INTRODUCTION – OUR WINDOW TO THE COLOURFUL WORLD
Our eyes help us experience the beauty of the world around
us. We see colourful flowers, stars, rainbows, sunrise and sunset because of
our eyes.
Even if our eyes are closed, we may identify things using:
- Touch
- Smell
- Taste
- Sound
However, colours cannot be identified without eyes.
Therefore, among all sense organs, the human eye is extremely important.
10.1 THE HUMAN EYE
The human eye is one of the most valuable and sensitive
sense organs.
It works similarly to a camera.
|
Camera |
Human Eye |
|
Lens |
Eye Lens |
|
Film/Sensor |
Retina |
|
Aperture |
Pupil |
The eyeball is approximately:
2.3 cm in diameter
The eye forms an image on a light-sensitive screen called
the retina.
PARTS OF HUMAN EYE AND THEIR FUNCTIONS
1. Cornea
- Transparent
thin membrane at front of eye
- Light
enters through cornea
- Major
refraction occurs here
2. Iris
- Dark
muscular diaphragm
- Controls
the size of pupil
3. Pupil
- Small
opening in iris
- Controls
amount of light entering the eye
Example
In bright sunlight:
- Pupil
contracts
In dim light:
- Pupil
expands
4. Eye Lens (Crystalline Lens)
- Transparent
flexible convex lens
- Helps
focus images on retina
5. Ciliary Muscles
Functions:
- Hold
eye lens
- Change
curvature of eye lens
- Change
focal length
6. Retina
- Delicate
light-sensitive membrane
- Contains
a large number of light-sensitive cells
Functions:
- Image
formation
- Converts
light into electrical signals
Image formed on retina is:
- Real
- Inverted
7. Optic Nerve
Function:
- Carries
electrical signals from retina to brain
The brain interprets these signals and allows us to see
objects correctly.
POWER OF ACCOMMODATION
The eye lens is made up of jelly-like fibrous material.
Its curvature can be changed by ciliary muscles.
When seeing distant objects
- Ciliary
muscles relax
- Lens
becomes thin
- Focal
length increases
- Distant
objects become clear
When seeing nearby objects
- Ciliary
muscles contract
- Lens
becomes thicker
- Focal
length decreases
- Nearby
objects become clear
Definition:
Power of accommodation:
Ability of the eye lens to adjust its focal length to see nearby and distant
objects clearly.
LEAST DISTANCE OF DISTINCT VISION (NEAR POINT)
Definition:
Minimum distance at which objects can be seen clearly
without strain.
For a normal eye:
Near point = 25 cm
FAR POINT
Definition:
Maximum distance at which objects can be seen clearly.
For a normal eye:
Far point = Infinity
Thus, a normal eye sees clearly between:
25 cm → Infinity
CATARACT
Sometimes in old age:
- Eye
lens becomes cloudy or milky
- Vision
becomes partially or completely lost
This condition is called:
Cataract
Correction:
- Cataract
surgery
10.2 DEFECTS OF VISION AND THEIR CORRECTION
Due to loss of accommodation power, images become blurred.
Main defects:
- Myopia
- Hypermetropia
- Presbyopia
1. MYOPIA (Near-sightedness)
Definition:
Person sees nearby objects clearly but distant objects
appear blurred.
Cause:
- Excessive
curvature of eye lens
- Elongation
of eyeball
Image formation:
- Image
forms before retina
Correction:
- Concave
lens
Example:
A student unable to read blackboard from last row.
2. HYPERMETROPIA (Far-sightedness)
Definition:
Person sees distant objects clearly but nearby objects
appear blurred.
Cause:
- Focal
length becomes too large
- Eyeball
becomes too short
Image formation:
- Image
forms behind retina
Correction:
- Convex
lens
Example:
Person keeps newspaper farther away while reading.
3. PRESBYOPIA
Definition:
Defect caused due to ageing where accommodation power
decreases.
