Human Eye and the

Colorful world notes

Easy understanding notes

ST. Coaching Centre – Sourav Maity

9/24/2022
 Human Eye
The Human Eye: It is a natural optical instrument which is used to see the
objects by human beings. It is like a camera which has a lens and screen
system.

Structure of the Human Eye

The various parts of eye and their functions :

 Retina: It is a light sensitive screen inside the eye on which image is

formed. It contains rods and cones.
 Cornea: It is a thin membrane which covers the eye trail. It acts like a
lens which refracts the light entering the eye.
 Aqueous humour: It is fluid which fills the space between cornea and
eye lens.
 Eye lens: It is a convex lens made of transparent and flexible jelly like
material. Its curvature can be adjusted with the help of ciliary muscles.
 Pupil: It is a hole in the middle of iris through which light enters the
eye. It appears black because light falling on it goes into the eye and
does not come back.
 Ciliary muscles: These are the muscles which are attached to eye lens
and can modify the shape of eye lens which leads to the variation in
focal lengths.
 Iris: It controls the amount of light entering the eye by changing the
size of the pupil.
  Optical nerve: These are the nerves which take the image to the brain
in the form of electrical signals.

The human eye is roughly spherical in shape with a diameter of about 2.3
cm. It consists of a convex lens made up of living tissues.

Some of the functions are-

S.No. Human Eye Part Functions

Opens and closes in order to regulate and control the amount

1. Pupil
of light.

2. Iris Controls light level similar to the aperture of a camera.

A thin membrane which provides 67% of the eye’s focusing

3. Cornea
power.

4. Aqueous humour Provides power to the cornea.

Captures the light rays focused by the lens and sends

9. Retina
impulses to the brain via the optic nerve.

10. Optic nerve Transmits electrical signals to the brain.

 Contracts and extends in order to change the lens shape for
11. Ciliary muscles
focusing.

How Pupil works?

For Example, You would have observed that when you come out of the
cinema hall after watching the movie in the bright sunlight, your eyes get
closed. And when you entered the hall from the bright light, you won’t be
able to see and after some time you would be able to see. Here, the pupil
of an eye provides a variable aperture, whose size is controlled by iris.
(a) When the light is bright: Iris contracts the pupil, so that less light enters
the eye.
(b) When the light is dim: Iris expands the pupil, so that more light enters
the eye.
Pupil opens completely when iris is relaxed.

Persistence of Vision
It is the time for which the sensation of an object continues in the eye. It is
about 1/16th of a second.

Power of Accommodation
The ability of the eye lens to adjust its focal length accordingly as the
distances is called power of accommodation.
 Color blindness
A person having defective cone cells is not able to distinguish between the
different colors. This defect is known as Color Blindness.

Defects of Vision and their Correction

Myopia (Short-sightedness): It is a kind of defect in the human eye due to
which a person can
see near objects
clearly but he cannot
see the distant
objects clearly.
Myopia is due to-
(i) Excessive
curvature of the
cornea.
(ii) Elongation of
eyeball.
Correction: Since a concave lens has an ability to diverge incoming rays, it
is used to correct this defect of vision. The image is allowed to format the
retina by using a concave lens of suitable power as shown in the given
figure.

Hypermetropia (Long-sightedness): 

It is a kind of defect in the human eye due to which, a person can see
distant objects properly but cannot see the nearby objects clearly. It
happens due to
(i) Decrease in the power of eye lens i.e., increase in focal length of eye
lens.
(ii) Shortening of eyeball.

A Hypermetropic eye has its least distance of distinct vision greater than 25
cm.
Correction: Since a convex lens has the ability to converge incoming rays,
it can be used to correct this defect of vision, as you already have seen in
the animation. The ray diagram for the corrective measure for a
Hypermetropic eye is shown in the given figure.

Presbyopia: 

It is a kind of defect in human eye which occurs due to ageing. It happens

due to the following reasons
(i) decrease in flexibility of eye lens.
(ii) gradual weakening of ciliary muscles.
In this, a person may suffer from both myopia and Hypermetropic.

