Overview
The eye is the sensory nervous system part through which one can see their surroundings, other humans, animals, birds, and possibly everything. In a nutshell, the human eye is a light-sensitive organ that allows vision. The retina consists of multiple cells that enable sight and help conscious light perception, including depth perception and colour distinction. These are known as rod and cone cells.
In this article, we shall have a detailed discussion about the human eye, its composition, the defects in human eyes, and learn about different physics-related concepts like reflection, refraction, scattering of light, and all-important topics for Board examination.
| Table of Contents |
| Overview |
| Human Eye: Structure and Composition |
| Defects in Human Eye |
| Refraction Through Prism |
| Scattering of Light and its Applications |
| Frequently Asked Questions |
Did you know: The human eye is possibly capable of detecting a single photon and can differentiate between about 10 million colours.
Human Eye: Structure and Composition
The eye is one of the human body’s five major senses. It is a genuinely magical organ, and the world would be a dismal place without it. With the help of the rod and cone cells, it aids us in gaining our vision. A cell known as the non-image-forming photosensitive ganglion cell is present in the human eye. Like those of other animals, it is responsible for receiving light signals that regulate pupil size adjustment. The eye is also responsible for releasing and controlling the melatonin hormone.
Although it looks simple, human eyes have a very complex structure. Let us learn about the structure and composition of an eye.
Human Eye Structure
- The human eye is spherical with a little bulge on its front.
- It is placed inside the eye socket.
- The human eye is covered by two membranes known as Scleroid and Choroid.
- The Scleroid is the outermost layer of the eye made up of white fibres. Its work is to protect all parts of the eye.
- The grey colour membrane that connects the inner and outer sides of an eye is Choroid. Its purpose is to darken the inside of the eye. This is done to prevent internal reflection.
Now that we have learned about the structure of the eye, let us discuss the parts of a human eye.
Parts of a Human Eye
- Cornea: This is the transparent, outermost layer. It is responsible for the majority of light refraction.
- Lens: The lens is made of a fibrous, jelly-like substance. All the images placed on the retina are inverted images by the lens. The human eye lens is a convex lens that causes light to converge on the retina.
- Iris: The iris is the colourful muscular diaphragm. Its function is to control pupil size.
- Pupil: It is the eye’s window. The amount of light that enters the eye is regulated and controlled by the pupil.
- Retina: The retina is a delicate membrane present at the back of the eye. It consists of a large number of light-sensitive cells.
Curiosity Enough: The range of a human eye without defects is 25 cm to infinity. Till 25 cm, a normal person can see anything clearly without straining or squinting their eyes.
Defects in the Human Eye
Humans are not flawless beings. Even the human eye has defects. These defects can be congenital or accidental during one’s lifetime. Let us look at some defects which are important concerning the CBSE Board examination.
- Myopia or Short-Sightedness
A person with this impairment fails to see far objects yet can perceive nearby objects.
The defect caused on the Eye: Because the eyeball is excessively long, the power of the converging lens becomes too high. As a result, the image is produced ahead of the retina, and the human brain cannot recognize it.
Remedy: It can be avoided by wearing glasses with a concave lens. These lenses help diverge the light first. Then our (human) eye lens can appropriately focus on the retina.
- Hypermetropia or Long-Sightedness
A person with this impairment cannot perceive nearby items yet can see far away objects. In Hypermetropia, the ciliary muscles do not contract adequately. The lens does not thicken and shorten. As a result, the image generated is blurry, and the brain cannot recognize it.
The defect caused on the Eye: The defect on the eyeballs is that the converging power of the lens becomes very short. As a result, the image appears further behind the retina.
Remedy: It can be overcome by wearing spectacles with a convex lens that boosts the eye lens’s ciliary strength. So that rays can correctly converge on the retina.
- Presbyopia or Old age Hypermetropia
It’s a visual problem in which an adult can’t see adjacent objects well due to a decrease of ocular accommodation power. The near-point of a presbyopic older person steadily recedes until it is more than 25 km away.
Cause: Ciliary muscles gradually deteriorate, which causes presbyopia. The flexibility of the eye lens is deteriorating. A variety of factors can cause presbyopia.
Remedy: Use a convex lens with the appropriate power.
Curiosity Enough: A person can have both Myopia and Hypermetropia simultaneously. Bifocal lenses are required for such persons.
We have now covered every important human eye point up to this point in the article. Now we shall study different physics-related topics related to the human eye and the colourful world.
Refraction Through Prism
Prism: A prism is a transparent substance or a piece of glass in a pyramid shape. A prism has two triangular and three rectangular surfaces joined together. Refracting surfaces are rectangular surfaces that refract light. The angle of the prism, or refracting angle, is the angle formed by two refracting surfaces. Refracting the edge refers to the line that connects the two refracting surfaces. The primary portion of the edge refers to any prism segment perpendicular to the refracting edge.
