NCERT Class X Science Class: Chapter –11. Human Eye and Colorful World – Part-7

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Long Answer Questions

Question 25:

Explain the structure and functioning of Human eye. How are weable to see nearby as well as distant objects?

Answer:

Cornea: The front part of the eye is covered by a transparent spherical membrane called the corona. Light enters the eye through cornea. The space behind the cornea is filled with a liquid called aqueous humour.

Iris: Just behind the cornea is a dark colored muscular diaphragm which has a small circular opening in the middle.

Pupil: Pupil is the small circular opening of iris. The pupil appears black because no light is reflected from it.

Eye Lens: The eye lens is a convex lens made of a transparent jelly-like protein aceous material. The eye lens is hard at the middle and gradually becomes soft towards the outer edges. The eye lens is held in position by ciliary muscles. The ciliary muscles help in changing the curvature and focal help length of the eye lens.

Retina: The inner back surface of the eye ball is called retina. It is a semi-transparent membrane which is light sensitive and is equivalent to the screen of a camera. The light sensitive receptors of the retina are called rods and cones. When light falls on these receptors they send electrical signals to the brain through the optic nerve. The space between the retina and eye lens is filled with another fluid called vitreous humor

Blind spot: It is a spot at which the optic nerve enters the eye and is insensitive to light and hence the name.

Working: The light coming from an object enters the eye through corona and pupil. The eye lens converges these light rays to form a reals, inverted and diminished image on the retina.

The light sensitive cells of the retina get activated with the incidence of light and generate electric signals.

These electric signals are sent to the brain by the optic nerves and the brain interprets the electrical signals in such a way that we see an image which is erect and of the same size as the object.

Before we go into the defects of vision, let us be the ophthalmologists like least distance of distinct vision, far point and power of accommodation of the eye.

Question 26:

When do we consider a person to be myopic or hypermetropic? Explain using diagrams how the defects associated with myopic and hypermetropic eye can be corrected?

Answer:

When a person is not able to see distant objects clearly but can see nearby objects clearly then he is considered to be myopic. If it is otherwise, he is hypermetropic.

The short- sightedness is corrected by using a concave lens, which diverges and shifts the image to the retina.

Image Shifts the image to the retina

Image Shifts the Image to the Retina

Image Shifts the image to the retina

Long-sightedness is corrected by using a convex lens, which converges and shifts the image to the retina beyond.

Image Shifts the image to the retina beyond

Image Shifts the Image to the Retina Beyond

Image Shifts the image to the retina beyond

Long sighted eye with correction

Question 27:

Explain the refraction of light through a triangular glass prismusing a labelled ray diagram. Hence define the angle of deviation.

Answer:

The refraction of light through a triangular glass prism is shown below

Image Refraction of light a triangular glass prism

Image Refraction of Light a Triangular Glass Prism

Image Refraction of light a triangular glass prism

A ray of light PE is entering from air to glass at the first surface AB. The light ray EF on refraction has bent towards the normal. At the second surface AC, the light ray FS has entered from glass to air and bent away from normal. The angle made by extending incident ray with the emergent ray is called angle of deviation.

Question 28:

How can we explain the reddish appearance of sun at sunrise orsunset? Why does it not appear red at noon?

Answer:

At the time of sunrise or sunset, the position of the sun is very far away from us. The sunlight travels longer distance through the atmosphere of the earth before reaching our eyes. Scattering of blue light is more than the scattering of red light. As a result of this, more red light reaches our eyes than any other color. Hence sunset and sunrise appear red.

During noon, the sun is overhead and sunlight travels less distance through the earth's atmosphere to reach our eyes. In this case, the scattering of almost all colors is very small. Hence, the sun appears white.

Image sun nesrly overhead and sun near Horizon

Image Sun Nesrly Overhead and Sun Near Horizon

Image sun nesrly overhead and sun near Horizon