Chapter 11 Human Eyes and Colourful World Class 10 Science Notes
Students should read Chapter 11 Human Eyes and Colourful World Class 10 Science Notes provided below. These notes have been prepared based on the latest syllabus and books issued by NCERT, CBSE and KVS. These important revision notes will be really useful for students to understand the important topics given in the chapter Human Eyes and Colourful World in Class 10 Science. We have provided class 10 science notes for all chapters.
Revision Notes Chapter 11 Human Eyes and Colourful World Class 10 Science
Chapter 11 Human Eyes and Colourful World is an important chapter in Class 10 Science. The following notes will help you to understand and easily learn all important points to help you score more marks.
Important Terms & Concepts
Eye: The natural optical device, using which, man could see objects around him. It forms an inverted and real image of la light sensitive surface called the retina.
Cornea: It is a thin membrane, covering the surface of eye ball, through which light enters. It acts as a primary lens, which provides the refraction for light rays entering the eye.
3. Aqueous Humour: It is a transparent gelatinous fluid, secreted form ciliary muscles, fills the space between cornea and eye lens. It provides nutrition to the eye tissues and increases the protection against dust, wind, pollen grains, etc.
Iris: It is a dark muscular diaphragm that controls the size of the pupil and is located just behind the cornea in the eye.
Pupil: The black opening between the aqueous humour and the eye lens. Since, light does not get reflected from it, its appearance is dark. The amount of light entering the eye controlled by the size of the pupil. In dim light, it opens up completely through the iris, but in bright light, it becomes very small.
Ciliary Muscles: The shape of the eye-lens can be modified by the presence of ciliary muscles. This leads to variation in focal length. When the eye looks at distant object, then the ciliary muscles are relaxed, the focal length of the lens has its maximum value, equal to the distance from the optical centre of eye lens to the retina. So, parallel rays coming into the eye get focused on the retina. When the eye looks at nearby objects, the ciliary muscles are contract, the focal length of eye lens decreases but increase its converging power to form an image of the retina of the eye again.
Eye lens: It is converging in nature, made by the jelly like proteinaceous material. The focal length of eye lens is changed by the ciliary muscles. Its function is to focus incoming light rays on the retina using its refractive property.
Aqueous Humour-It provides Power to Cornea
Vitreous Humour: It is a transparent, colourless, gelatinous mass that fills the space between the eye lens and retina of the eye ball. It helps to keep retina in place by pressing it against the choroid.
Blind Spot: The spot on the retina where no receptors are found. The image formed on this region will not be seen and from here, a bundle of sensory fibres called optic nerves leads back into the brain.
Optic Nerve: IT acts like a cable, connecting the eye with the brain. It transmits the visual information in the form of electrical signals generated at retina to the brain.
Near Point of Eye: The minimum distance between the object and the eye (lens) so that a clear image is formed on the retina is called the least distance for clear vision or near point of the eye. This distance is 25 cm for human eye (average value). In young age, it is around 7-8 cm and in old age it is 1-2 m or even more.
Far Point of Eye: The farthest point upto which an eye can see the object clearly is called the far point of the eye. For a normal eye, it is at infinity.
Retina: Retina is a delicate membrane having enormous number of light-sensitive cells on which real, inverted and diminished image is formed by the crystalline lens of eye. These receptors are known as rods and cones. It is equivalent to the photographic film in a camera.
Rods and Cones: The cells in retina, which are light sensitive. Rods respond to the intensity of light.
Cones respond to the illumination. The cones are sensitive to different extent in different primary colours; Red, Blue and Green. Specialised cones respond to specific colours. There are around 125 million rods and cones cells. The cells generate electrical signals which are transmitted to the brain through optical nerves.
Range of Vision: The distance between for point and near point of the eye is called range of vision.
Accommodation: The ability of an eye lens to adjust its focal length by the action of ciliary muscles to get a clear and sharp image of the distant object as well as nearby object is called accommodation.
There is always a limit upto which ciliary muscles can increase or decrease the focal length of eye lens.
For normal eye =infinity
Near point =25 cm
Power of Accommodation: The maximum variation in the converging power (focal length) of eye lens so that the far-off and nearby objects are viewed clearly is called power of accommodation. For a person having normal vision, it is about 4 dioptres.
Persistence of Vision: The time for which the impression or sensation of an object continues in the eye is called the persistence of vision. It is about 1/16th of a second.
Motion Picture: Motion picture projection is done to give us the feeling of moving images of an event.
The event is projected at the rate of 24 frames per second. Any projection more or less than this will lead to mixed-up or blurred image.
Two eyes of human beings are positioned such that:
• The field of view is more
• Three dimensional views is possible
• Coordination with the brain can give the concept of distance.
Colour-Blindness: Cones with specific colours are there in the retina. If some cones are absent, the distinction of colours is not possible. In such case, the person is said to be colour-blind. This defect arises due to (i) absence of colour responding cones cells in the retina and (ii) due to genetic disorder.
