Std 7 Light

Question 1:

Fill in the blanks .

(a) When the beams from the headlights of  a car fall on an object in the night , the shadows called ............. and  ..................... can be seen.

(b) During a lunar eclipse the shadow of the ............. falls on the ......................... .

(c) During a solar eclipse the shadow of the .............. falls on the ................ .

(d) Various shades of colour are seen in the sky at sunrise and sunset due to ........................ .

ANSWER:

(a) When the beams from the headlights of  a car fall on an object in the night , the shadows called umbra and penumbra can be seen.

(b) During a lunar eclipse the shadow of the Earth falls on the Moon.

(c) During a solar eclipse the shadow of the Moon falls on the Earth.

(d) Various shades of colour are seen in the sky at sunrise and sunset due to scattering of sunlight.

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Question 2:

Give reasons.

(a) Space beyond the earth's atmosphere appears dark.

(b) We are able to read while sitting in the shade.

(c) We should not observe the solar eclipse with naked eyes.

ANSWER:

(a) When sunlight is scattered by the particles present in the atmosphere, the surrounding gets illuminated and appears bright. In space, there is no atmosphere i.e. no source for scattering the sunlight. Thus, as sunlight is not scattered in different directions in the space, it appears dark.

(b) Light, which is required for seeing and reading, will reach us even when we are sitting under the shade. This light comes to us upon reflecting from various objects around us.

(c) We should not observe solar eclipse with naked eyes because by doing so we might damage our eyesight. We may suffer from permanent vision impairment or serious eye injuries. Thus, we should always use some device such as binoculars while watching the sun during solar eclipse.

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Question 3:

Give some example of scattering of light that we come across in day-to-day life.

ANSWER:

Some example of scattering of light that we come across in day-to-day life are:

• Blue colour of the sky: Out of the seven components present in sunlight, blue colour is scattered the most by the particles present in the atmosphere and hence, the sky appears blue.
• Reddish colour of the sun during sunrise and sunset: At sunrise or sunset, the sun is located near the horizon of the Earth. Hence, light has to travel a long distance through the Earth’s atmosphere. At the time of sunrise or sunset, when white sunlight falls on suspended atmospheric particles, blue colour light scatters out in deep space, while red colour light scatters less, and reaches the observer on the surface of the Earth. Hence, when this less scattered red light reaches our eyes, the sun and its surroundings appear to be reddish.

• Danger signals in red colour:​ Red colour is scattered least by the atmospheric particles and it can travel longer distance as compared to other colours. Thus, it alarms a person of the potential danger from far distance.

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Question 4:

Why is the shadow of bird flying high not seen on the earth?

ANSWER:

When a bird flies high up in the sky, the darkest part of its shadow, called umbra, does not reach the Earth. Hence, the shadow of bird flying high is not seen on the Earth.

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Question 5:

Why is a penumbra not obtained from a point source?

ANSWER:

We do not obtain penumbra from a point source because all the light rays falling on the shadowing object are completely blocked by it. This is illustrated in the figure below.

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Question 6:

Answer the following questions in your own words.

(a) What is meant by scattering of light ?

(b) Does the shadow really vanish in the zero shadow condition?

(c) Will the laser beam be seen if it passes through a glass box which contains a lighted incense stick?

ANSWER:

(a) The ability of light to spread in various directions is known as scattering of light.

(b) In the zero shadow condition, the shadow does not really vanishes. In fact, it is formed just beneath the object because of which we are not able to see it.

(c) Yes, the laser beam will be seen if it passes through a glass box which contains a lighted incense stick. This is because the laser beam will be scattered by the smoke particles formed in the glass box due to burning of incense stick. Due to this scattering, the path of laser beam will become visible.

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Question 7:

Discuss and write.

(a) Write a science based paragraph on 'What if the sun did not rise?'

(b) What efforts will you make to remove the misconceptions about eclipses?

(c) Various eclipses and the conditions during the period.

ANSWER:

(a) If the Sun did not rise, the Earth will not get illuminated. Due to this, we won't be able to see objects around us if any other temporary source of light is not used. Also, we know, Sun is the main source of energy to all living beings. So, plants won't be able to prepare their food because of absence of sunlight and soon they will die. This will create imbalance in the food chain which in turn will result in the destruction of all kinds of organisms on Earth.  Also, all the production of electrical energy will get terminated because of absence of this solar energy. Thus, it is true to day that Sun is main source of our survival.

(b) To remove the misconceptions about eclipses, such as we should not eat during eclipses etc., following efforts can be made:

• Social media such as Newspapers, T.V., radio,etc. should launch some programmes and debates discussing the myths existing related to eclipses. These mediums should highlight the superstitious talks prevailing from the past and how we can remove this. These mediums should discuss why science do not support these misconceptions.
• At the school level, discussion should be held in classrooms regarding these misconceptions. They should be asked whether they have seen such thing existing in their home. If yes, they should be taught how to change the mindset of their parents and grandparents to eradicate these misconceptions.

(c) There are two types of eclipses known as

• Solar eclipse: It occurs when the Moon comes in between the Earth and the Sun and the Moon's shadow falls on the Earth. The solar eclipse occurs only on the new moon day. Two types of solar eclipse are:
  Total Solar eclipse: When the solar disc is completely covered by the Moon. 
  Partial Solar eclipse: When the solar disc is partially covered by the Moon. 

 

• Lunar eclipse: It occurs when the Earth comes in between the Moon and the Sun and the Earth's shadow falls on the Moon. The lunar eclipse occurs on the full moon night. Two types of lunar eclipse are:
  Total lunar eclipse: When the total Moon comes under the shadow of the Earth. 
  Partial lunar eclipse: When the partial Moon or some part of the Moon comes under the shadow of the Earth.

 

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Question 8:

Explain the difference:

(a) Point sources and extended sources of light.

(b) Umbra and penumbra.

ANSWER:

(a)  

Point Source of Light	Extended Source of light
It is that source of light which forms only the umbra region on the screen of the shadowed object.	It is that source of light which forms umbra as well as penumbra region on the screen of the shadowed object.
eg: A pin hole through which sunlight is entering is a point source of light	eg: Sun is an extended source of light

(b)

Umbra	Penumbra
It is the part of the shadow where all of the light from the source is blocked by the shadowing object.	It is the region around the umbra where the light from the source is partially blocked by the shadowing object.
It is formed using an extended source of light.	It is formed using point source of light.