This chapter is very important from examination point of view. In this chapter, we have discussed various concepts like human eye and its structure and functioning, perception of vision and colours, defects of vision and their correction, atmospheric refraction and its effects, dispersion of light and recombination of light. These concepts are very important as questions related to them are being asked in almost every exam.
Human eye and colorful world class 10 notes
In the forthcoming section, you will get notes for Class 10 science chapter Human eye and colorful world. These notes are in the form of questions and answers, so that you can find them easy to understand. These notes have been prepared by a group of experienced teachers, and you will find them very useful for your NCERT and CBSE board exams for class 10.
Structure Of Human Eye
The human eye is more or less like a photographic camera. The essential parts of human eye are Cornea, Iris, Pupil, Eye lens, Ciliary muscles, retina and optic nerve.
- Cornea: The front transparent part of the eye is called cornea which is bulged outwards. The light coming from the objects enter the eye through cornea, thus it serves as a window of the eye.
- Iris:The coloured diaphragm behind the cornea is called as iris.
- Pupil: A small hole in the centre of iris is called as pupil. It appears black as no light is reflected from it.
- Eye lens: Eye lens is a double convex lens which is transparent and flexible material (like a jelly) made of proteins.
- Ciliary muscles: The special type of muscles which hold the eye lens in a proper position. The focal length of the eye lens is varied and regulated with the help of ciliary muscles.
- Retina: A screen behind the eye lens and at the back of the eye ball, on which the image of the object is formed. It is a delicate membrane consisting of a large number of light sensitive cells in the form of rods and cones. The rods respond to the intensity of light and the cones respond to the colour of objects by generating electric in pulses.
- Optic nerve: The optic nerve receives the electric impulses from retina and passes them to the brain where the information is processed and we perceive the objects as they are.
- Aqueous humour: The space between the cornea and the eye lens is filled with a viscous liquid called aqueous humour.
- Vitreous humour: The space between the eye lens and the retina is filled with a specific fluid called as vitreous humour.
- Blind spot: It is the least sensitive spot on the retina through which the optic nerve enters the retina.
Diameter of eye ball: The whole eye ball is a sphere of diameter about 2.3cm.
Working Of A Human Eye
Light rays coming from an object to be seen enter the eye through Cornea and fall on the eye lens through the pupil of the eye.
The eye-lens, being convex, converges the rays of light, forming a real and inverted image of the object on the retina.
The large number of rod and cone shaped cells of the retina get activated by the light falling on them, they generate electric signals which are sent to the brain via optic nerve.
The brain processes this message and it gives rise to the sensation of vision. Although the image formed on the retina is inverted, our mind interprets the image as that of an erect object.
Persistence Of Vision.
One of the important characteristics of human eye is that the image formed on the retina of an object is neither permanent nor it fades away instantly . But actually the image of any object seen persists on the retina for 1/16th of a second, even after the removal of the object.
This ability of an eye to continue to see the image of an object for a very short duration, even after the removal of the object is called persistence of vision. This property of persistence is used in cinematography i.e. in projection of motion pictures.
How do we see different colours?
The light-sensitive retina of our eyes contains individual receptor cells, called “cone” cells because of their shape, that are sensitive to three different bands of color. One set is most sensitive to red light, another set is most sensitive to green, and another is most sensitive to blue.
Different colors of light from the scenes we look at have different frequencies (color is nothing more than the frequency of light, as light is a form of electromagnetic radiation).
Colour with a frequency equal to red, activate the red-type cell alone, and we see red. Colour with a frequency equal to green activate the green-type cell alone, and we see green. Colour with a frequency equal to blue, activate the blue-type cell alone, and we see blue.
Similarly colors with a frequency close to red, for example. stimulate the red-sensitive cones a lot, and stimulate the green-sensitive cones a little bit, and hardly affect the blue-sensitive cones at all. When the brain receives a strong “red” signal, combined with a weak “green” signal and hardly any “blue” signal, it interprets that as a reddish color. Other colors work in a similar way.
The additive primary colors are red, green, and blue. One can recreate the appearance of any color by appropriately mixing red, green, and blue light. The color one produces won’t actually be the color one see. It will still be just a mixture of red, green, and blue, but it will fool one’s brain into thinking that it is some other color. For example, one can recreate yellow by mixing green and red light. The light is actually nothing more than red and green light put together in the same spot, but one’s brain will interpret this as yellow. Real yellow light actually has its own frequency, but human vision cannot tell the difference between a pure spectral yellow (light of one frequency) and a mix of red and green light. When we see “yellow” light, we cannot be sure that it is really yellow. It could be yellow for real (yellow photons), or it could just be a mix of red and green.
