The Human Eye and the Colourful World Notes Class 10
The Human Eye and the Colourful World Notes Class 10: CBSE Notes Class 10 Science Chapter 11 Notes. The Human Eye and the Colourful World Notes PDF. Class 10 Science Chapter 11 Revision Notes – The Human Eye and the Colourful World.
The Human Eye and the Colourful World Notes Class 10: Overview
The Human Eye and the Colourful World Notes Class 10 |
|
Board |
CBSE / NCERT |
Class |
10 |
Subject |
Science |
Chapter Number |
11 |
Chapter Name |
The Human Eye and the Colourful World |
Topic |
Notes |
The Human Eye and the Colourful World Notes Class 10 Science
Human eye:
Most sensitive and important organ of our body is an eye. When ray of light falls on eye, we gets sensation and forms image of the object with the help of lens inside our eye. It works almost similar to camera. It has different part and each part has specific function, explained below.
- Eyeball: It is muscular and spherical ball of diameter about 2.3 cm as average. It is white in colour due to sclera.
- Cornea: It is the thin transparent bulgy layer over an eyeball. The first refraction of light is takes place through cornea, and then it light enters in eye.
- Iris: The dark muscular diaphragm beside cornea is known as Iris. It may different colours for different person (blue, green, brown, black etc).Iris controls the size of pupil.
- Pupil: The stretchable muscular opening inside iris is known as pupil. The function of pupil is to regulate or control the amount of light entering the lens. When intensity of light is less, pupil gets stretched and expanded to provide more exposure for light to enter the lens. For excess intensity of light pupil gets contracted and blocks the unnecessary light to fall on lens.
- Convex lens: The crystalline semisolid (jelly like) convex lens made up of large number of small fibrous is mounted behind iris has focal length around 2.5 cm. The regulated light from pupil falls on convex lens. Lens converges’ all the rays and sends to retina.
- Retina: Retina is screen like membrane, made up of large number of light sensitive cells which gets activated on illumination of light. Retina collects all the rays coming from convex lens, and forms real and inverted image of the object.
- Optics nerves: Optical nerves are a kind of transducer which converts light energy into electric pulse and carries these pulses to brain. Finally brain reads the frequency/wavelength of the signal and we see object as it is.
Refer the figure for description of eye diagrammatically.
Power of accommodation:
Unlike camera, we don’t need to focus separately to see the object placed at different distances. Our eyes have an important ability that we can focus all the objects clearly from the same positions, irrespective of their distances from eyes. This ability is known as power of accommodation.
The ability of human eye lens to contract or expand so that the focal length of eye can increase or decrease as per the need is called as Power of accommodation.
Ciliary glands are present around the lens which helps eyes to change its focal length. 1) When we want to see nearby object, focal length of eye lens should be small, to achieve this ciliary muscles pull the lens, makes it thicker so that provide us ability to see nearby object. 2) If we want to see distinct objects, ciliary muscles contracts the eye lens, due to this the focal length of lens is increased and we can see distinct objects clearly. This ability plays very vital role in our visual ability. Note that, the focal length cannot be increased or decreased beyond certain limited value.
The Human Eye and the Colourful World Notes Class 10
Defects in human eye:
When ciliary muscles don’t work properly, we often lose the power of accommodation, which creates two types of defects in our eyes viz.
1) Myopia (Near sightedness):
The defect in human eye in which one can see nearby objects clearly, but cannot see distinct object clearly is known as myopia or near sightedness.
In Myopia, ciliary muscles expands the lens, but doesn’t relaxed, so that the focal length of eye lens decreases, eyeball gets elongated. Image of object converges before retina. Due to this we can see nearby objects clearly but can’t see distinct objects.
To correct this defect, ophthalmic experts can prescribe the concave lens of suitable power (or focal length) to use in spectacles.
Due to the concave lens, the rays’ gets slightly diverge before entering eyelens, so that after refraction through eye lens, they converge at the retina. Refer the diagram shown below explains the corrected vision due to concave lens.
As we already know that the power of concave lens is negative, the myopic person uses spectacles with negative powers (e.g. -0.5 D, -1.25 D etc) as per requirement.
2.) Hypermetropia (Far sightedness):
The defect in human eye in which one can see distinct objects clearly, but cannot see nearby object clearly is known as hypermetropia or far sightedness.
In Hypermetropia, ciliary muscles contracts the lens, but doesn’t relaxed, so that the focal length of eye lens increases, eyeball gets flattered. Image of object converges behind retina. Due to this we can see far objects clearly but can’t see nearby objects.
To correct this defect, ophthalmic experts can prescribe the convex lens of suitable power (or focal length) to use in spectacles.
Due to the convex lens, the rays’ gets slightly converge before entering eyelens, so that after refraction through eye lens, they converge at the retina. Refer the diagram shown below explains the corrected vision due to convex lens.
The power of the convex lens is taken as positive, hence the spectacles of the hypermetropic person has positive power (e.g. +1.00 D, +2.00 D etc)
3) Presbyopia: This defect usually appears due to old age. The power of accommodation of eyes decreases due aging so the person may find difficulties to see nearby objects as well as distinct objects too. This is known as Presbyopia. Such people require bi-focal lenses. A bi-focal lens consists of both concave and convex lenses. The upper portion consists of a concave lens and lower part is convex lens, so that both the difficulties in vision of nearby as well as distinct object can be easily overcome.
Distance of distinct vision (DDV) or near point: For normal human eye, the minimum distance up to which one can see the object clearly without any difficulty is called as distance of distinct vision (DDV). It is about 25 cm for normal human eye.