Cause:
- Weakening
of ciliary muscles
- Reduced
flexibility of eye lens
Effects:
- Near
point shifts farther away
- Difficulty
reading nearby objects
Correction:
- Bifocal
lenses
Bifocal lens contains:
Upper part:
- Concave
lens
- Helps
distant vision
Lower part:
- Convex
lens
- Helps
near vision
Nowadays correction can also be done by:
- Contact
lenses
- Surgery
EYE DONATION (IMPORTANT TEXTBOOK EXAMPLE)
Important facts:
- Eyes
should be removed within 4–6 hours after death
- Removal
takes 10–15 minutes
- No
disfigurement occurs
- Spectacle
users can donate eyes
- Cataract-operated
persons can donate eyes
- Diabetic
and asthma patients can donate
People with these diseases cannot donate:
- AIDS
- Hepatitis
B/C
- Rabies
- Cholera
- Meningitis
- Tetanus
- Acute
leukaemia
One pair of eyes can provide vision to up to FOUR corneal
blind people.
10.3 REFRACTION OF LIGHT THROUGH A PRISM
A prism has:
- Two
triangular bases
- Three
rectangular surfaces
Angle between refracting surfaces:
Angle of Prism (A)
Terms:
PE → Incident ray
EF → Refracted ray
FS → Emergent ray
Angles:
∠i → Angle of incidence
∠r
→ Angle of refraction
∠e
→ Angle of emergence
∠D
→ Angle of deviation
Angle of Deviation
The angle between incident ray and emergent ray is called:
Angle of deviation
10.4 DISPERSION OF WHITE LIGHT BY A PRISM
Definition:
Splitting of white light into its constituent colours.
The seven colours are:
VIBGYOR
- Violet
- Indigo
- Blue
- Green
- Yellow
- Orange
- Red
Red colour:
- Deviates
least
Violet colour:
- Deviates
most
Band of colours formed:
Spectrum
ISAAC NEWTON'S EXPERIMENT
Newton used:
- First
prism → split white light
- Second
inverted prism → recombined colours
Conclusion:
Sunlight consists of seven colours
RAINBOW FORMATION
Rainbow is:
Natural spectrum appearing after rainfall
Formation steps:
- Refraction
of sunlight
- Dispersion
- Internal
reflection
- Refraction
again
Important points:
- Water
droplets act like tiny prisms
- Rainbow
forms opposite to Sun
Example:
Rainbow seen through:
- Waterfalls
- Water
fountains
10.5 ATMOSPHERIC REFRACTION
Definition:
Refraction of light due to Earth's atmosphere.
Examples:
- Twinkling
of stars
- Advance
sunrise
- Delayed
sunset
TWINKLING OF STARS
Reason:
- Continuous
atmospheric refraction
Stars appear to twinkle because:
- They
are point-sized sources
- Atmospheric
conditions keep changing
WHY PLANETS DO NOT TWINKLE
Planets:
- Are
closer to Earth
- Act
as extended sources
Hence fluctuations average out.
ADVANCE SUNRISE AND DELAYED SUNSET
Because of atmospheric refraction:
- Sun
appears about 2 minutes earlier
- Sun
appears about 2 minutes later at sunset
The Sun also appears flattened during sunrise and sunset.
10.6 SCATTERING OF LIGHT
Scattering occurs when light interacts with particles.
Examples:
- Blue
sky
- Deep
sea colour
- Red
sunrise
- Red
sunset
TYNDALL EFFECT
Definition:
Scattering of light by colloidal particles.
Examples:
✔ Beam of sunlight entering
smoky room
✔ Sunlight through dense forest
✔ Mist scattering sunlight
Very fine particles:
- Scatter
blue light
Large particles:
- Scatter
long wavelengths
WHY IS THE SKY BLUE?
Reason:
- Atmospheric
particles scatter shorter wavelengths more strongly
Blue light:
- Short
wavelength
- Scattered
more
Hence:
Sky appears blue
If Earth had no atmosphere:
Sky would appear black
WHY ARE DANGER SIGNALS RED?
Reason:
- Red
colour scatters least
- Visible
from large distance
- Easily
seen through smoke and fog
Examples:
- Traffic
signals
- Warning
lights
- Railway
signals
