Introduction
Let us discuss the concept, the human eye, and the colourful world in brief. The human eye is an organ that allows vision and reacts to light. The Rod and Cone cells present in the retina allow vision and conscious light perception, including the perception of depth and colour differentiation. The human eye is possibly capable of detecting a single photon and can differentiate between about 10 million colours. The eye is the sensory nervous system part.
The human eye's non-image-forming photosensitive ganglion cells in the retina similar to other mammals' eyes, receive light signals which affect the adjustment of the size of the pupil, suppression, and entrainment of the body clock and regulation of the hormone melatonin.
The eye is one of the five major senses in the human body. It is truly a magical organ, and without this, the world will be dark. The eye comes under the sensory nervous system and can recognize up to 10 million different colours. It helps us gain our vision with the help of the Rod and Cone cells.
Let us talk about the structure of the eyes as it is somewhat a complex organ.
It is in the shape of a spherical ball.
The socket of the eye holds it.
Two layers named Scleroid and Choroid cover it.
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It is a naturally occurring optical body element as we know that we have a pair of eyes, and its function is to allow us to notice. Without that part, the whole world would have been a dark place for us.
Structure of a Human Eye
It is a circular or spherical ball with a small bulge on the front
It is located in the socket of the eye
It has two layers that cover it which are, scleroid and choroid
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Function of Scleroid
Scleroid is the outermost covering layer that consists of white fibres, and its functionality is to protect all parts of the eye.
Function of Choroid
The choroid is a grey membrane that is attached to a choroid from the inner side. Its functionality is to darken the eye from the inside. So, no internal reflection takes place.
Eye Defects
Let us have a look at a few eye defects that occur very generally for more human beings.
Myopia (Short-Sightedness)
In this defect, a person is unable to see far objects clearly whereas can see the nearby objects. This is because the ciliary muscles don't relax, and the lens doesn't elongate properly because of which the focal length does not properly increase. Resultantly no clear image is formed.
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Defect on Eyes: Eyeball, as being too elongated, the converging lens power is also too high. Because of this, the image is formed in front of the retina, and the brain cannot identify it. It can be overcome using spectacles containing a concave lens that diverges the rays first. Then, our (the human) eye lens can converge them on the retina properly.
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Hypermetropia (Long-Sightedness)
In this defect, a person is unable to see nearby objects but can see far off objects. This is because the ciliary muscles don't contract properly, the lens doesn't become thick and short because of which the focal length doesn't decrease. Resultantly, the image formed is not clear, and the brain can't identify it.
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Defect on Eye
Eyeballs being converging power and too short of the lens is too low. Due to this, the image forms behind the retina. It can be overcome by using spectacles containing a convex lens that increases the ciliary power of the eye lens. So that it can converge rays on the retina properly.
Refraction through Prism
Prism: Prism is a piece of any transparent material or a piece of glass bonded by triangular and three rectangular surfaces. The rectangular surfaces are referred to as refracting surfaces. Where the angle between two refracting surfaces is called the angle of the prism or refracting angle.
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The line along where the two refracting surfaces meet is referred to as refracting the edge. Any of the prism sections that are perpendicular to the refracting edge is called the principal section of the edge.
The refraction via glass prism and glass slab differences can be given below.
The emergent ray lies parallel to the incident ray in the slab but considering a prism, the emergent ray won't be parallel to the incident ray due to the opposite faces of the prism eye are not parallel to one other.
Dispersion
It is the phenomenon of splitting light into seven colours. When light (Sunlight or bulb light) is allowed to pass via glass prism, it splits into seven colours. It happens because we know white light is a combination of seven colours and on entering the prism, each colour gets refracted by different angles because of which different colour (spectrum) is obtained on the screen.
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The obtained colours are Red, Orange, Yellow, Green, Blue, Indigo, Violet.
The human eye, the colourful world, can be obtained by the below explained colours, which are a part of the spectrum.