We have absolutely no way to tell the difference with our eyes. We need special instruments to look at the spectrum of the light in order to find out if it’s really yellow or just a mix of some other colors.
Write a short note on color blindness?
Colour blindness is said to be occurring when a person can not distinguish between different colours, though his vision may otherwise be normal. This is because the retina of the eye of such a person does not posses some cone cells. e.g. a person who is blind to red-green colour may be deficient in cone shaped cells having red and green pigment in the retina of his eyes.
It is a genetic disorder which occurs by inheritance. It has not been cured so far.
What are the various defects of vision?
The inability of an eye to see the objects clearly is known as defect of vision.
There are five defects of vision or defects of eye which can be corrected by using suitable spectacles. These are:
- Myopia Or short sightedness
- Hypermetropia or long sightedness.
Out of these five defects the first two are the most common ones.
What is Myopia? How can it be corrected?
Myopia, also called short sightedness is that defect due to which an eye can see nearby objects clearly but can not see the distant objects clearly.
This defect is caused either;
- Due to the high converging power of the eye lens or
- due to the eyeball being too long.
The far point of an eye suffering from this defect is less than infinity.
In some cases, in an eye suffering from myopia, the ciliary muscles attached to the eye lens do not relax completely to make the eye lens thinner, in order to reduce its converging power. So due to the greater converging power of the eye lens; the image of the distant obiect is formed in front of the retina and hence the eye can not see it clearly.
In some other cases, in an eye suffering from this defect, the eyeball is too long due to which the retina is at longer distance from the eye lens. This also leads to the formation of image of distant object in front of retina.
This is shown in the following fig. (b). This defect can be corrected by using spectacles containing concave lenses of suitable focal lengths as shown in fig .(c).
Parallel rays of light from infinity first diverge on passing through concave lens and appear to come from F, then the rays are converged by the eye lens and the image is formed on the retina and hence the eye can see it clearly.
What is hypermetropia? How can it be corrected?
Hypermetropia or long sightedness is that defect of eye due to which an eye can see the distant objects clearly but can not see the nearby objects clearly.
The near point of a hypermetropic eye is more than 25cm away So a person suffering from this defect can not read or write easily,
Hypermetropia is caused either:
- due to the low converging power of the eye lens
- due to the eye-ball being too short.
Thus the image of a nearby object in a hypermetropic eye is formed behind the retina instead on retina as shown in the following fig(b).
This defect can be corrected by using spectacles containing convex lens. Due to the combined converging action of this lens and eye lens the image is formed on the retina and the eye can see the nearby object clearly.
What is Presbyopia? How can it be corrected?
It is that defect of eye due to which an old man can not read comfortably and clearly without spectacles.
Presbyopia is a sort of hypermetropia where near point of eye recedes to a distance more than 25cm from the eye.
Correction of Presbyopia: it is corrected in the same way as hypermetropia by using spectacles of convex lenses of suitable focal lengths.
What is Astigmatism? How can it be corrected?
Astigmatism is that defect of eye due to which a person can not focus on both horizontal and vertical lines simultaneously.
This defect arises due to the irregularities in the surface of the cornea. The cornea surface of a normal eye is a part of the surface of a perfect sphere. A person suffering from this defect has its cornea having different curvatures in different directions in horizontal and vertical planes. Due to this eye can focus the object well in one direction while those in the perpendicular direction to it are not well focused. That is, if the eye can focus well on horizontal lines, it can not focus well on vertical lines at the same time and vice-versa.
Correction of astigmatism: This defect can be corrected by super imposing cylindrical lenses upon the spherical shape of spectacle lenses.
What is cataract? How can it be corrected?
Cataract is that defect of eye that comes usually in old age of a person. Cataract develops when the eye lens of a person becomes cloudy (or even opaque) due to the formation of a membrane over it. It decreases the vision gradually leading some times to total loss of vision.
Correction of cataract: This problem is solved by cataract surgery i.e. removal of the eye lens and its replacement by a lens of suitable focal length.
Use of bifocal lens.
When a person suffers from both myopia and hypermetropia his spectacles for correction have bifocal lenses. The upper half is a concave lens for distant vision and lower half is a convex lens for reading.
