The human eye is one of the most valuable and sensitive sense organs in the human body. It enables us to see the wonderful world and colours around us. Structure of eye: The human eye has the following main parts: Cornea: It is the transparent spherical membrane covering the front of the eye. Light enters the eye through this membrane. Crystalline lens: The eyes lens is a convex lens made of a transparent, soft and flexible material like a jelly made of proteins. Iris: It is a dark muscular diaphragm between the cornea and the lens. It controls the size of the pupil. It is the colour of the iris that we call as the colour of the eye. Pupil: It is a small hole between the iris through which light enters the eye. In dim light, it opens up completely due to expansion of iris muscles, but in bright light it becomes very small due to contraction of iris muscles.

1. Human Eye and Colourful World
Dr.Shivakumar HP
School of Science

2. THE HUMAN EYE
The human eye is one of the most valuable and sensitive sense organs in the human
body. It enables us to see the wonderful world and colours around us.
Structure of eye:
The human eye has the following main parts:
 Cornea: It is the transparent spherical membrane covering the front of the eye.
Light enters the eye through this membrane.
 Crystalline lens: The eyes lens is a convex lens made of a transparent, soft and
flexible material like a jelly made of proteins.
 Iris: It is a dark muscular diaphragm between the cornea and the lens. It
controls the size of the pupil. It is the colour of the iris that we call as the colour
of the eye.
 Pupil: It is a small hole between the iris through which light enters the eye. In
dim light, it opens up completely due to expansion of iris muscles, but in bright
light it becomes very small due to contraction of iris muscles.

3.  Ciliary Muscles: They hold the lens in position and help in modifying the curvature of
the lens.
 Retina: It is the light sensitive surface of the eye on which the image is formed. It
contains light sensitive cells rods and cones. Rod cells respond to the intensity of light
and cone cells respond to the illumination. i.e. primary colours Number of rod cells is
greater than number of cone cells. These cells generate signals which are transmitted to
the brain through optic nerves.
 Optic Nerve: It transmits visual information from the retina to the brain.
 Sclera: It is an opaque, fibrous, protective, outer layer of an eye containing collagen
and elastic fibre. It is also known as white of the eye.
 Blind spot: It is the point at which the optic nerve leaves the eye. It contains no rods
and cones, so an image formed at this point is not sent to the brain.
 Aqueous Humour: Between the cornea and eye lens, we have a space filled with a
transparent liquid called the aqueous humour which helps the refracted light to be
focussed on retina. It also maintains intraocular pressure.

4.  Vitreous Humour: The space between eye lens, and retina is filled with another
liquid called vitreous humour.
Note
• Iris regulates the amount of light entering the eye by adjusting the size of the
pupil.
• The pupil appears black because no light is reflected by it.
• The eye ball is nearly spherical in shape with a diameter of 2.3cm.

5. Formation of an Image
An image is formed on the retina by successive refractions at the cornea, the aqueous humour, lens and
vitreous humour. It is real, diminished and inverted in nature.
The light-sensitive cells of retina get activated upon illumination and generate electrical signals. These
signals are then sent to the brain via the optic nerve. The brain interprets these signals and finally
process the information, so that we perceive objects as they are.
Colour of Objects
The rod shaped cells of retina respond to the intensity of light, i.e. the degree of brightness or darkness
but do not respond to colours. The cone cells are sensitive to the different extent of primary colours
such as red, blue and green.
Note:
 Eyesight of Bees The retinal cones of bees are sensitive to the ultraviolet light which we cannot see
with our eyes.
 Eyesight of Chicks Chicks wake up earlier in the morning than humans because their retina have
mostly cones which are very sensitive to bright light and rods are very few.

6. Terms Related to Human Eye
1) Accommodation : It is the ability or the property of the eye lens to focus both near and distant objects by adjusting its
focal length. However, the focal length cannot be decreased or increased beyond a certain limit, due to which a healthy
person cannot view clearly, If the object is held too close(i.e. less than 25cm) or too far from the eye.
The ciliary muscles help in changing the curvature of the eye lens. When muscles are relaxed, the lens becomes thin and
its focal length is increased. This enables us to see distant object clearly. While viewing nearby objects, the ciliary muscles
contract. So the lens becomes thicker and its focal length decreases.

7. 2) Power of accommodation: It is the maximum variation in power of eye lens for focussing near by or far objects,
clearly at retina. For a young adult with normal vision, the power of accommodation is about 4D. The eye loses its
power of accommodation at old age.
3) Far point of the eye: It is the farthest point up to which the eye can see clearly. It is infinity for normal eye.
4) Near point of the eye: The minimum distance, at which an object can be seen most distinctly without any strain is
called the least distance of distinct vision. For a normal eye of an adult, it is 25cm. It is also called near point of the
eye.
5) Persistence of vision: The time for which impression or sensation of an object continues in the eye is called
persistence of vision. It is about
1
16
𝑡ℎ of a second which means that the minimum time for which we should view
an object, so that its clear image is formed on retina is
1
16
𝑡ℎ of a second .
Note: when the eye is looking at the nearby objects, eye lens becomes more convex .(i.e. focal length decreases).
Why do We have Two Eyes for Vision and Not Just One?
A human being has a horizontal field of view of about 150° with one eye and of about 180 ° with two eyes. So, two
eyes give a wider field of view. The ability to detect faint objects is enhanced with two eyes.
Our eyes are separated by a few centimetres and each eye sees a slightly different image. Our brain combines the two
images into one tells us how close or far away things are. Keeping both the eyes open provides the third dimension of
depth.