In red colour, wavelength decreases and frequency increases
Red colour - Least Deviated
Violet colour- Maximum Deviated
All seven colours can recombine to produce white light
An application of dispersion is the formation of a Rainbow.
Rainbow
It is an example of dispersion (Spectrum produced by the dispersion of sunlight). "It is formed by the dispersion of white sunlight by the raindrops that are present in the atmosphere.” Each and every raindrop acts as a tiny glass prism. A Rainbow is always formed opposite the sun. White light enters these raindrops. Different coloured rays are refracted through different angles where the rainbow is formed thus.
Scleroid and Choroid
The scleroid is the first layer from the outside that covers the eye. It protects all parts of the eye. The choroid is the second layer of the eye and is filled with a grey membrane. It helps in darkening the eyes from the inside so that there is no reflection.
Myopia Vs Hypermetropia
Myopia is a defect that doesn't let people see far placed objects. It doesn't affect near objects. There are certain scientific reasons for this. Though it can be cured by wearing spectacles and it has all the possible treatments with the doctors. This generally happens because the ciliary muscles don't relax, which doesn't let our eyes form a clear image. Whereas Hypermetropia is a defect that is just the opposite of Myopia. Here, the ciliary muscle doesn't contract properly. In this, a person is unable to see nearer objects clearly whereas has no problem in seeing the faraway objects. This also has all the cures in today's world and wearing spectacles can help.
What is Prism?
Prism is a piece of any glass material or a piece of glass fortified by three-sided and three rectangular surfaces. The rectangular surfaces are alluded to as refracting surfaces. Where the point between two refracting surfaces is known as the point of the prism or refracting point. The concept of a rainbow is also a part of the study of Prism. Every single raindrop goes about as a small prism. The Rainbow is constantly framed against the sun. White light enters these raindrops. Distinctive hued beams are refracted through various points and hence a beautiful rainbow is formed.
When we do the prism concept, the colours achieved are:
Red - Wavelength decreases and frequency increases. It is also the least deviated colour.
Violet - It is the maximum deviated colour, unlike red.
If we combine all seven colours, we will form white colour.
FAQs on The Human Eye and the Colourful World
1. What are the main topics covered in the CBSE Class 10 chapter 'The Human Eye and the Colourful World'?
This chapter primarily explains optical phenomena and the functioning of the human eye. Key topics include:
- The structure and function of the human eye, including the roles of the cornea, iris, pupil, ciliary muscles, lens, and retina.
- The concept of power of accommodation.
- Common defects of vision like Myopia (near-sightedness), Hypermetropia (far-sightedness), and Presbyopia, along with their correction using lenses.
- Refraction of light through a glass prism.
- Dispersion of white light, leading to the formation of a spectrum and phenomena like rainbows.
- Atmospheric refraction, which explains why stars twinkle and the apparent position of the sun at sunrise and sunset.
- Scattering of light, which explains the blue colour of the sky and the reddish appearance of the sun during sunrise and sunset.
2. What are the functions of the key parts of the human eye?
The human eye works like a camera, focusing light onto a light-sensitive screen. The main functions of its parts are:
- Cornea: The transparent outer layer that refracts most of the light entering the eye.
- Iris: A muscular diaphragm that controls the size of the pupil, thus regulating the amount of light that enters.
- Pupil: The opening in the centre of the iris through which light enters.
- Crystalline Lens: A convex lens that fine-tunes the focus of light onto the retina. Its focal length is adjustable.
- Ciliary Muscles: These muscles hold the lens in place and change its shape to adjust the focal length, a process called accommodation.
- Retina: The light-sensitive screen at the back of the eye where the image is formed. It contains rod and cone cells that convert light into electrical signals.
- Optic Nerve: Transmits the electrical signals from the retina to the brain for interpretation.
3. What is the power of accommodation of the human eye?
The power of accommodation is the ability of the eye lens to adjust its focal length to see both nearby and distant objects clearly. This is achieved by the action of the ciliary muscles. When viewing distant objects, the muscles are relaxed, and the lens is thin. To view nearby objects, the muscles contract, making the lens thicker and increasing its converging power.