No cure is developed till now by the science.
Night-Blindness: Some persons have the difficulty to see the objects in dim light during night. This defect of eye is called night-blindness. This defect arises due to (i) lack of vitamin A n the food and (ii) improper functioning of rods-shaped cells.
The rods-shaped cells respond to intensity variation in light. So, by taking the proper amount of vitamin A in the diet, the functioning of rods-shaped cells may be improved.
Cataract: Sometimes, the crystalline lens of eye, at old age, becomes milky land cloudy due to growth of thin membrane over it. This causes partial or complete loss of vision. This condition is called cataract. This is corrected by the surgical removal of extra growth.
Myopia of Short-sightedness or Near-sightedness: The defect of vision due to which an eye cannot see distant object distinctly but can see nearby objects clearly. This defect of eye is called myopia. The image in this case, falls before the retina. For every myopic eye, there exists a far point beyond which clear image cannot be seen. Short-sightedness is caused due to
(i) excessive curvature in cornea or
(ii) elongation of eye-ball.
The short-sightedness is corrected by using a concave lens, which diverges and shifts the image to the retina.
Hypermetropia of Long-sightedness or Far-sightedness: The defect of vision due to which an eye cannot see nearby object distinctly but can see distant objects clearly. This defect of eye is called hypermetropia. The image in this case, falls beyond the retina. For a hypermetropic eye, there exists a near point. Long-sightedness is caused due to (i) greater focal length of the eye lens or (ii) smaller eyeball. It is corrected by using a convex lens, which converges and shifts the image to the retina from beyond.
The following numerical examples help the students in calculating the power of corrective lens in hypermetropia and myopia.
Ex. 1. Least distance of distinct vision of a long-sighted man is 40 cm. he wishes to reduce it to 25 cm by using a spectacle. What is the power and type of lens used by him?
Ex. 2. A person cannot see distinctly any object placed beyond 40 cm from his eye. Calculate the power of the lens which will help him to see distant objects clearly.
Sol. A lens which can bring an object at ∞ to 40 cm is required for the person.
Presbyopia: In human eye, with age, the near point recedes and the far point get reduced. The eye becomes both myopic and hypermetropic. This is caused due to (i) weakening of ciliary muscles and (ii) reducing ability of eye lens to change its curvature. It can be corrected by using bifocal lens.
Bifocal Lens: It is used to correct presbyopic eye. It contains a lens with upper concave and lower convex lens character. The concave lens is used for viewing long distant objects and the convex lens is used for reading purpose.
Astigmatism: The inability of an eye in focusing objects in both horizontal and vertical lines clearly is called astigmatism. This is caused due to varying curvature in eye lens in horizontal and vertical lines. It is corrected by using cylindrical lenses.
Contact Lenses: The lenses used in contact with eye to correct the defects in the image formation are called contact lenses. Soft contact lenses allow more of oxygen to reach than hard lenses. The lenses vend the rays of light to correct their (refractive) angle, to enable a sharp image being formed at the retina. It is to be used with care and should be cleaned or replaced over a period of time.
Prism: Prism is a homogeneous, transparent, refracting material, such as glass, enclosed by two inclined plane refracting surfaces, at some fixed angle, called refracting angle or angle of prism. It has two triangular bases and three rectangular lateral surfaces which are inclined to each other.
Refraction of Light through a Glass Prism: Let ABC is the principal section of a glass prism of refractive index ‘n’. The angle ‘A’ is the refracting angle of the prism.
When a ray of light PQ from rarer medium, such as air is incident on a refracting surface AB, it gets refracted and bends towards the normal to the plane of face AB. It is refracted along QR. At the second surface AC, the light ray has entered from glass to air, i.e., denser to rarer medium and emerges out along RS as it has bent away from the normal which is shown in figure. Here, PQ is incident ray, QR is refracted ray, RS is called emerged ray, ∠A is the angle of prism, ∠i is angle of incidence, ∠r is angle of refraction,and ∠e is angle of emergence and ∠δ is the angle of deviation.
Thus, the incident ray PQ suffers two refraction in passing through the prism.
Angle of Refraction: The angle between the refracted ray and the normal is called angle of refraction (∠r).
Angle of Emergence: The angle between the emergent ray and normal at the second refracting face of the prism is called angle of emergence (∠e).
Angle of Deviation: The angle formed between the incident ray PQ produced in the forward direction and emergent ray RS produced in the back ward direction in the refraction through the prism is called angle of deviation (∠δ).
Dispersion: The splitting up of white light into its constituent colours is called dispersion. The colour sequence is given by the acronym V I B G Y O R – Violet, Indigo, Blue, Green, Yellow, Orange and Red as shown in the figure. This colour pattern is called a spectrum.
