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The human eye enables us to see objects around us when lights relect from the object and fall on eye.

The human eye is the most significant one becsuse it enables us to see the beautiful and colourful world around us.

The ability to see an object  is called vision. It is also termed as  eyesight. 

The human eye acts a camera. lense system of eye forms an image on a light-sensitive screen called the retina.

Light enters inside the eye through a thin membrane called the cornea.

Refraction for the light rays entering the eye occurs at the outer surface of the cornea in majority.

The crystalline lense only provides the finer adjustment of focal length required to focus objects on the retina.

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

Iris is a dark muscular diaphragm that help to control the size of the pupil.

The  lense of eye made an inverted real image of the object on the retina.

The retina is a very delicate membrane having enormous number of light-sensitive cells (rod or cone cells ). The light-sensitive cells get activated upon illumination and produced an electrical signals,Which goes to the brain via the optic nerves.

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Defects of vision and their orrection

When human eye  cannot focus the image of an object on the retina properly. Then the vision of a person becomes blurred and they cannot see either the distant objects or nearby objects or sometimes  both clearly and comfortably. The person is said to have a defect of vision. 

There are three kind of defdct of vision . These are

(i) myopia or near-sightedness:

Myopia is also called as near-sightedness. A person sufrring with myopia can see nearby objects clearly but He cannot see distant objects distinctly.

A person with myopic defect in his eye  has the far point nearer than  infinity.

The image of a distant object is formed before or  in front of the retina and not at the retina itself.

This kind of defect is corrected by using Concave lenses of suitable power  so that the lens will bring the image back on to the retina.

(ii) Hypermetropia or far – sightedness

Hypermetropia is also called as far-sightedness. A person sufrring with hypermetropia can see far away objects clearly but He cannot see nearer objects distinctly.

The image of a nearer object is formed in behind  the retina and not at the retina itself.

Hyper metropia is a kind of  defect of eye in which least distance of distinct vision is more than that of normal eye or 25 cm. 

This kind of defect is corrected by using  Convex lenses of suitable power so that the lens will bring the image back on to the retina.

(iii) Presbyopia.

Presbyopia is a eye defect of vision in which the human eye loses its ability to focus and making it difficult to see nearby objects.

It is a natural process related to age  and is often called as “old sight”.

Presbyopia is  as similar to hypermetropia, because in both cases the near point recedes to a distance more than 25 cm from the eye.

Refraction of light through a prism

When light ray passes through the prism, it slows down  and bends, but different wavelengths  of light rays bend at different angles.

Visible light is the part of spectrum of electromagnetic waves which have wavelength that can be detected by the human eye and the wavelength of visible light shows the color that the light appears. 

light with the longest wavelength appears as red, and light with the shortest wavelength appears  as violet and in-between  all others colors lies .

A prism, can be used to separate visible light into its seven different colors.

A prism is a pyramid-shaped object made of transparent material usually clear glass or plastic.

When light enters from air to the glass of the prism, the change in speed causes the light to change direction and bend. As different  color of light possess different  wavelenth so they they have different  speed and bend differently.

The violet  light ray  bends maximum  and red light rays least.

Dispersion of white light throgh a glass prism

The phenomenon of splitting of white light ray into its constituent colours is called dispersion of light. The band of seven colours of obtained from the splitting of white light is called the spectrum.

All the constituent colours of a white light have unifrom (same) velocity in vacuum, but when it passes through a transparent medium like glass prism, their velocity changes.

Atmoshperic refraction

The deviation of light from a straight line as it passes through the atmosphere is known as atmospheric refraction.

This happens due to by varying optical densities of diffrent layre of atmoshpere.

Following events are observed due to atmoshperic refraction.

Advance sunrise and delay sunset.

The apparent position of stars.

Formation of rainbow.

Twinkling of stars

Twinkling of stars

The starlight coming from star undergoes refraction continuously when entering the earth’s atmosphere, before it reaches the earth. This atmoshperic refraction of starlight cause twinkling of stars.

Advance sunrise and delayed sunset

Advance sunrise and delayed sunset

The Sun is visible about 2 minutes before the actual sunrise, and about 2 minutes later the actual sunset because of atmospheric refraction.

Actual sunrise means the actual crossing of the horizon by the Sun.

The time difference between actual sunset and the apparent sunset is about 2 minutes.

Scattering of light

When light from sun enters the earth's atmosphere, the light spreads in all directions by the due to strike of dust particles, free water molecules and the molecules of the gases present in the atmosphere. This phenomenon is termed scattering of light.

We see the blue colour of the sky, the colour of the rainbow, etc due to scattering of light.

The light goes in a straight line in reflection whereas in the scattering of light the light ray gets scattered into different directions by the medium through which it passes.

Tyndall Effect

The scattering of light rays by particles in a colloid or in a very fine suspension is called Tyndall Effect.

Example: 1. When a sreach light/torch is switched on in a foggy environment, the path of the light becomes visible due to Tyndall Effect effect .

Example: 2. Beam of sun light coming from holr in roof becomes visible due to Tyndall Effect effect .

Why is the colour of the clear Sky Blue?

When light rays strike with air molecules,It is scattered becuase particles size is very very small as compared to wavelength of light.

The amount of scattering is inversely proportional to 4th power of wavelenth of light accoding to Rayleigh's law.

When light coming from sun travels through earth's atmosphere, the different wavelengths of light get scattered from their path through different amounts.

Since wavelength of blue color is approximately half the wavelength of red color so blue colours light scrattered more than red color thats why sky appers blue than red.

Colour of the Sun at Sunrise and Sunset

The colour of the sun appears red at sun at sunrise and sunset becuse red color has longer wavelength which produces more intensity on observer's eye as compared to other colors.

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HUMAN EYE AND COLORFUL WORLD - CLASS 10 - Questions & answers


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