Notes

Notes For Class X

Human Eye and the colourful world

 

THE HUMAN EYE :

  • The human eye is one of the most valuable and sensitive sense organs.
  • It enables us to see the wonderful world and the colours around us.

Important parts of human eye and their work are

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PART NAME

FUNCTION

Retina

l  Retina is a light sensitive screen.

l  It Captures the light rays focused by the lens and sends impulses to the brain via optic nerve.

l  It has rod cells and cone cells.

l  The eye lens forms an inverted real image of the object on the retina.

Cornea

Ø  The transparent spherical membrane covering the front of the eye.

Ø  Most of the refraction for the light rays entering the eye occurs at the outer surface of the cornea.

Iris

Iris is a dark muscular diaphragm that controls the size of the pupil.

Pupil

The pupil regulates and controls the amount of light entering the eye.

Ciliary muscles

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

Optical nerves

signals from eye are sent to the brain via the optic nerves.

 

 

Power of Accommodation:

The ability of the eye lens to adjust its focal length is called accommodation.

The human eye can clearly see a nearby object as well as an object on infinity. This ability of the human eye is called the power of accommodation of human eye.

Defects In Vision and Correction

1) Myopia

  • Myopia is also known as near-sightedness.
  • Person with myopia can see nearby objects clearly but cannot see distant objects distinctly.
  • The image of a distant object is formed in front of the retina and not at the retina itself.

CAUSE :

(i) excessive curvature of the eye lens, or

(ii) elongation of the eyeball.

CORRECTION:

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using a concave lens of suitable power.

 

2) Hypermetropia

  • Hypermetropia is also known as far-sightedness.
  • Person with hypermetropia can see distant objects clearly but cannot see nearby objects distinctly
  • The image of a distant object is formed in front of the retina and not at the retina itself.

CAUSE :

(i) the focal length of the eye lens is too long, or

(ii) the eyeball has become too small.

CORRECTION:

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using a convex lens of suitable power.

  • Presbyopia
  • generally occurs at old age.
  • Person suffering from this type of defect of vision cannot see nearby objects clearly and distinctively.

CAUSE:

  1. i) weakening of the ciliary muscles
  2. ii) reduction in the flexibility of the eye lens

CORRECTION:

Using bi-focal spectacles.

The upper portion consists of a concave lens. It facilitates distant vision. The lower part is a convex lens. It facilitates near vision.

 

DISPERSION:

It is the phenomena of splitting of white light into its constituent seven colours on passing through a Glass prism.

Visible spectrum: The band of seven colours obtained due to the dispersion of white light is called a visible spectrum (VIBGYOR).

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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.

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.

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ATMOSPHERIC REFRACTION

Refraction of light due to earth’s atmosphere ( because of difference between density of air layers ) is called atmospheric refraction .

Examples :

Twinkling of stars

Advance sunrise and delayed sunset

 

 

SCATTERING OF LIGHT :

Scattering of light takes place when the size of the scattering molecule is very very small when compared to the wavelength of light.

In earth’s atmosphere as the sun light travels through it gets scattered by a large number of molecules.

Examples:

Colour of the clear sky is blue.

White colour of Clouds

Danger signals are Red..

The sun looks reddish at the time of sun rise and sun set