Friday, December 26, 2014

CBSE I NCERT Topic_ Interesting Atmospheric Refraction Phenomena (Light)

1. A student sitting at the back of the classroom cannot read clearly the letters written on the blackboard. What advice will a doctor give to her?

Ans: The student is suffering from myopia (near sightedness). Doctor advises her to use a concave lens of appropriate power to correct this defect.

2. How are we able to see nearby and also the distant objects clearly?

Ans: Human eye is able to see nearby and distant objects clearly by changing the focal length of the eye lens using its power of accommodation.

3. A person needs a lens of power –4.5 D for correction of her vision.
(a) What kind of defect in vision is she suffering from?
(b) What is the focal length of the corrective lens?
(c) What is the nature of the corrective lens?

Ans: (a) Negative sign of power of lens indicates that focal length is negative that is only possible in concave lens that is used for correction of Myopia
(b) f =1/-4.5 =-2/9= – 0.22 m,
(c) The nature of the corrective lens is diverging rays of light.

4. Is the position of a star as seen by us its true position? Justify your answer.
Ans: No. light from stars undergoes atmospheric refraction which occurs due to the variation in air density.

5. Why do we see a rainbow in the sky only after rainfall?
Ans: We see a rainbow in the sky only after rainfall because the water droplets behave like prisms and disperse sunlight. due to atmospheric refraction and total internal refraction. A rainbow is always formed in a direction opposite to that of the sun. The water droplets act like small prisms. They refract and disperse the incident sunlight, then refract it internally and finally refract it again when it comes out of the raindrop.

6. Why is the colour of the clear sky blue?

Ans: Blue coloour gets scattered the maximum due to less wave velocity.

7. What is the difference in colours of the Sun observed during sunrise/sunset and noon? Give explanation for each.

Ans: During sunrise and sunset the sun appears reddish whereas at noon the sun appears white.
At sunrise and sunset the light coming from the sun has to travel a longer distance through the atmosphere to reach us. Ttherefore the blue and green components of white light are gets scattered away (removed) almost completely leaving the longer wavelength. Hence, during sunrise and sunset the sun appears reddish.
When the sun is overhead at noon, then the light coming from the sun has to travel a relatively shorter distance through the atmosphere to reach us. As a result, only a little of the blue colour of the white light is scattered (most of the blue light remains in it). Since the light coming from the overhead sun has almost all its components colours in the right proportion, therefore, the sun appears white

8. Chicken can see only in bright light. What type of cells is present in its retina?

Ans: Its retina has only rod cells and no cone cells.

9. Give reason: What will be colour of the sky in the absence of atmosphere?

Ans: In the absence of any atmosphere, there will be no scattering of sunlight and the sky will appear dark.

10. Give reason: Why are the traffic light signals (or danger signals) of red colour?

Ans: In the visible spectrum, the red color has the largest wavelength. The red colour is least scattered by fog or dust particles. Therefore, we can observe red color easily even in foggy and dusty conditions.
11. Why does the sky appear dark and black to an astronaut instead of blue?

Ans: This is because there is no atmosphere containing air in the outer space to scatter light. Since there is no scattered light, which can reach our eyes in outer space, therefore, the sky looks dark and black there. This is why the astronauts who go to outer space find the sky to be dark and black instead of blue.

12. What is Tyndall Effect? Explain with an example.

Ans. The scattering of light by particles in its path is called Tyndall Effect.
When a beam of sunlight enters a dusty room through a window then its path becomes visible to us. This is because the tiny dust particles present in the air of room scatter the beam of light all around the room. And when thus scattered light enters our eyes, we can see the beam of light. Thus, an example of Tyndall effect is the way a beam of sunlight becomes visible as it passes through dust particles in the air of a room.

13. The sun near the horizon appears flattened at the sun set and sun rise. Explain why.

Ans: - This is due to atmospheric refraction. The density and refractive index of the atmosphere decreases with altitude, so the rays from the top and bottom portion of the sun on horizon are refracted by different degrees. This causes the apparent flattening of the sun. But the rays from the sides of the sun on a horizontal plane are generally refracted by the same amount, so the sun still appears circular along the sides.  

14.Why We can see the sun about 2 minutes before the actual sunrise, and about 2 minutes after the actual sunset 

Ans: We can see the sun about 2 minutes before the actual sunrise, and about 2 minutes after the actual sunset because of atmospheric refraction. By actual sunrise, we mean the actual crossing of the horizon by the Sun.

When the sun just below the horizon its rays enters earths atmosphere and refracted towards the earth. Refracted ray that reach us making it appears the sun reaches to horizon. This is the apparent sunrise. But actual sunrise occur when sun reach to horizon. Conversely apparent sunset occur slightly latter than actual sun set . Since light from sun already below the horizon it refract through atmosphere enabling us to see apparent sun set even sun has already set.

15.What is night blindness? How is it caused and can it be corrected ?

Ans. Some persons have difficulty in seeing the objects in dim light during night. This defect of eye is called night-blindness.
Causes are :
(1) Lack of vitamin A in the food.
(2) Improper functioning of rod-shaped cells.
It is corrected by using contact lenses. Soft contact lenses allow more of oxygen to reach than hard lenses. They bend the rays of light to correct their refractive angle, which enables a sharp image to be formed at the retina. 
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