What is a glass prism? Write a short note on refraction through a glass prism?
A Prism is a transparent glass medium bounded by two triangular and three rectangular faces as shown in the figure. One of three rectangular faces is called the base and the other two are called as refracting faces.
The line joining the two triangular faces is called the refracting edge of the prism and the angle made by two opposite faces is called angle of the prism (denoted by A).
The refraction through a prism is different from that through a glass slab in the sense that the emergent ray in case of prism is not parallel to the incident ray so we call it as deviated ray. Thus light after passing through a prism is deviated from its original path.
Refraction through a glass prism.
Consider ABC as the prism with its base BC, and AB and AC being its refracting faces. Let a ray of light PQ incident on face AB after refraction at Q it is refracted through QR and it emerges out as RS. Draw N1O and N2O normal at point Q and R.
Producing PO forwards and RS backwards. the two lines meet at D giving rise to an angle & called as the angle of deviation denoted by δ.
Angle of deviation (δ), therefore is defined as the deviation occurred to the incident light due to a prism placed in the path of incident light.
Prism Formula: ∠i+ ∠e = ∠A + ∠δ.
What is dispersion of light? What is the reason behind dispersion of light?
When a beam of white light is passed through a prism, it is split into its constituent colours. The phenomenon of splitting of white light into its constituent seven colours is called dispersion of light.
The seven colours are violet, indigo, blue, green, yellow, orange and red. If the patches of these seven colours are obtained on a screen it is called spectrum of white light. The spectrum of white light shows the white light is made up of seven colours. It can be remembered by the word “VIBGYOR’ where V stands for violet.I for indigo. B for blue. G for green, Y for yellow, O for orange and R for red.
Cause of dispersion: We know that white light has range of wave length from 3800°A to 7800°A. Thus when white light passes through the prism, different colours are refracted through different angles, with the result the seven colours are spread out to form a spectrum.
The red colour having maximum wavelength gets deviated least and the violet having minimum wavelength gets deviated most.
Thus in this case dispersion of white light occurs due to the refraction of different colours through different angles through a glass prism.
Recombination of spectrum colors.
Newton was the first to obtain the spectrum of white light. He showed that white light can be dispersed into seven colours. He also observed that the reverse of it is also true i.e. seven coloured light can be recombined to give back white light.
This can be done by placing two glass prisms in such a way that the first prism disperses the white light into its constituent seven colours and all these seven coloured lights are received by the 2nd (which has been placed upright down) which is recombined into the original white beam of light.
This recombination of seven colours is due to the fact that the second prism refracts these rays equally and oppositely to that produced by the first prism.
What is rainbow? How is it formed?
The rainbow is the best example of dispersion of sunlight in nature. It is an arch of seven colours visible in the sky when the sun shines on rain drops during or after the shower of rain or water.
The essential condition for observing a rainbow is that the observer must stand with his back towards the sun.
During or after a shower, the rain drops suspended in air act like many small prisms. The incident sun light falling on these tiny drops is refracted and dispersed by them. It then suffers total internal refraction and is finally refracted out of the rain drops to reach the observers eye.
This spectrum of light having red colour at the top and violet at its bottom is observed by the eye when it makes 41°and 44° with the horizontal. This is called primary rainbow.
Some times a faint rainbow along with this rainbow is also observed making 51° and 530 angle with the horizontal. That is called as secondary rainbow.
What is atmospheric refraction?
The phenomenon of bending of light while passing through different layers of atmosphere is called atmospheric refraction.
We know that different layers of atmosphere are at different temperatures, and hence they posses different optical densities. Thus the sun light or the light coming from stars on passing through these layers having different optical densities suffer multiple refractions before reaching the surface of earth. This is called atmospheric refraction. Some natural phenomena like ”twinkling of stars” are explained on the basis of atmospheric refraction.
What is scattering of light? State and explain Tyndall effect.
Scattering of light: The phenomenon of spreading or throwing of light in different directions when it strikes an obstacle like atom, molecule, dust particle, water drop etc. is called as scattering of light. Lord Rayleigh studied scattering of light and he established a law called as Rayleigh’s law according to which the intensity of scattered light (Is) varies inversely as the fourth power of wavelength (λ) of incident light i.e. Is ∝ 1/λ4.
Tyndall Effect: The scattering of light by the colloidal particles in its path is called Tyndall effect e.g. when a fine beam of sunlight enters a smoke filled room, smoke particles become visible due to scattering of light by these particles. Similarly when sunlight passes trough the canopy of a dense forest, tiny water droplets in the mist scatter sunlight.