8. Defects of Vision and their Correction:
The defects due to which a person cannot see the object distinctly and comfortably are called defects of vision.
The main defects of vision are:
 Myopia or near/ short sightedness
 Hypermetropia or far/long sightedness
 Presbyopia

9. MYOPIA OR NEAR /SHORT SIGHTEDNESS
In this defect, a person can see nearby object distinctly but cannot see
distance objects clearly. In this case image forms before retina.
Causes: A person with this defect has a far point nearer than infinity. This
defect arises due to the decreases in focal length of the lens because of
• Excessive curvature of eye lens
• Elongation of the eye ball
As a result , the image is formed before retina.
Remedy This defect can be corrected by using concave lens. A concave
lens of suitable power will bring back the image on retina.

10. HYPERMETROPIA OR FAR/LONG SIGHTEDNESS
In this defect , a person can see distant objects clearly
but cannot see nearby objects clearly. A person with this
defect has the near point farther away from normal near
point[25cm].In this case, the image is formed beyond
retina.
Causes: This defect arises due to following reasons
 Focal length of eye lens becomes large.
 Eyeball becomes too short, so that the image is
formed behind retina.
Remedy: This defect can be corrected by using a
convex lens of suitable power. This will bring the
image back on retina.

11. Presbyopia
It is found in old people. For most of the people, the near point gradually recedes away
with age. Sometimes, a person may suffer from both myopia and hypermetropia.
Causes This defect arises due to the following reasons
 Weakness of ciliary muscles.
 Hardening or loss of elasticity of eye lens.
Remedy: This defect can be corrected by using bifocal or varifocal lenses which consist
of both convex and concave lenses. The upper portion consist of a concave lens ( for
myopia) and lower portion consists of convex lens (for hypermetropia).
Note:
 These days, it is possible to correct the eye defects with contact lenses or through
surgical interventions.
 Contact lenses are small lenses placed in contact with the eye which is a replacement
for the traditional spectacles.
Cataract :
It is a condition in which crystalline lens of eye becomes milky and cloudy due to growth of
membrane over it. Generally occurs among people at old age. This causes partial or complete
loss of vision. It is possible to restore vision through a cataract surgery.

12. Numerical Problems
There are many numerical problems which are generally based on myopia and hypermetropia on finding the focal length and power of the lens
which are used to correct the defect. These problems are solved by using lens formula,
1
𝑓
=
1
𝑣
−
1
𝑢
Where, f= focal length of the lens (if f is + ve, then lens is convex and if f is – ve then lens is concave lens.)
v = image distance and u = object distance.
In these problems, always remember that for healthy human eye, the far point is infinity and near point is least distance of distinct vision, i.e.
25cm
1) A person cannot see the object beyond 3m distinctly. State the nature and focal length of the lens required to correct this defect of
vision.
Ans: Object distance, u= -∞
Image distance, v = -3m =-300cm (far point)
Focal length f= ?
1
𝑓
=
1
𝑣
−
1
𝑢
1
𝑓
=
1
−300
−
1
(−∞)
1
𝑓
=
1
−300
f = - 300 cm or f=-3m
According to the sign of given focal length, the lens is diverging in nature, i.e. it is a concave lens.

13. Refraction of light Through a Prism
Prism is a transparent refracting medium bounded by at least two lateral surfaces, inclined to each other at a certain
angle. It has two triangular bases and three rectangular lateral surfaces. The angle between two lateral surfaces is
called angle of prism(A).
In the diagram given above, a ray of light (incident beam) is entering from air to glass at the first surface AB. The
light ray on refraction is bent towards the normal. At the second surface AC, the light ray enters from glass to air,
so it bents away from the normal. The above diagram shows refraction through a prism.
Angle of Deviation (D)
It is the angle at which the emergent ray (extended backward) makes with the incident ray ( extended
forward). It depends upon angle of prism, i.e. ⦟A, angle of incident and angle of emergence and is given
by ⦟D = ⦟i + ⦟e - ⦟A

14. Dispersion of white light by a Glass Prism
The phenomenon of splitting of white light into its constituent colours, when it passes through a prism is called dispersion.
This band of seven colours so obtained, the VIBGYOR (V= Violet, I= Indigo, B = Blue, G = Green,
Y = Yellow, O = Orange, R = Red) is called spectrum.
Isaac newton was the first one to use a glass prism to obtain the spectrum of light.
Note : A similar band of seven colours is produced when a beam of white light from an electric bulb falls on a triangular
glass prism.

15. Cause of Dispersion
Light rays of different colours, travel with the same speed in vacuum and air but in any
other medium, they travel with different speeds and bend through different angles, which
leads to the dispersion of light.
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
i.e. wavelength ∝ Velocity ∝
1
Deviation

16. Recombination of white light
Newton showed that the reverse of dispersion of light is also possible.
He kept two prisms close to each other, one in erect position and the other in an inverted position.
The light gets dispersed when passes through the first prism.
The second prism receives all the seven coloured rays from first prism and recombines them into
original white light. This observation shows that sunlight is made up of seven colours. Any light
that gives spectrum similar to that of sunlight is called white light

17. Rainbow
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. A rainbow can also
be seen on a sunny day by looking at the sky through a waterfall or through a water fountain, with the Sun
behind you.
Note: Red colour appears on the upper side of the rainbow and violet on the lower side, in case of primary rainbow