Correction: By using a bifocal lens with appropriate power. Bifocal lenses

consist of both concave and convex lens, upper position consists of the
concave lens and lower portion consists of a convex lens.

Cataract: 

Due to the membrane growth over eye lens, the eye lens becomes hazy or
even opaque. This leads to a decrease or loss of vision. This problem is
called a cataract.

Correction: It can be corrected only by surgery.

Power of the correcting convex lens

1 1 1
The Lens formula   v + u = f can be used to calculate the focal length and
hence, the power of the myopia correcting lens.
In this case,
Object distance, u = ∞
Image distance, v =
person’s far point Focal length, f =?
Hence, lens formula becomes

In case of a concave lens, the image is formed in front of the lens i.e., on
the same side of the
object.
Focal length = Far point
1
Now, power of the required lens (P) =  f ( ¿ m.)

Power of the correcting convex lens

1 1 1
Lens formula,  v + u = f  can be used to calculate focal length f and hence,
power P of the correcting convex lens, where,
Object distance, u = -25 cm, normal near point
Image distance, v = defective near point

1 1 1
Hence, the lens formula is reduced to v + 25 = f

Refraction of light through a prism:

When a ray of light is incident on a rectangular glass slab, after refracting
through the
slab, it gets
displaced
laterally. As a
result, the
emergent ray
comes out
parallel to the
incident ray.
Unlike a
rectangular
slab, the sides
of a glass
prism are
inclined at an
angle called the angle of prism.

Prism
Prism: A prism has two triangular bases and three

Angle of Deviation: The angle between the incident deviation.

Refraction of Light
 Dispersion of white light by a glass prism
The phenomenon of splitting of white light into its seven constituent colors
when it passes through a glass prism is called dispersion of white light. The
various colors seen are Violet, Indigo, Blue, Green, Yellow, Orange and
Red. The sequence of colors remembers as VIBGYOR. The band of seven
colors is called the
spectrum. The different
component color of light
bends at a different
angle with respect to
the incident angle. The
violet light bends the
least while the red
bends most.

Composition of white
light: White light
consists of seven colors i.e., violet, indigo, blue, green, yellow, orange and
red.

Monochromatic light: Light consisting of single color or wavelength is

called monochromatic light, example; sodium light.

Polychromatic light: Light consisting of more than two colors or

wavelengths is called polychromatic light, example; white light.

Recombination of white light: Newton found that when an inverted prism

is placed in the path of dispersed light then after passing through the prism,
they recombine to form white light.

Isaac Newton
He was the first, who obtained spectrum of sunlight by using glass prism.
He tried to split the spectrum of white light more by using another similar
prism, but he could
not get any more
colors.
He repeated the
experiment using
second prism in
inverted position
with respect to the
first prism. It
allowed all the
colors of spectrum
to pass through
second prism. He
found white light emerges on the other side of second prism.
He concluded that Sun is made up of seven visible colors VIBGYOR.

Rainbow
It is the spectrum of sunlight in nature. It is formed due to the dispersion of
sunlight by the tiny water droplet, present in the atmosphere.

Formation of the rainbow: The water droplets act like small prism. 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 colors reach the
observer’s eye.
Red color appears on top and violet at the bottom of rainbow.
A rainbow is always formed in a direction opposite to that of Sun.
At ‘A’ – Refraction and dispersion take place.

At ‘B’ – Internal reflection takes place.

At ‘C’ – Refraction and dispersion take place.

Atmospheric Refraction: The refraction of light caused by the Earth’s

atmosphere (having air layers of varying optical densities) is called
Atmospheric Refraction.

Appearance of Star Position: It is due to atmospheric refraction of star

light.
The temperature and density of different layer of atmosphere keeps
varying. Hence, we have different medium.
Distant star act as point source of light. When the starlight enter the Earth’s
atmosphere, it undergoes refraction continuously, due to changing
refractive index i.e. from Rarer to denser. It bends towards the normal.
Due to this, the apparent position of the star is different from actual
position. The star appear higher than its actual position.

