Here I am going to provide you CBSE Class 10 Science Chapter 11 The Human Eye and the Colourful World Notes. By going through The Human Eye and the Colourful World Class 10 Notes you can revise the Human Eye chapter in a very effective way. I hope that this will certainly help you in your studies!
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Structure of a Human Eye
- Of all the sense organs, the human eye is the most significant one as it enables us to see the beautiful, colourful world around us.
- The eye is spherical in shape and has a diameter of 2.3 cm on average.
The internal structure of the eye includes- the cornea, iris, pupil, lens, ciliary muscles, retina, nerve cells, optic nerve, yellow spot, aqueous and vitreous humor, and suspensory ligament.
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- Cornea: The cornea is the thin membrane of the eye through which light enters. Most of the light refraction occurs at the outer surface of the cornea.
- Iris: A dark muscular diaphragm that controls the size of the pupil, known as Iris is present behind the cornea.
- Pupil: The pupil is a structure in the eye that controls and regulates the amount of light entering the eye.
- Lens: It is composed of a fibrous, jelly like material. Provides the focused real and inverted image of the object on the retina. This is convex lens that converges light at retina.
- Retina: The lens system of the human eye forms an image on the light-sensitive screen called the retina.
- Optic Nerves: The light-sensitive cells get triggered upon illumination and induce electrical signals. These signals are sent to the brain through the optic nerves.
Near point or Least distance of distinct vision
- The minimum distance at which objects can be seen most distinctively without strain.
- For a normal adult eye, its value is 25 cm.
Range of human vision – 25 cm to infinity.
Accomodation: The ability of the eye lens to adjust its focal length is called accommodation.
- Focal length can be changed with the help of ciliary muscles.
- Focal length increases when Ciliary muscles get relaxed and lens get thin.
- Focal length decreases when Ciliary muscles get contract and lens get thick.
Defects of a Human Eye
- There are few common eye disorders seen in all individuals and are caused by several factors.
- These conditions can be improved by the corrections.
- The defects include:
Myopia
- This is also called as the short-sightedness.
- A person with this eye defect can only see the nearby objects clearly compared to distant objects.
Causes:
- Excessive curvature of eye lens.
- Elongation of eye ball
Correction:
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- This condition can be corrected using a concave lens.
Hypermetropia
- This is also called as the far-sightedness.
- A person with this eye defect can only see the distant objects clearly compared to near objects.
Causes:
- Focal length of the eye lens becomes too long.
- Eye ball becomes too small.
Correction:
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- This condition can be corrected using a convex lens.
Presbyopia
- This is an age-related condition caused due to the weakening of ciliary muscles, hardening of the lens, and reduced lens flexibility.
- A person with this defect usually finds difficulties to focus on nearby objects unable to read or write.
Causes:
- Gradual weakening of ciliary muscles.
- Diminishing flexibility of eye lens.
Correction:
- Use of convex lens of suitable power.
- Sometimes a person may suffer from both myopia and hypermetropia.Such people require bifocal lens for correction.
Cataract
- This is an age-related condition caused due to the loss of transparency of the lens by erosion of lens proteins.
- It usually results in blurry vision, cloudy lenses.
Correction:
It can be corrected by replacing the old lens with an artificial lens.
Dispersion of white light by a glass prism
- A prism splits the incident white light into a band of seven colours.
- Isaac Newton was the first to use a glass prism to obtain the sunlight spectrum.
- The band of the coloured components of a light beam is called its spectrum.
- The group of alphabets that represent sequential colours in spectrum is called as acronym. i.e. V I B G Y O R
- Angle of Prism: The angle between two adjoining lateral surfaces. Refraction through a glass prism.
- Angle of deviation (d): It is the angle between incident ray and emergent ray.
- Angle of deviation ∝ 1/wavelength
- Different colours of light bend through different angles with respect to the incident ray, as they pass through a prism.
- The red light bends the least while the violet the most.
- Thus the rays of each colour emerge along different paths and thus become distinct.
Total Internal Reflection
- When light enters obliquely from a denser medium to a rarer medium and the angle of incidence exceeds critical angle, the light reflects in the denser medium.