Dispersion Through a Prism
It’s the phenomenon of light breaking into seven different colours. Light (White light from the Sun) splits into seven colours through a glass prism. It happens because white light comprises seven colours, and as it enters the prism, each colour is refracted at different angles, resulting in different shades on the surface. This splitting of seven colours forms a band which is termed as Spectrum.
You can remember the names of the different colours using the acronym VIBGYOR. Here the first letter from the name of each colour has been taken. The colours are:
V – Violet
I – Indigo
B – Blue
G – Green
Y – Yellow
O – Orange
R – Red
Curiosity Enough: Sir Isaac Newton was the first man to observe and prove the splitting of white light into seven distinct colours.
Listed below are some of the important properties of these colours, which appears many times on paper:
- When combined, all the seven colours again form white paint or white light.
- The frequency of each colour decreases in travelling from Red to Violet. We can say that Red has the highest frequency and Violet has the least.
- The wavelength increases from Red to Violet. Violet has the highest wavelength, and Red has the shortest one. Due to this phenomenon, Red colours deviate the least, and Violet deviate the maximum.
Note: Frequency and wavelength are inversely proportional to each other, which means that if one increases, the other decreases and vice versa.
One of the best examples of dispersion in nature is the formation of a rainbow. Let us study in detail how a rainbow is formed.
Rainbow
The formation of a rainbow is a very simple yet beautiful process in nature. After the rain, the suspended particles of water vapour are left in the atmosphere. These particles act as small glass prisms, although they are microscopic. When sunlight passes through these water particles, it gets divided into the seven VIBGYOR colours.
First, the light refracts inside a water droplet and then disperses. Then the dispersed light is reflected internally and again refracts after coming out of the water droplet. This refracted light reaches the human eye and is visible in the shape of a rainbow.
A rainbow is always formed near a water body and in the opposite direction of the sunlight. The refraction, dispersion, and internal refraction of light cause the formation of a rainbow.
Scattering of Light and its Application
The sky appears blue: In the Earth’s atmosphere, Nitrogen and Oxygen molecules are present in abundance. These particles are responsible for scattering blue light far more than red light. This is the reason why the sky appears blue from the Earth. The atmosphere scatters just a small amount of the blue light present in white sunshine, making the sky appear blue. Because most blue light is not scattered, the composition of sunshine is essentially unchanged. As a result, the direct sunlight reaching through the blue sky seems white.
At sunrise or sunset, the sky appears red: At sunrise and sunset, the Sun is near the horizon. The light from the Sun has to travel a relatively larger distance to reach our eyes. During this time, most of the shorter wavelength (blue colour) present in sunlight is scattered out. Also, the blue colour gets away from our line of sight. As a result, the light that reaches us straight from the rising or setting Sun is primarily of the longer wavelength, i.e., red colour, which causes the Sun to look red. The sky seems to be red around the rising and setting suns.
Twinkling of stars: The twinkling of stars is due to a phenomenon known as atmospheric refraction. Since stars are light years far away from us, their light takes years to reach us. This light meeting the Earth’s atmosphere has to travel through multiple layers with different refractive indices. Before reaching our eyes, the light wave is refracted numerous times. This makes us see the star at an apparent position rather than the actual one. The light ray bends differently every time it reaches our eyes, changing the star’s apparent position, which causes the twinkling.
Frequently Asked Question
- Are the topics mentioned in NCERT enough to study ‘The Human Eye and the Colourful World’?
Ans. Yes! NCERT has covered all the important topics related to this chapter in the board examinations. The topics are written in well comprehensive language.
- Which part of the human eye is responsible for the colour of the eye?
Ans. The Iris is responsible for providing the colour of the eyes. Different human beings can have different Iris colours like black, brown, amber, blue, etc.
- Why does the sky appear black from space and not blue like Earth?
Ans. This is due to the absence of an atmosphere in outer space that scatters sunlight. Since the sunlight reaches our eyes without scattering in outer space, the sky seems dark and black.
- What is the Tyndall effect is the scattering of light?
Ans. We can see the path of a beam of sunshine approaching us in a dusty room through a window due to the small dust particles (colloids) in the room’s air, which disperse the light beam all over the place. This effect is termed the Tyndall effect.
- Why do we see the Sun 2 minutes earlier than sunrise and 2 minutes after sunset?
Ans. This is due to atmospheric refraction. The sunlight reaches our eyes after being refracted through the layers of the atmosphere, giving us an apparent position of the Sun.
