Chapter 10 - Human Eye and The Colourful World
Chapter 10: The Human Eye and the Colourful World
1.
The Human Eye
The human eye is a natural
optical device that helps us to see objects around us.
Draw Human Eye from figure 11.1 Ncert Book
Structure
of Human Eye:
- Eye
Ball : Its spherical in shape with
diameter 2.3 cm.
- Cornea:
Transparent front part where light enters. It bends light towards the
lens.
- Aqueous Humour:
A transparent liquid filled in the space between cornea and eye lens.
- Iris:
It is the dark muscular diaphragm between the cornea and the lens.
- Pupil:
It is the small hole between the iris through which light enters.
- Lens:
Convex, transparent, flexible structure located behind the iris.
- Ciliary Muscles:
Muscles holding the lens in position.
- Vitreous Humour :
A liquid filled in the space between the eye lens and retina.
- Retina:
Screen where the image is formed. It contains light-sensitive cells (rods
and cones).
- Optic Nerve:
Sends visual signals from the retina to the brain.
- Blind Spot :
It is the spot in the retina where the optic nerve connects
2.
Functions
- Cornea:
-
It refract most of the light rays.
- Aqueous Humour:
-
It helps the refracted light to be
focused on retina.
-
It maintains the intraocular pressur
- Iris:
-
It controls the size of the pupil.
-
Its colour forms the colour of our
eyes.
-
It regulates and controls the amount
of light entering the eye by adjusting the size.
- Pupil:
-
It allows the light to enter inside
the eyes. It appears black as no light is reflected.
- Crystalline Lens:
-
It provides adjustment of focal
length required to focus objects at different distances
- Ciliary Muscles:
-
It helps in modifying the curvature
of eye lens
- Vitreous Humour :
-
It provides nutrients to the eye.
-
It keep our eye in spherical shape
- Retina:
-
It forms image of the object.
- Optic Nerve:
-
It helps to transmit visual information
from the retina to the brain.
3.
Image Formation in Human Eye
- The lens forms a real, inverted, and diminished
image on the retina.
- The brain processes the signals and allows us to see
the object upright.
4.
Accommodation
The ability of the eye to adjust its
focal length to see near and distant objects clearly by changing the shape of
the lens using ciliary muscles is known as accommodation.
- For near objects:
Ciliary muscles contract, lens becomes thicker, focal length decreases.
- For distant objects:
Ciliary muscles relax, lens becomes thinner, focal length increases.
How we can see far and nearby objects?
When the muscles are relaxed, the lens becomes thin. Thus,
its focal length increases. This enables us to see distant objects clearly.
When you are looking at objects closer to the eye, the ciliary muscles
contract. This increases the curvature of the eye lens. The eye lens then
becomes thicker. Consequently, the focal length of the eye lens decreases. This
enables us to see nearby objects clearly.
5.
Range of Normal Human Eye
- Near Point:
Minimum distance for clear vision = 25 cm
- Far Point:
Maximum distance for clear vision = Infinity
6.
Persistence of Vision
- The image of an object remains on the retina for about 1/16th
of a second even after the object is removed.
- It is the basis for motion pictures.
7.
Defects of Vision
a)
Myopia (Near-sightedness)
- Definition:
A person can see nearby objects clearly but distant objects appear blurry.
- Causes:
- Elongation of eyeball.
- Lens becomes too curved.
- Remedy:
Use of concave lenses.
Draw diagram from figure 11.2 Ncert Book
b)
Hypermetropia (Far-sightedness)
- Definition:
A person can see distant objects clearly but cannot see nearby objects.
- Causes:
- Eyeball is too short.
- Lens becomes too flat.
- Remedy:
Use of convex lenses.
Draw diagram from figure 11.3 ncert Book
c)
Presbyopia
- Definition:
Defect in old age where the eye cannot focus on near objects.
- Causes:
- Loss of flexibility of the eye lens.
- Weakening of ciliary muscles.
- Remedy:
Use of bifocal lenses or progressive lenses.
d)
Cataract
- Definition:
Formation of cloudy layer on the eye lens that leads to blurred vision.
- Remedy:
Surgical removal of the cloudy lens and replacement with an artificial
lens.
8.
Refraction of Light Through Prism
Prism is transparent refracting
medium bounded by at least two lateral surfaces, inclined to each other. It has
two triangular bases and three rectangular lateral surfaces
When light passes through a glass
prism, it bends twice:
- Once when entering the prism.