Dispersion occurs because refraction or bending differs with the colour. The speed of light of different colours in a medium like glass, water, etc. is different. The speed of light, however is the same for all colours in free space or vacuum. Varying speed for different colours lead to different refractive indices for different colours. It has been observed that the refractive index of glass for violet colour is more than that for red colour. Therefore, they emerge out through a prism along different directions and become distinct.
Recombination of Spectrum of White Light: Two prisms, one upside down will recombine the colours to give a white light if white light falls on the first prism. This is due to the fact that the refraction or bending produced by the second prism is equal and opposite of the refraction or bending produced by the first prism. This was explained by Sir Isaac Newton.
Rainbow: It is an optical natural spectrum, produced by the nature in the sky, in the form of a multicoloured arc. It is formed by the dispersion of sunlight by the tiny water droplets present in the atmosphere after the rain. These water droplets act like a prism. Rays of sunlight enter the water droplets. At the point of incidence, they refract and disperse, then reflect it internally and finally refract it again at the point of emergence. Thus, the different colours of this spectrum called rainbow is reached the observer’s eye.
Necessary Conditions for the Formation of a Rainbow:
I) The presence of water droplets in the atmosphere, and
II) The sun must be at the back of the observer, i.e., observer must stand with his back towards the sun.
Sometimes, we may get two rainbows. They are called primary and secondary rainbows. In primary rainbow, red colour appears on the top of the rainbow and violet colour appears at the bottom of the rainbow. In secondary rainbow, red colour appears on the inner edge and violet on the outer edge.
Atmospheric Refraction: Change in the direction of propagation of light rays travelling through the atmosphere due to change in density of the different layers of air is called
atmospheric refraction. There are several natural events which can be explained on the basis of atmospheric refraction, such as twinkling of stars, advanced sunrise and delayed sunrise and delayed sunset, etc.
(i) Twinkling of Stars: This is due to the refraction of light coming from the stars by atmosphere. Light from stars passing through the atmosphere, bends towards the normal. So, the stars appear slightly higher than they actually are. Due to atmospheric variations, they do not appear stationary but there is fluctuation in its position. This also causes increase or decrease in the luminous flux of the star light entering our eyes. As a result, stars appear twinkling.
(ii) Plants do not Twinkle: Planets being of larger size can be taken as a collection of large number of point sized objects/sources of light, which nullify the twinkling effect of each other.
(iii) Advanced Sunrise and Delayed Sunset: Sun is visible 2 minutes before sunrise and 2 minutes after sunset because of atmospheric refraction.
Scattering of Light: The phenomena of change in the direction of propagation of light caused by the large number of molecules, such as smoke, tiny water droplets, suspended particles of dust and molecules of air present in the earth’s atmosphere is called scattering of light. The colour of the scattering light perceived by us depends on the size of the particles. Very fine particles scatter mainly of longer wavelengths. Large sized particles scatter light of longer wavelengths. Enough larger particles scatter the light which may appear white.
Rayleigh Scattering: According to Rayleigh “The amount of scattering is inversely proportional to the fourth power of the wavelength.” Therefore, the light of shorter wavelength is scattered much more than the light of longer wavelength. There is no change in the wavelength of light rays during scattering. The most beautiful phenomena of nature, such as ‘Blue Colour of Sky’, ‘White Colour of Clouds’, ‘Red Hues of Sunrise and Sunset’ can be explained in terms of scattering of light.
Blue Colour of Sky: Blue colour has a shorter wavelength than red. So, according to Rayleigh scattering law, blue colour of sunlight scattered much more strongly by the large number of molecules present in the earth’s atmosphere. Hence, the sky appears blue. On the surface of moon, there is no atmosphere. Therefore, the scattering phenomenon does not occur on the moon. Hence, the sky of the moon appears dark.
White Colour of Clouds: Large particles like rain drops, dust or ice particles present in the atmosphere scattered all the wavelength of light almost equally. Hence, the clouds which have droplets of water scattered all colours equally to give the white appearance. So, clouds generally appear white.
At Sunset or Sunrise, the Sun looks almost reddish: The sunrays have to travel through a larger atmospheric distance. As λb<λr , most of the blue light and shorter wavelengths are removed by scattering. Only red colour, which is least scattered is received by our eye and appears to come from the sun. Hence, the appearance of sun at sunset or sunrise, full moon near the horizon may look almost reddish.
Danger Signals are Red: The wavelength of red colour is longer among the other colours of visible spectrum of sunlight. According to Rayleigh scattering law (Scattering ∝1/λ4 ), red colour is least scattered while passing through the atmosphere and therefore, travels large distance, i.e., red colour can be seen through a large distance. Hence, the danger signals make use of red light.
Tyndall Effect: The phenomenon of scattering of light by the colloidal particles is known as Tyndall effect. This effect can be observed when
a) A fine beam of sunlight enters a room containing suspended particles of dust, the path of the beam, of light visible. It is due to the scattering of light.
b) Sunlight passes through a canopy of dense forest. In the forest, mist contains tine water droplets, which act as a colloidal particles. These tiny water droplets scatter the sunlight.