The colour of the scattered light depends on the size of the scattering particles. If the particle size is much larger than the wavelength range of visible light, all wavelengths are scattered almost equally and the scattered light appears white.
However, when the particle size is very small as compared to wavelength range of visible light, the shorter wavelength region (i.e. blue light) is scattered most and the longer wavelength region (i.e. red light) is scattered least.
Human eye and the colorful world text questions and answers
In this section you will find answers to the textual questions of NCERT science chapter titled ”human eye and the colorful world”. These answers have been prepared by a group of experts who have an experience of more than 10 years of teaching at secondary level.
What is meant by power of accommodation of the eye?
A normal eye can see nearby as well as the distant objects clearly. The ability of an eye to focus the distant objects as well as the near by objects on the retina by changing the focal length of its lens is called accommodation or power of accommodation of eye.
Accommodation acts like a reflex, but can also be consciously controlled.
When the object is at infinity, the ciliary muscles are relaxed and the eye lens becomes very thin. In this way, the lens acquires maximum focal length and minimum converging power. The image of the distant object is formed on the retina. Thus in this case the eye is relaxed and is called to be unaccommodated.
For observing nearby objects the ciliary muscles increase the thickness of the eye lens, and the focal length of lens decreases and its converging power increases. Thus the image of the nearby objects is formed on the retina. In this case the eye is in a state of tension and is said to be accommodated.
The maximum accommodation is reached when the object is at a distance of 25cm from the eye.
Far point of eye: The most distant point at which an object is seen clearly by an eye is called far point (F) of the eye. It lies at infinity.
Near point of eye: The closest point, at which an object is seen clearly by an eye is called Near point (N) of the eye. The distance between the Near point and the eye is called least distance of distinct vision. It is denoted by “d”. For normal eye its value is 25cm. The distance between the Near point and the Far point of the eye is called range of vision.
A person with myopic eye cannot see objects beyond 1.2 m distinctly. What should be the type of the corrective lens used to restore proper vision?
A person with a myopic eye should use a concave lens of focal length 1.2m so as to restore proper vision.
What is the far point and near point of the human eye with normal vision?
For a normal human eye. the far point is at infinity and near point is at 25cm from the eye.
A student has difficulty reading the black board while sitting in the last row. What could be the defect the child is suffering from? How can it be corrected?
The child is suffering from myopia also called short sightedness. The defect can be corrected by using spectacles with concave lens of suitable focal length.
The human eye can focus object at different distances by adjusting the focal length of the eye lens. This is due to:
- Near Sightedness
- Far Sightedness
The human eye forms the image of an object at its;
The least distance of distinct vision for a young adult with normal vision is about;
The change in focal length of an eye lens is caused by the action of the:
- Ciliary muscles
A person needs a lens of power -5.5 diopters for correcting his distant vision. For correcting his near vision he needs a power +1.5 diopter. What is the focal length of the lens required for correcting (i) distant vision and (ii) near vision?
(i) Focal length of the lens required for correcting vision is given by F= 1/P.
or F = -1 /5.5 = 0.18 meters = 18centimeters.
Therefore focal length of the lens will be F = 18 cm.
(ii) Power of the lens required for correcting near vision = + 1.5 D
Focal length of the lens required for correcting vision is given by; F = 1/P
=> F = + 1 /1.5 =+0.67 meters = 67 centimeters.
Therefore focal length of the lens will be F = 67 cm.
The far point of a myopic person is 80 cm in-front of the eye. What is the nature and power of the lens required to correct the problem?
We know from lens formula 1/f = -1/u + 1/v …..(i)
Here v = – 80cm and u = ∞.
Putting these values in equation (i), we get,
1/f = -1/∞ + 1/(-80)
Or 1/f = -1/80
or f= -80cm = -0.80meter.
To correct myopia, a person concerned should use concave lens (diverging lens) of focal length 0.80 m, so that for an object at infinity (u=∞), the virtual image is formed at the far point of myopic person, i,e, v = -80 cm.
Now the power of this lens is given by P=1/f.
or P = 1/(-0.80)
Or P = 1.25 Diopter.
Therefore the power of lens will be P = -1.25 Diopter.
Why is a normal eye not able to see clearly the objects placed closer them 25cm?
This is because the focal length of eye can not be adjusted below a certain minimum limit, which in case of human eye is 25 cms.
