1. What do you understand by 'far point' and 'near point' of a normal eye?

The farthest point from the eye which can be seen clearly is called the far point of the eye.For the normal eye it is at infinity.The nearest point up to which an eye can see the objects clearly is called the 'near point' of the eye.The near-point of a normal human eye is at a distance of 25 cm from the eye.

2. Mention the common defects of vision of the human eye.

Defects of vision :A normal eye can see objects clearly at any distance between 25 cm and infinity from the eye.Sometimes,a human eye gradually loses its power of accomodation.Then we cannot see the objects clearly.Our vision becomes defective.There are mainly four common defects of vision which can be corrected by the use of suitable eye glasses.These defects are :

   (i) Myopia or Shortsightedness
   (ii) Hypermetropia or longsightedness
   (iii) Presbyopia
   (iv) Astigmatism

3. What is astigmatism?How will you correct this defect?

An eye which cannot simultaneously focus (see distinctly) all objects or lines making different inclinations,is an astigmatic eye.Example :an astigmatic eye may see distinctly vertical lines on a graph paper but at the same time horizontal lines are hardly seen.The defect is in eyes in which either the cornea or the retina becomes aspherical ,i.e., eyes with distortions in the cornea such that its curvature is not the same everywhere.The defect is corrected by using cylindrical lenses.

4. Why the sun looks almost reddish at the sunrise and sunset ?

During sunrise or sunset,the sun is near the horizon(Horizon is the boundary between earth and the sky which we cannot seen).Therefore,the light from the sun has to travel a large distance through atmosphere than at noon when the sun is overhead.Consequently,blue colour waves are scattered away by the atmosphere due to their shorter wavelength.Red waves of longer wavelength are least scattered and reach the observer.For this reason,the sun looks reddish at the sunrise and sunset.

5. A hypermetropic person prefers to remove his spectacles,while driving.Explain.Why?

Wearing the spectacles (used for near vision),when the person looks at the incoming distant objects,the parallel rays from the objects get further converged.As a result,the image is formed in front of retina and it appears blurred.So that it does not happen,he prefers to remove his spectacles.

6. The clouds are seen white.Give reason.

The clouds are nearer the earth surface and they contain dust particles and aggregates of water molecules of a size bigger than the wavelength of visible light.Therefore,the dust particles and water molecules present in clouds scatter all colours of incident white light from sun to the same extent and hence when the scattered light reaches our eye ,the clouds are seen white.

7. What are the possible causes of shortsightedness?What corrective measure will you adopt for a myopic eye?

A myopic eye is able to see nearby objects but far off objects appear blurred.The point beyond which the objects cannot be seen clearly is called far point.

Myopia can be due to :
(i)Elongation of the length of the eye ball;when the eye ball elongates,the distance between the eyelens and the retina increases,this results in the image being focussed in front of the retina.So it is blurred.
(ii)Shortening of the focal length of the eye lens.

Correction of Myopia

In this case,the ciliary muscles of the eye become weak and are not able to increase its focal length by suitably thinning the lens.So the eyelens will have small focal length which will converge the distant rays infront of the retina.To focus these rays at the retina,the rays must be diverged suitably.This is done by using a concave lens of suitable focal length (as spectacles) in front of the eye.

8. What will be the colour of sky in the absence of atmosphere?

In the absence of any atmosphere,there will be no scattering of sunlight and the sky will appear dark.This is what astronauts actually observe at heights at 20 km above the earth where the atmosphere becomes thin.

9. How does the colour of the scattered light depend on the size of the colloidal particles?

The colour of the scattered light depends on the size of the scattering particles.Very fine particles having sizes less than the wavelength of visible light mainly scatter blue light of shorter wavelength of the visible spectrum.The particles of relatively larger size scatter light of longer wavelengths.The large particles like raindrops,dust and ice particles scatter white light.

10.Why are the danger signals red ?

The scattering of light is found to be inversely proportional to the fourth power of the wavelength of light.Thus scattering of red light is very much less than that of yellow light(therefore λ_r > λ_v).Therefore red light signals can be seen upto a longer distance.This is why danger signals are red.

11.What is meant by Tyndall Effect?

The scattering of light by particles in its path  is called Tyndall effect.

eg:-When a beam of sunlight enters a dusty room through a window,then its path becomes visible ,because the tiny dusty particles present in the air of room scatter the beam of light all around the room.

12.Define about Persistence of vision of the eye.

The phenomenon of the continuation of the impression of an image on the retina for sometime even after the light from the object is cutoff is called persistence of vision.The impression of the image retains on the retina is about (1/16)^th of a second.