The human eye is an exceptionally sensitive sense organ that works similarly to a
camera. It uses a lens system to form images on a light-sensitive screen.
Important Parts of the Human Eye
Cornea:
A thin, transparent membrane forming a bulge on the front of the eyeball.
Most of the refraction of light entering the eye occurs at the outer surface
of the cornea.
Eyeball:
The eyeball is roughly spherical and has a diameter of approximately
2.3 cm.
Iris and Pupil:
The iris is a dark muscular diaphragm situated behind the cornea.
It adjusts the size of the pupil. The pupil regulates the amount of light
entering the eye.
Crystalline Lens:
It is made of fibrous, jelly-like material. It makes finer adjustments
in focal length to focus objects at different distances. It forms an
inverted real image on the retina.
Retina:
A delicate membrane containing a very large number of light-sensitive cells.
These cells become activated by light and generate electrical signals.
Optic Nerve and Brain:
The optic nerve carries electrical signals from the retina to the brain.
The brain interprets these signals, allowing us to perceive objects in
an upright form.
Camera Analogy:
The crystalline lens acts like the camera lens, the retina acts like the
camera screen, and the brain processes the received image.
2. Power of Accommodation
Accommodation is the ability of the eye lens to adjust its
focal length with the help of the ciliary muscles.
Distant Vision
Ciliary muscles relax.
The lens becomes thin.
Curvature of the lens decreases.
Focal length increases.
Faraway objects are focused clearly.
Nearby Vision
Ciliary muscles contract.
The lens becomes thicker.
Curvature of the lens increases.
Focal length decreases.
Nearby objects are focused clearly.
Limits of Vision
Near Point:
The minimum distance at which an object can be seen clearly and comfortably
without strain. For a normal young adult, it is approximately
25 cm.
Far Point:
The farthest point up to which an object can be seen clearly.
For a normal eye, it is infinity.
The ability of the eye to change its focal length is possible because of
the flexibility of the crystalline lens and the action of the ciliary muscles.
3. Defects of Vision and Their Correction
Refractive defects occur when the eye loses its normal power of accommodation,
causing images to form at incorrect positions and resulting in blurred vision.
Myopia — Near-sightedness
Condition: Nearby objects are clear, but distant objects appear blurred.
Cause:
The image forms in front of the retina because of excessive curvature of
the eye lens or an elongated eyeball.
Correction:
A concave lens of suitable power is used to bring the image back onto the retina.
Myopia is corrected using a concave lens.
Hypermetropia — Far-sightedness
Condition:
Distant objects are clear, but nearby objects appear blurred.
The near point is farther than 25 cm.
Cause:
The image forms behind the retina because the focal length is too long
or the eyeball is too short.
Correction:
A convex lens provides the additional focusing power required.
Hypermetropia is corrected using a convex lens.
Presbyopia
Presbyopia is an age-related defect of vision.
The near point recedes due to weakening of the ciliary muscles and
reduced flexibility of the eye lens.
It is commonly corrected using bifocal lenses.
Upper Portion
Concave lens for distant vision.
Lower Portion
Convex lens for nearby vision.
Cataract
In old age, the crystalline lens may become milky and cloudy.
This condition is called cataract.
It can lead to partial or complete loss of vision.
Vision can generally be restored through cataract surgery.
Comparison of Vision Defects
Defect
Clear Vision
Image Formation
Cause
Correction
Myopia
Nearby objects
In front of retina
Long eyeball or excessive lens curvature
Concave lens
Hypermetropia
Distant objects
Behind retina
Short eyeball or insufficient lens power
Convex lens
Presbyopia
Difficulty seeing nearby objects
Near point moves farther away
Weak ciliary muscles and less flexible lens
Bifocal or suitable corrective lenses
4. Refraction and Dispersion through a Prism
Prism Refraction
A triangular glass prism has two triangular bases and three rectangular
lateral surfaces.
Angle of Prism (∠A):
The angle between the two lateral refracting surfaces of the prism.
Angle of Deviation (∠D):
The angle through which the emergent ray is deviated from its original path
due to refraction through the prism.
Dispersion of White Light
Dispersion is the splitting of white light into its component
colours when it passes through a prism.
The seven colours are represented by the word:
VioletIndigoBlueGreenYellowOrangeRed
Different colours bend through different angles.
Red light bends the least.
Violet light bends the most.
Newton’s Experiment
Isaac Newton used an inverted second prism to recombine the dispersed
colours into white light.
Newton’s experiment proved that sunlight is composed of seven colours
and that a prism does not create colours; it only separates them.
Rainbow Formation
A rainbow is a natural spectrum formed due to the combined effects of:
Dispersion
Refraction
Internal reflection
Raindrops act as tiny prisms that split sunlight into its component colours.
A rainbow is always formed in the direction
opposite to the Sun.
5. Atmospheric Refraction Phenomena
Atmospheric refraction is the bending of light as it passes through layers
of the Earth's atmosphere having different refractive indices.
Twinkling of Stars
Starlight undergoes continuous refraction while passing through the atmosphere.
Stars are point-sized sources of light.
Their apparent position and brightness keep changing due to changing
atmospheric conditions.
This fluctuation in brightness is called twinkling.
Why Planets Do Not Twinkle
Planets are extended sources of light.
They appear as collections of many point-sized sources.
The fluctuations from different points average out.
Therefore, planets generally do not twinkle.
Advance Sunrise and Delayed Sunset
Due to atmospheric refraction, the Sun is visible approximately:
2 minutes before actual sunrise.
2 minutes after actual sunset.
The atmosphere bends sunlight around the horizon, allowing us to see the
Sun even when it is technically below the horizon.
6. Scattering of Light
Tyndall Effect
The scattering of light by colloidal particles such as dust, smoke,
and water droplets is called the Tyndall Effect.
It makes the path of a beam of light visible.
Why Does the Sky Appear Blue?
Fine particles in the atmosphere scatter shorter wavelengths more strongly.
Blue light has a shorter wavelength than red light.
Therefore, blue light is scattered more effectively in all directions.
This makes the sky appear blue.
If Earth had no atmosphere, there would be no scattering of sunlight
and the sky would appear dark.
Why Are Danger Signals Red?
Red light has the longest wavelength among visible colours.
It is scattered the least by fog, smoke, and dust.
Hence, red light remains visible from a greater distance.
Why Does the Sun Appear Reddish at Sunrise and Sunset?
During sunrise and sunset, sunlight travels through a thicker layer
of the atmosphere.
Most of the blue light is scattered away before reaching our eyes.
The remaining light is rich in red and orange colours.
Therefore, the Sun appears reddish.
Phenomenon
Reason
Blue sky
Greater scattering of shorter blue wavelengths
Red danger signals
Red light is scattered the least
Reddish sunrise and sunset
Blue light is scattered away through a longer atmospheric path
Tyndall Effect
Scattering by colloidal particles
7. Eye Donation — Think It Over
Eye donation is a noble act that can help restore vision to people suffering
from corneal blindness.
Important Facts
Timeframe:
Eyes should be removed within approximately 4–6 hours
after death.
Eligibility:
People of any age or sex can donate their eyes. Even people with
diabetes or hypertension may donate.
Ineligibility:
People suffering from certain communicable diseases such as
AIDS, rabies, or tetanus cannot donate.
Impact:
One pair of donated eyes can help restore vision to up to
four corneal-blind people.
Eye donation does not restore the complete eyeball to another person.
The donated corneas are used to restore vision in people with corneal blindness.