What happens to the image distance in the eye when we increase the distance of an object from the eye?
When we increase the distance of an object from the eye, the focal length of eye lens is changed due to accommodation power of the eye so as to keep image distance constant.
This happens in a normal eye, because the image distance in the eye is fixed and equal to the distance of retina from the eye lens.
Why do stars twinkle?
Stars twinkle when we see them from the Earth’s surface because we are viewing them through thick layers of varying optical densities of air at various altitudes in the Earth’s atmosphere.
When the light coming from a star enters the earth’s atmosphere it undergoes bending (refracted) many times and in random directions. Due to the continuously varying optical densities of air at various altitudes, the light from the stars gets refracted by different amounts from one moment to the next.
Thus when the atmosphere refracts more star light towards us. the star appears to be bright and when the atmosphere refracts less star-light towards us, the star appears to dim. It is due to this continuous increase and decrease in brightness of star light, that stars appears to twinkle. (it looks as though the star moves a bit, and our eye interprets this as twinkling).
Stars closer to the horizon appear to twinkle more than stars that are overhead, this is because the light of stars near the horizon has to travel through more air than the light of stars overhead and so is subjected to more refraction.
Explain why the planets do not twinkle?
The planets do not usually twinkle, because they are so close to us, they appear big enough that their twinkling is not noticeable.
Stars twinkle because they are so far away from Earth that, even through large telescopes, they appear only as pin points. And it’s easy for Earth’s atmosphere to disturb the pin-point light of a star. Thus the stars twinkle.
However planets don’t look like pinpoints, instead they look like tiny disks (when seen through a telescope). And while the light from one edge of a planet’s disk might be forced to show dimming effect by Earth’s atmosphere, it is nullified by the brighter effect produced from the opposite edge of the planet’disk by Earth’s atmosphere Thus, on the whole, the brightness of a planet always remains the same and hence it does not appear to twinkle.
Why does the sun appear reddish early in the morning and at the time of sunset?
As the white light contain seven colours (VIBGYOR) and when all the seven colours reaches to us in right proportion, the sunlight appears white to us.
At the time of sunrise and sunset the sun is near the horizon, the sunlight has to travel a much larger distance through the atmosphere to reach the observer on the earth. Therefore most of the blue light whose wave length is shorter as compared to other colours is scattered away. So the light reaching us directly consists mainly of longer wavelength red colour due to which the sun appears reddish at the time of sun rise and sunset.
However when the sun is nearly overhead (at noon). the sun light has to travel relatively shorter distance through the atmosphere to reach us. During this only a little of the blue light is scattered. So the sun light contains almost all component colours in the right proportion, therefore the sun in the sky overhead appears white to us.
Why does the sky appear dark instead of blue to an astronaut?
As sky appears blue due to scattering of blue light by dust particles present in atmosphere.
But sky appears dark to an astronaut when he is at a huge height from the surface of earth because, there is nothing which could scatter the sunlight. Therefore, the sky appears dark.
FAQs on Human Eye and The Colourful World
Sky appears blue to us because of the scattering of the light. During scattering of light, its low wavelength components (bluish components) scatter the most and the large wavelength components (reddish components) scatter least. Due to this high scattering of bluish components, blue light spreads in the sky and hence it appears blue to us.
The sun appears white to us during the day time because the light has to travel a shorter distance to reach us and while doing so, all the colors get equally scattered and hence the light reaching us comprises of all the colors in almost equal quantities. This almost equal proportion of all the colors in the light reaching our eyes make the sun appear as white in color.
Red color is used as a sanger signal because it scatters the least among the seven constituent colours of the white light. Due to its least scattering, an observer can see the danger signal from a very far off distance very clearly.
When white light is passed through a glass prism, the violet deviates the most and the red deviates the least.
The image formed on the retina of a human eye is real and inverted.
The change of focal length of eye lens is caused by the action of the Ciliary muscles. When ciliary muscles are relaxed, the lens is thinner and when the ciliary muscles are contracted, the lens becomes thicker. In this way the ciliary muscles help the eye in its accomodation.
The least distance of distinct vision for a young adult with normal vision is about 25cm. It means that the person can see the objects at a distance of at least 25cm clearly without any kind of strain in his eye. If an object is brought closer than 25cm, the eye cannot see it clearly and experiences strain.
These were the “human eye and colourful world notes for 10th class science”. We are sure that you will find these notes very useful for your exams.