Far distance of distinct vision or far point: For normal human eye, the maximum distance up to which one can see the object clearly without any difficulty is called as far point. It is infinity for normal human eye.
Rod cells and cone cells: The rod cell and cone cells are present at the outermost layer of retina. The function of rod cell is to detect the intensity of light i.e. whether the light is bright or dim detected using rod cells. Whereas the function ofcone cells is to detect the colour or wavelength of light reached to retina, for this there are three different cone shaped cells are present. Three cone cells mainly detects red, blue and yellow colours.
The Human Eye and the Colourful World Notes Class 10
Dispersion of light:
The phenomenon of splitting of white light in to its component colours (seven colours) is called as dispersion of light.
The sequential pattern of colours obtained at the output is called as dispersion spectrum. Dispersion pattern consist of following seven colours in sequential manner viz.
Red, Orange, Yellow, Green, Blue, Indigo, Violet (VIBGYOR)
Sir Isaac Newton used an instrument called prism through which he sent a rays of white light from one face. It observed that when beam of white light is allowed to fall on prism. It gets split in to seven colours as mentioned above. Now he placed second identical prism inverted in the path of dispersed beam, and observed that the beam of white light is emerging from either side of second prism.
The phenomenon of dispersion can be seen in formation of rainbow in rainy season.A rainbow is always formed in a direction opposite to that of the Sun. The water droplets act like small prisms. They refract and disperse the incident sunlight, due to the dispersion of light and internal reflection, different colours reach towards us and we observe the rainbow.
Atmospheric refraction:
When light travels through different layers of atmosphere it gets refracted multiple times. This is due to different conditions of atmosphere around earth. Sometime atmospheric particles may be hotter so acts as rarer medium, whereas the particles with less temperature act as denser medium. When light travels through such layers, it’s direction changes again and again due to refraction from rarer medium to denser medium or vice versa. Because of this the positions of objects are appear to be changed which can be called as atmospheric refraction. The main example of atmospheric refraction is twinkling of star.
Twinkling of stars:
Almost all stars have their own light. Light emitted by stars enters the atmosphere and suffers multiple refractions due continuous changes in refractive indices of medium and then reaches to observer. When the refraction is takes place more toward normal, position of star appears slightly higher than its original position. But this position of the star is not stationary and keeps on changing slightly, since the physical conditions of the earth’s atmosphere like temperature and air are not stationary. Due to this the position of stars continuously appeared fluctuating which makes star sometimes brighter or dim. This phenomenon is known as twinkling of stars.
On other hand, planets do not twinkle because planets are closer to earth than stars. Apart from this, planets can be considered as composed of large number of stars in it. Due to this the average intensity numbers of stars appears bright is always equals to average number of stars appears dim, which cancels the effect of twinkling and planets acts as steady light source.
Advance sunrise and delayed sunset:
During sunrise in horizon, sunrays appear to travel in curved path and reached the observer approximately 2 min before actual sunrise travels this is called as advanced sunrise. During sunset also, after setting of sun completely below horizon sunray gives us feeling of presence of sun till 2 minute later of actual sunset, this is known as delayed sunset. This increases total duration of day by 4 minutes daily.
The Human Eye and the Colourful World Notes Class 10
Scattering of light:
Light travels in atmosphere through different layers. Each layer consisting of dust particles, molecules of gases, water vapours. When rays of light strikes on these particles it gets reflected from these particles but, all reflected rays travels in all possible direction in the surrounding. This phenomenon is known as scattering of light. Blue colour of the sky, colour of water in deep sea takes place due to scattering of light. Scattering can be produced using some solutions known as colloidal solution.
Tyndall Effect:
Earth’s atmosphere is composed of very fine particles such as dust, water droplets. When a beam of light falls on these small particles, the ray of light becomes almost visible. This continuous and diffused reflection of light due to such tiny particles makes the path of light visible. This phenomenon is known as Tyndall effect.
e.g. sunrays are visible in dense forest through canopy different branches of trees, and we may feel the presence of particles in the sunrays. Thus, scattering of light makes the particles visible.
Colour of the scattered light depends on the size of the particles from which it scatters. Very fine particles scatter mainly blue light and particles of larger size scatters mainly colours having longer wavelength e.g. red, orange etc. Particles of considerable size can scatter white light too.
Blue colour of sky:
The molecules of air and other particles are having very fine in size, such that their size is almost equivalent to the lower wavelengths of visible spectrum. These lower wavelengths are close to wavelengths of blue and indigo colours. The excess quantity of these small size particles scatters blue light more than any other wavelengths, hence sky appears blue to observer.
Astronauts in space will see the atmosphere completely dark. As no particles are present beyond atmosphere, scattering of light will not takes place and so that the space will completely appear dark.
Colour of the Sun at Sunrise and Sunset:
A sunray has to travel longer distance during sunrise and sunset to reach the observer as compared to sun over head to observer. During this most of the blue light scatters in atmosphere. Thick and dense layers of particles near to sun scatters almost all blue light except light of larger wavelength i.e. red, orange and yellow which reaches to observer and the region around sun during sunrise and sunset appears reddish, yellowish to observer.
Sir C.V. Raman is the only physicist in India, who received a Noble price for his tremendous work and efforts to discover the scattering by colloidal solution. The effect discovered by him is proudly known as “Raman Effect”. In his memory we celebrate 28th February as National Science day.
The arrangement to study scattering of light using colloidal solution.