HUMAN EYE AND THE COLOURFUL WORLD CLASS 10 | NCERT | CBSE | SEBA |

HUMAN EYE AND THE COLOURFUL WORLD

            

                             Question and answers

Page 190

1. 

Ans: The ability of the eye lens to adjust its focal length so that for any object distances sharp images are always formed at its retina is called power of accommodation.

2. 

Ans: As the person cannot see objects clearly beyond 1.2m and is suffering from myopia the corrective lens should be a concave or diverging lens.

3.

Ans:For an adult human eye with normal vision the near point is 25cm and the far point is infinity.

If a student has difficulty reading the blackboard while sitting in the last row, he/she is suffering from myopia.

             It can be corrected by using a concave or diverging lens of suitable power.

                                   EXERCISE

Option (b)

Option (d)

Option (c)

Option (c)

Work out using equation of power, p= 1/f (use +,- symbol as given in question)

Use lens formula to work out f and then find p                (take v=-80cm, u=∞).

             [If p is +ve nature is convex, if p is -ve nature is concave]

Draw the diagram of ‘correction of hypermetropia’ given to you. 2nd part work out using lens formula to find f, then find out P (but take v=-1m or -100cm, u=-15cm).

8. 

Ans: The focal length of the normal eye can be reduced to a minimum of 25cm and not less than that. So objects upto 25cm can be seen clearly but for objects closer than 25cm images will be formed behind the retina and appear blurred.

9.

For any increase or decrease in the distance of an object from the eye, the image distance does not change but remains constant at the retina.

10. 

Light from the stars reach us as point sources. When it travels through our atmosphere it suffers multiple refractions due to the different layers of the atmosphere and this causes us to see the stars only in their apparent position. Their apparent positions also keep changing due to the chaotic movement of the atmospheric layers.  At the same time the amount of light that reaches us also fluctuates to give us the flickering effect. As a result of the change in apparent position and flickering effect of light the stars appear to twinkle. 

11. 

Planets are closer to than the stars. Light from planets reach us as a group of point sources. The change in the apparent position of a particular point source cannot be detected by our eyes because such position will lie within the same group of point sources. So the position of the planet appears to be fixed. At the same time the amount of light reaching us also remains constant. So a planet does not twinkle.

12. 

Early in the morning or late in the afternoon the sun is at  a greater distance to us than during the daytime. As sunlight travel a longer distance most of the colours get scattered. Of all the colours, red colour has the longest wavelength and it is least scattered. So only the red coloured light reaches us more prominently than the other colours. That is why the sun appears reddish early in the morning or late in the afternoon.

13. 

Astronauts are space travelers and in the space there is no atmosphere. In the absence of an atmosphere and its particles there can be no scattering of light. Thus the sky appear dark instead of blue to an astronaut.