- This is called internal reflection.
Conditions necessary for Internal Reflection
- Light should enter obliquely from a denser to a rarer medium.
- The angle of incidence should exceed critical angle, the light reflects in the denser medium.
Critical angle: The angle of incidence for which the angle of refraction is 90º.
Rainbow
- It is a natural spectrum appearing in the sky after rain showers. Rainbow is observed in the direction opposite to the sun.
- Three phenomenon which are involved in rainbow formation are :(i)Dispersion, (ii) Refraction & (iii) Internal reflection
- Some water droplets remain suspended in air after rain. These droplets behave as glass prism.
- When light enters the rain drop, it first refracts and disperses.
- Then it reflects internally and again refracts as it come out of the drop and the seven colours reach the eye of observer in form of rainbow.
Atmospheric Refraction
The refraction by different layers of atmosphere is called atmospheric refraction.
Some of the examples of atomspheric refraction are following:
An object placed behind the fire or a hot surface appears to flicker when seen through the air.
- The air above hot surface becomes hot and rises. The space is occupied by cool air.
- The refractive index of hot air is less than that of cool air.
- So, the physical condition of the medium are not constant.
- Due to changing Refractive Index (RI) of medium, the light appears to come from different directions.
- It results in fluctuation in apparent position of object.
Stars when seen near the horizon appear slightly higher than their actual position due to atmospheric refraction.
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- The refractive index of earth’s atmosphere in general increases from top to bottom.
- So, the light coming from a star near the horizon has to travel from rarer to denser medium and it bends towards the normal.
- As a result the star appears higher.
Advanced sunrise
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- The sun appears about two minutes earlier than actual sunrise and the sun remains visible for about two minutes after actual sunset.
- When the sun is below horizon, the rays have to pass from rarer to denser medium.
- So rays bend towards the normal. As a result the sun appears higher than its actual position.
Twinkling of stars
- Stars are very far from us, so they behave as point source of light.
- Since the physical conditions of the earth’s atmosphere are not constant the light from stars appears to come from different directions.
- This results in fluctuation of apparent position of star.
- The amount of light coming from stars also vary due to changing Refractive Index of atmosphere.
- The star appears bright when more light from star reaches our eyes and the same star appears dull when less amount of light reaches our eyes.
- Both these effects are responsible for twinkling of stars.
Scattering Effect of Light
- Spreading of light in various directions by colloid particles.
- Scattering ∝ 1/wavelength
Tyndall effect
When light passes through a colloid its path becomes visible. This is called Tyndall effect.
Example:
(i) Path of light becomes visible when light enters a dark and dusty room through a slit or ventilator.
(ii) Path of light becomes visible when light passes through dense canopy of trees in a forest.
Dependence of colour of scattered light
- If particles are very fine, they scatter mainly the blue colour of light (shorter wavelength).
- Medium sized particles scatter mainly the red colour (longer wavelength).
- Even larger particles scatter all the colours of light that is why it appears white.
- Wavelength of red light is about 1.8 times to that of blue light.
Danger signs are made in red colour
- Red is the least scattered colour.
- It is least scattered by fog and smoke and can be seen in the same colour over a long distance.
- So, danger signs are made in red colour.
Colour of sky appears blue on a clear day
- The upper layer of atmosphere contains very fine particles of water vapours and gases.
- These particles are more effective in scattering of light of shorter wavelength mainly blue than larger wavelength.
- So, the sky appears blue.
Appearance of sky to an astronaut in the space
- The sky would appear dark to an astronaut in the space as scattering is not very prominent at such high altitude due to absence of particles.
Clouds appear white
- Clouds are formed by water vapours. Water vapours condense to form water droplets due to larger size of droplets, all colours of light are scattered and clouds appear white.
Colour of sun appear red during sunrise and sunset
- While sunset and sunrise, the colour of the sun and its surrounding appear red.
- During sunset and sunrise, the sun is near horizon and therefore the sunlight has to travel larger distance in atmosphere.
- Due to this most of the blue light (shorter wavelength) are scattered away by the particles.
- The light of longer wavelength (red colour) will reach our eye. This is why sun appear red in colour.