- Once when exiting the prism.
- Angle of Deviation:
The angle between the original path of the incident ray and the emergent
ray.
The peculiar shape of the prism makes the emergent ray bend
at an angle to the direction of the incident ray. This angle is called the
angle of deviation. In this case ∠D is the
angle of deviation.
Draw the diagram from 11.4 Ncert Book
9.
Dispersion of White Light by Glass Prism
- Dispersion:
Splitting of white light into its seven colours (VIBGYOR i.e Violet,
Indigo, Blue, Green, Yellow, Orange, Red ) when passed through a prism.
- Cause of Dispersion:
Different colours of light have different speeds in the prism material.
Each colour bends by a different amount.
Draw diagram from figure 11.5 ncert Book.
Red light has the maximum wavelength
and violet light has the minimum wavelength. So in any medium, red light
travels fastest and deviates least, while violet light travels slowest and
deviates maximum.
Wavelength α Velocity α 1/ deviation
10.
Recombination of White Light
When a second identical prism is
placed in an inverted position after the first prism, the seven colours
recombine to form white light.
Draw the diagram from figure 11.6 Ncert Book
11.
Rainbow Formation
- Caused by dispersion, total internal reflection, and
refraction of sunlight by water droplets in the atmosphere.
- Primary rainbow:
Light enters a droplet, bends, reflects inside, and disperses on exit.
- The observer sees the rainbow when the sun is behind
them.
Draw diagram from figure 11.8 Ncert Book
How
is rainbow formed?
A rainbow is a natural spectrum appearing in the sky after a rain
shower. It is caused by dispersion of sunlight by tiny water droplets, present
in the atmosphere. 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, then reflect it internally, and finally refract it again
when it comes out of the raindrop . Due to the dispersion of light
and internal reflection, different colours reach the observer’s eye.
12.
Atmospheric Refraction
Bending of light when it passes
through layers of air of different densities.
Effects:
- Twinkling of Stars:
Due to continuous refraction by moving air layers causing starlight to
bend in different directions.
Draw the diagram from figure 11.9 Ncert
- Advance Sunrise and Delayed Sunset: Sun appears before it actually rises and appears for a
longer time during sunset because of the bending of sunlight.
Draw the diagram from figure 11.10 Ncert Book
13.
Scattering of Light – Tyndall Effect
- Scattering of light occurs when it strikes small particles
in the atmosphere.
- Tyndall Effect:
The scattering of light by colloidal particles or very fine dust in the
air.
The earth’s atmosphere is a heterogeneous mixture of minute particles. These particles include smoke, tiny water droplets, suspended particles of dust and molecules of air. When a beam of light strikes such fine particles, the path of the beam becomes visible. The light reaches us, after being reflected diffusely by these particles. The phenomenon of scattering of light by the colloidal particles gives rise to Tyndall effect.
This phenomenon is seen when a fine beam of sunlight enters a smoke-filled room through a small hole. Thus, scattering of light makes the particles visible. Tyndall effect can also be observed when sunlight passes through a canopy of a dense forest. Here, tiny water droplets in the mist scatter light.
The colour of the scattered light depends on the size of
the scattering particles. Very fine particles scatter mainly blue light while
particles of larger size scatter light of longer wavelengths. If the size of
the scattering particles is large enough, then, the scattered light may even
appear white.
Why
is the Sky Blue?
- Blue light is scattered the most by air molecules due
to its shorter wavelength.
- As a result, we see the sky as blue.
The molecules of air and other fine particles in the atmosphere have size smaller than the wavelength of visible light. These are more effective in scattering light of shorter wavelengths at the blue end than light of longer wavelengths at the red end. The red light has a wavelength about 1.8 times greater than blue light. Thus, when sunlight passes through the atmosphere, the fine particles in air scatter the blue colour (shorter wavelengths) more strongly than red. The scattered blue light enters our eyes.
If the earth had no atmosphere, there would not have been
any scattering. Then, the sky would have looked dark. The sky appears dark to
passengers flying at very high altitudes, as scattering is not prominent at
such heights.
Why
are Danger Signals Red?
- Red light is scattered the least due to its longer
wavelength.
- It can travel long distances and remain visible, making
it ideal for warning signs and signals.
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