4. What is the difference between myopia and hypermetropia?
Myopia and hypermetropia are common refractive defects of the eye. The key differences are:
- Vision: A person with myopia (near-sightedness) can see nearby objects clearly but not distant ones. A person with hypermetropia (far-sightedness) can see distant objects clearly but finds it difficult to see nearby objects.
- Image Formation: In myopia, the image of a distant object is formed in front of the retina. In hypermetropia, the image is formed behind the retina.
- Cause: Myopia occurs due to the excessive curvature of the eye lens or elongation of the eyeball. Hypermetropia is caused by a short focal length of the eye lens or the eyeball being too short.
- Correction: Myopia is corrected using a concave lens. Hypermetropia is corrected using a convex lens.
5. How is a rainbow formed through the dispersion of light?
A rainbow is a natural spectrum appearing in the sky after a rain shower. It is caused by the dispersion of sunlight by tiny water droplets present in the atmosphere. These droplets act like small prisms. When sunlight enters a water droplet, it first undergoes refraction and dispersion, splitting into its seven constituent colours. Then, the light is internally reflected from the inner surface of the droplet. Finally, the light is refracted again as it comes out of the droplet, forming a vibrant rainbow in the sky, always opposite to the direction of the Sun.
6. What is the Tyndall effect and how is it related to the scattering of light?
The Tyndall effect is the phenomenon where a beam of light passing through a colloid or a fine suspension is scattered by the particles, making the path of the light visible. This is a direct example of the scattering of light. The blue colour of the sky is a large-scale example of this effect, where particles in the Earth's atmosphere scatter the shorter-wavelength blue light from the sun more than the longer-wavelength red light.
7. Is 'The Human Eye and the Colourful World' a chapter in Physics or Biology?
This is a common point of confusion for students. In the CBSE Class 10 Science syllabus, this chapter is part of the unit on 'Natural Phenomena' and is officially taught under Physics. While the 'Human Eye' is a biological organ, the chapter's focus is on the principles of optics, light, refraction, and dispersion, which are core concepts in physics.
8. Why do stars twinkle but planets do not?
Stars twinkle due to atmospheric refraction, but planets do not. The reason for this difference is their apparent size as seen from Earth:
- Stars are so far away that they act as point-sized sources of light. As their light travels through the Earth's constantly moving atmosphere with varying densities, it gets refracted multiple times. This causes the apparent position of the star to fluctuate, creating a twinkling effect.
- Planets are much closer and appear as extended sources of light (a collection of many points). While the light from each point on the planet also refracts, the variations from all points average out, cancelling the twinkling effect and making the planet appear as a steady source of light.
9. Why does the Sun appear reddish during sunrise and sunset?
The Sun appears reddish during sunrise and sunset due to the scattering of light. At these times, sunlight has to travel through a much thicker layer of the Earth's atmosphere to reach our eyes. During this long journey, most of the shorter wavelength light (like blue and violet) is scattered away by the air molecules. The longer wavelength light (like red and orange) is scattered the least and is able to pass through, reaching our eyes and making the Sun and the surrounding sky appear reddish.
10. If the Earth had no atmosphere, what changes would we observe in the sky and daylight?
If the Earth had no atmosphere, our view of the sky and daylight would be dramatically different. Firstly, the sky would appear pitch black instead of blue, even during the day, because there would be no particles to scatter sunlight. Secondly, there would be no diffused daylight; shadows would be extremely sharp and dark. Phenomena like twinkling of stars, advanced sunrise, delayed sunset, and rainbows would not occur.
11. What is the fundamental difference between refraction and dispersion of light?
While related, refraction and dispersion are distinct phenomena. Refraction is the bending of light when it travels from one medium to another (e.g., from air to glass). It affects the entire beam of light. Dispersion, on the other hand, is the splitting of white light into its constituent colours (VIBGYOR). This happens because the angle of refraction is slightly different for each colour or wavelength. In essence, dispersion is a result of refraction, where different colours refract by different amounts.
