Human Eye and Colourful World Class 10 Notes

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Introduction to the Human Eye

  • The human eye is a complex organ that allows us to perceive the world around us through the sense of sight.
  • It functions like a camera, capturing light and forming images.

Structure of the Human Eye

1. Cornea
The cornea is the transparent front surface of the eye that refracts light entering the eye.

2. Iris
The iris is the colored part of the eye that controls the amount of light entering the eye by adjusting the size of the pupil.

3. Pupil
The pupil is the opening in the center of the iris through which light enters the eye.

4. Lens
The lens is a transparent, flexible structure that focuses light onto the retina. Its focal length can be adjusted by the ciliary muscles.

5. Retina
The retina is the light-sensitive layer at the back of the eye where the image is formed.

Power Of Accomadation

  • The human eye is capable of focusing on objects at various distances. However, there is a minimum distance at which objects can be seen clearly without any strain to the eye. This distance is referred to as the least distance of distinct vision.
  • The least distance of distinct vision, also known as the near point, is the minimum distance from the eye at which an object can be viewed clearly without any strain. If an object is brought closer than this distance, the eye cannot focus on it properly, and the object appears blurred.
  • For a normal human eye, the least distance of distinct vision is typically about 25 centimeters (cm). This value can vary among individuals and may change with age, as the ability to focus on close objects decreases with age.
  • Accommodation is the ability of the eye's lens to change its shape to focus on objects at different distances. The ciliary muscles control this change in shape. When viewing objects closer than the least distance of distinct vision, the eye must exert more effort to focus, which can lead to eye strain.

Defects Of eye

Common defects of vision include myopia (nearsightedness), hypermetropia (farsightedness), and Presbyopia.


  • Myopia, commonly known as nearsightedness, is a refractive error of the eye that affects the ability to see distant objects clearly.
  • Myopia is a condition where the eye can see nearby objects clearly but has difficulty focusing on objects that are far away. In a myopic eye, distant objects appear blurred.
  • A person with this defect has the far point nearer than infinity. Such a person may see clearly upto a distance of a few metres.
  • In a myopic eye, the image of a distant object is formed in front of the retina and not at the retina itself
  • Myopia often occurs when the eyeball is elongated, or the cornea is too curved.
Correction of Myopia
Wearing glasses or contact lenses with concave lenses helps diverge the incoming light rays, allowing them to focus on the retina.


  • Hypermetropia, also known as farsightedness or long-sightedness, is a common refractive error of the eye where distant objects can be seen more clearly than close ones
  • It is a condition where the eye can see distant objects clearly but has difficulty focusing on nearby objects. In a hypermetropic eye, close objects appear blurred.
  • The near point, for the person, is farther away from the normal near point (25 cm).
  • In a Hypermetropia eye,image of a closeby object are focussed at a point behind the retina
  • Hypermetropia often occurs when the eyeball is shorter than normal, or the cornea is too flat.
Correction of Hypermetropia
Wearing glasses or contact lenses with convex lenses helps converge the incoming light rays, allowing them to focus on the retina.


A prism is a transparent optical object with flat, polished surfaces that refract light. The most common type of prism is a triangular prism, which has a triangular base and rectangular sides. When light passes through a prism, it undergoes refraction, bending, and dispersion.

Process of Refraction in a Prism

(i)Incidence of Light: A beam of white light is incident on one of the prism's surfaces at an angle to the normal.
(ii)Refraction at the First Surface: As the light enters the prism, it slows down and bends towards the normal due to the change in medium from air to glass (or another transparent material). This is the first refraction.
(iii)Traveling Through the Prism: The light travels through the prism, and its path may be affected by the angles and shape of the prism.
(iv)Refraction at the Second Surface: As the light exits the prism, it speeds up and bends away from the normal, moving from the glass into the air. This is the second refraction.
(v) This angle between incident ray and emergent ray is called the angle of deviation

Dispersion of light through prism

Dispersion of light occurs when a beam of white light passes through a medium, such as a glass prism, and is separated into its constituent colors. This separation happens because different colors of light travel at different speeds in the medium.

Steps of Dispersion (i) When a beam of white light enters a dispersive medium like a prism, it consists of various colors, each with a different wavelength.
(ii)As the light enters the medium, each color slows down by a different amount. Shorter wavelengths (such as violet) slow down more than longer wavelengths (such as red).
(iii) This difference in speed causes each color to be refracted by a different angle, leading to the separation of the colors.
(iv) As the light exits the medium, the colors spread out further, forming a spectrum of colors ranging from violet to red.
(v) The various colours seen are Violet, Indigo, Blue, Green, Yellow, Orange and Red as remember by acronym VIBGYOR
(vi) The band of the coloured components of light beam is called its spectrum

Rainbow Formation

Rainbows are formed by the dispersion of sunlight in water droplets in the air. The light is refracted, reflected, and dispersed inside the droplets, creating the colorful circular arc of a rainbow.

Atmospheric Refraction

Atmospheric Refraction is the refraction of light by atmosphere .

Twinkling of stars

The mechanism of twinkling is given below
(i)As starlight passes through the Earth's atmosphere, it is refracted multiple times due to the varying refractive indices of the different atmospheric layers.
(ii)The Earth's atmosphere is turbulent, with constant motion and mixing of air masses. This turbulence causes rapid and random changes in the refractive index along the path of the starlight.
The continuous changes in the refractive index cause the path of the starlight to bend and shift rapidly. This leads to variations in the apparent brightness and position of the star as seen from the Earth's surface.
(iii)Stars are so far away that they appear as point sources of light. Even slight changes in the path of their light can lead to noticeable fluctuations in brightness, resulting in the twinkling effect.

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