light enters the eye, and it is located on the front of the surface of the eyeball. Moreover, refraction of light rays occurs on its outer surface.

The eyeball is diametrically 2.5 cm wide and is typically spherical. 

The iris, a dark muscular diaphragm, is located behind the cornea and regulates the amount of light entering the eye by controlling the size of the pupil. 

The video below explains this:

Human Eye One Shot Detailed Video Explanation:

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Process of Image Formation

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The process followed is that the photosensitive cells of the retina get activated upon illumination and begin to emit electrical signals, which further get relayed to the brain by the optic nerves. Upon interpreting these, the brain processes the information and allows humans to perceive objects. An image depicting the same components is as follows: 

Fig: Human Eye

Accomodation

The ability of the eye lens to adjust its focal length is referred to as accommodation. The lens, being constituted by a gelatinous, fibrous material can be modified by ciliary muscles. A change in its curvature causes a change in focal length. When the muscles are in a relaxed position, the lens becomes thin causing its focal length to increase and allowing us to see distant objects clearly. When objects closer to the eye are being looked at, the ciliary muscles contract, increasing the curvature of the eye lens and causing it to become thicker and causing the focal length of the eye lens to decrease, thus enabling us to see nearby objects clearly. 

Perception

The minimum distance at which objects can be seen most clearly without straining one’s eyes is called the near point of the eye or least distance of distinct vision. For humans to be able to see an object comfortably, it must be placed 25 cm from the eyes at all times. The far point of the eye is the farthest distance upto which the eye can see objects distinctly, and for a normal eye objects can be perceived that are anywhere between 25 cm and infinity. 

Read More About Tyndall Effect

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Defects of Vision and their Correction

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Some common refractive defects of vision, which can be corrected using suitable spherical lenses are:

1. Cataract
2. Myopia
3. Hypermetropia
4. Presbyopia

Cataract

The condition that arises when at old age, the crystalline lenses of people become cloudy and cause either a partial or complete loss of vision. The only way to restore vision remains surgery. 

Myopia/Near Sightedness

The condition that causes persons to be able to see nearby objects distinctly but not objects that are far away. In other words, the far point of persons affected by this condition is nearer than infinity, because only objects within a few metres can be viewed clearly. 

Some common causes are that the image of a distant object ends up being formed in front of the retina and not on it. This may further be due to an excessive curvature of the eye lens and the presence of an elongated eyeball. The means to correct this is through using a concave lens, which causes the image to be formed on the retina itself. An image depicting the same is as follows: 

Myopia

Hypermetropia/Far-Sightedness

The condition that causes people to only be able to see distant objects clearly but not ones placed nearby. In other words, the near point for persons afflicted with this becomes farther than 25 cm. Light rays end up getting focussed at a point behind the retina. This defect arises as a result of the focus lens being too long or the eyeball being too small. 

Corrective measures for the same include using spectacles with convex lenses in order to converge the rays with additional focussing power on the retina for the formation of an image. A visual depiction of the same is as follows, where N=Near Point of a Hypermetropic Eye, and N’=Near Point of a Normal Eye: 

Hypermetropia

Presbyopia

The condition in older people where the power of accommodation decreases rapidly with ageing, and causes the near point of the eye to recede. This results in them not being able to see nearby objects without the use of eye glasses. The cause is that there is a gradual weakening of ciliary muscles caused by a reduction of eye lens flexibility. 

Additionally, in persons with both myopia and hypermetropia, bi-focal lenses offer correction, because they contain both concave and convex lenses. The upper portion with the concave lenses enable distant vision within whereas the lower portion with convex lenses facilitates near vision. With the leaps and bounds science has taken in recent years, though, surgical intervention methods have also become readily available to correct for these conditions. 

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Refraction of Light Through a Prism

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A triangular glass prism is one that has two triangular bases and three rectangular lateral surfaces, which remain inclined to each other. The angle of the prism is the one created between its two lateral faces. 

Refraction of Light Through a Prism

In the image from above, the light ray enters from air at the glass surface AB, and upon refraction bends towards the normal. At AC, the second surface, the light ray moves from glass to air, bending away from the normal in the process. The unique shape of the prism allows the emergent ray to be bent at an angular direction to the incident ray, called the angle of deviation, which in this case is ∠D. 