Twinkling of stars
It is also due to atmospheric refraction.
Distant star act like a point source of light. As the beam of starlight keeps
deviating from its path, the apparent position of star keeps on changing
because physical condition of earth’s atmosphere is not stationary.
Hence, the amount of light enters our eyes fluctuate sometimes bright and

sometime dim. This is the “Twinkling effect of star”.

Q) Why stars don’t twinkle?

A) Planets are closer to earth and are seen as extended source of light
i.e. the collection of large number of point sized sources of light. Therefore
the total amount of light entering our eyes from all individual point source
will nullify the twinkling effect.

Why, the duration of day becomes approximately 4 minutes shorter if there

is no atmosphere on earth: Actual sun rise happens when it is below the
horizon in the morning. The rays of light from the sun below the horizon
reach our eyes because of refraction of light. Similarly, the sun can be seen
about few minutes after the actual sun set. Thus the duration of, day time
will increase by 4 minutes.
This is due to atmospheric refraction. Because of this sun is visible about 2
minutes earlier than actual sunrise and about 2 minutes after the actual sun
set.

Apparent flattering of the Sun’s disc at sunset and sunrise is due to

atmospheric refraction.

Scattering of light: According to Rayleigh’ Law of Scattering, the amount of

1
scattered light ∝  4  (λ = wavelength)
λ
Scattering of light decreases with increase in wavelength.

Tyndall Effect
When a beam of light strikes, the minute particle of earth’s atmosphere,
suspended particles of dust and molecule of air the path of beam become
visible. The phenomenon of scattering of light by the colloidal particle gives
rise to Tyndall Effect.
It can be observed when sunlight passes through a canopy of a dense
forest.
The color of the scattered light depends on the size of the scattering
particles.

Color of Sunrise and Sunset

While sunset and sunrise, the color of the sun and its surroundihg appear
red. During sunset and sunrise, the sun is near to horizon, and therefore,
the sunlight has to travel larger distance in atmosphere. Due to this, most
of the blue light (shorter wavelength) is scattered away by the particles.
The light of longer wavelength (red color) reaches our eye. This is why sun
appears red in color.

At noon sun appears white: At noon, the sun is overhead and sunlight
would travel
shorter distance
relatively through
the atmosphere.
Hence, at noon,
the sun appears
white as only little
of the blue and
violet colours are
scattered.
Q) Why the danger signal or sign is
made of red color?
A) Red color scatters the most when strikes the small particle of fog and
smoke because it has the maximum wavelength (visible spectrum). Hence,
from large distance also, we can see the red color clearly.

Assignment
Q1) Assertion (A): The human eye has more field of view.

Reason (R): For a normal eye, the farthest point up to which

the eye can see objects clearly are infinity.

Q2) Why does the Sun appear reddish early in the morning?

Q3)  Why there is no dispersion of light refracted through a

rectangular glass slab?

Q4)  Is the position of a star as seen by us in its true position?

Justify your answer.

Q5) How will you use two identical prisms so that a narrow beam of
white light incident on one prism emerges out of the second
prism as white light?

Q6) Draw a neat diagram to show the refraction of a light ray through
a glass prism, and label it.

Q7) Draw a ray diagram showing the dispersion through a prism

when a narrow beam of white light is incident on one of its
 refracting surfaces. Also, indicate the order of the colours of the
spectrum obtained.

Q8) How does refraction take place in the atmosphere? Why do stars
twinkle but not the planets?

Q9) How does refraction take place in the atmosphere? Why do stars
twinkle but not the planets?

Q10) Why does the Sun appear white at noon?

Q11) When a monochromic light having only one wavelength, passes

through a prism, will it show dispersion?

Q12) Demonstrate an activity with a well labeled diagram to prove that

white light is made up of seven colors.