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Dispersion of White Light By a Glass Prism

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A prism, when made to encounter with a ray of white light splits the incident ray into a band of colours. This happens as follows: 

Dispersion of White Light By a Glass Prism

The band of coloured components is referred to as the spectrum, and the process of splitting of light into its constituent colours is termed dispersion. However, different colours of light bend through different angles with respect to the incident ray, as they pass through a prism. Violet light bends the most, whereas red light bends the least. Thus the rays of each colour emerge along different paths and thus become distinct.

Isaac Newton used two similar prisms to split the colours of the spectrum, by placing one in an inverted manner. He did this following using a glass prism to obtain a spectrum of sunlight. He noticed that the spectrum recombined and emerged as one ray of white light after refraction through the second prism. 

A rainbow is the most commonly seen natural spectrum, caused by the dispersion of light as seen through miniscule water droplets, which act as tiny prisms. It is typically formed in the direction opposite to the sun. The prism-adjacent droplets refract and disperse the incident sunlight, proceed to reflect it internally and finally refract it again while emerging out of the raindrop. Due to these phenomena, different colours are viewed by the observer. 

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Atmospheric Refraction

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The apparently random flickering of objects seen through a turbulent stream of hot air rising above a fire or a radiator is caused due to the air just above the fire becoming hotter than the air further up. The hotter air is lighter (less dense) than the cooler air above it, and has a refractive index slightly less than that of the cooler air. Since these physical conditions are not stationary, the position of the object fluctuates. Some phenomena where this is observed is as follows: 

Advance Sunrise and Delayed Sunset

Normally, sunrise refers to the actual crossing of the horizon by the Sun. However, the sun remains visible to the world 2 minutes before sunrise and 2 minutes after sunset, owing to atmospheric refraction. The same causal factor also drives the Sun’s disc seemingly flattening at sunrise and sunset. 

Twinkling of stars

Starlight, upon entering the earth’s atmosphere, undergoes refraction constantly before reaching the earth’s surface. The phenomenon being studied, atmospheric refraction, happens through several mediums of evolving refractive indices. Due to the atmosphere bending starlight towards the normal, the star’s apparent position is not the same as its actual one. Moreover, the former keeps altering itself depending on the earth’s varying physical conditions. 

As light rays arising from the star start to differ even minutely, the star’s apparent position fluctuates and so does the amount of starlight entering the eye, causing the star to appear bright at some points of time, and fainter at others. This is what is commonly termed the twinkling effect. 

One might wonder then, how come planets don’t twinkle? This is simply because planets can be viewed as a collection of a large number of point-sized sources of light, which are inherently closer to the earth and are thus viewed as extended sources. As a result of this, the total variation in the amount of light entering our eye from all the individual point-sized sources will average out to zero, thereby cancelling out the twinkling effect. 

Scattering of Light

Apart from the scattering of light through colloidal particles, the interplay of light with surrounding objects gives rise to phenomena such as the blue colour of the sky, the colour of water in the depths of the sea, the predominantly red colour of the sun during sunrise and sunset and so on. 

Tyndall Effect

The earth’s atmosphere is composed of minute particles such as smoke, tiny water droplets, suspended particles of dust and molecules of air present heterogeneously. Upon a beam of light striking such fine particles, the path of the beam becomes visible. The light reaches us, after being reflected diffusely by these particles. This gives rise to the Tyndall effect, which is observed when a narrow beam of sunlight enters a smoke-filled room through a small hole. Thus, scattering of light makes the particles visible. The colour of the scattered light is affected by the size of the scattering particles. Very minute particles scatter mainly blue light while particles of larger size scatter light of longer wavelengths.

Colour of the Sun at Sunrise and Sunset

Sunlight near the horizon passes through thick air layers and large distances before reaching the eye. This leads to a predominantly reddish hue owing to its large wavelength. From overhead, however, it needs to travel a relatively shorter distance and hence reaches the eye sooner. At midday, the Sun appears white because only a small portion of blue and violet light manages to get scattered, due to their shorter wavelength. The same is shown visually as follows: 

Colour of the Sun at Sunrise and Sunset

Why the Sky Appears Blue

The molecules of air and other fine particles in the atmosphere are smaller than the wavelength of visible light. These are more effective in scattering light of shorter wavelengths at the blue end than light of longer wavelengths at the red end, because the latter is about 1.8 times greater than the former. 

When sunlight passes through the atmosphere, the fine particles suspended in the air scatter the blue shorter wavelengths more strongly than the red, and it is this scattered blue light that enters human eyes. 

It is because of this phenomenon that all danger and fog lights use red light because in the case of fog or smoke, red light with its long wavelength gets scattered the least and can be seen with minimum distortion even from great distances. 

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