Solutions-Class 7-Science-Chapter-17-Effects of Light-Maharashtra Board

(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.

When sunlight is scattered by the particles present in the atmosphere, the surrounding gets illuminated and appears bright. In space, there is a vacuum beyond earth’s atmosphere, due to which there is no medium for scattering of sunlight. Thus, it appears dark.

Light is required for seeing and reading. The sun is an extended source of light which is far away from the earth. The sunlight casts a shadow of the object, in its path, on the earth. This shadow has no umbra, but penumbra which is nothing but the shade. There is a sufficient amount of light available to read in the shade. Hence, we are able to read while sitting in the shade.
 

The earth receives the harmful ultra-violet rays from the sun. During a solar eclipse, even if brightness of the sun is less, the ultra-violet rays reach the earth. If the solar eclipse is seen/watched with naked eyes, the ultra-violet rays directly enter our eyes and damage vision. Hence, the solar eclipse should not be observed with naked eyes. We should always use some device such as binoculars or dark glass while watching the sun during solar eclipse.

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

• A beam of light emerging from a cinema projector on a screen.
• A beam of light emerging from head lamps of a car moving through a thick fog.

• 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 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.

• 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.

The sun is an extended source of light which is extremely bigger than the earth.

When a bird flies (at relatively shorter height from the ground), there is a formation of umbra and penumbra on the ground. But, as they fly high in the sky, their umbra becomes smaller and smaller and eventually disappears. At the same time their penumbra becomes more and fainter and then disappears. As a result, the shadow of the bird flying high is not seen on the earth.

Scattering of light emerging from a point source does not take place. Thus, only dark shadow of the object placed in front of the point source is obtained. (See figure)

The ability of light to spread in various directions is known as scattering of light.
The light rays hit the molecules, dust particles and other tiny particles present in the atmosphere and get scattered. When the scattered light rays enter our eyes, we perceive the light. A beam of light, blue sky, reddish appearance of sun are all effects of scattering of light.
 

In the zero shadow condition, the shadow does not really vanishes. On the zero shadow day the sun reaches exactly over our head Thus, the shadow formed just beneath the object because of which we are not able to see it and hence appears to be vanished.

 Yes, the laser beam will be seen if it passes through a glass box which contains a lighted incense stick. This is because Scattering of light is required/necessary, for a ray of light to be visible, 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.

If the earth stops revolving, the part of the earth opposite to the sun would never face the sun. Thus, the sun would never rise at that part.

Following are its effects/consequences :

• 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.
• Animals that feed on plants would become endangered. As a result, carnivorous animals that survive on these animals would become extinct.
• The part of the earth opposite to the sun would not receive sunlight. That part would always experience darkness. Artificial sources of light would have to be used continuously. For which, a large amount of electricity would need to be generated.
• Temperature of that part of the earth would decrease extremely. Water present in the liquid state on the earth would freeze completely. This would prove to be unfavorable for the living world.
• Water from the oceans would not undergo the process of evaporation, due to which there would be no rainfall. This would cause shortage of fresh water.
• Use of solar energy would not be possible.

 Temperature of the part of earth facing the sun, would increase tremendously. This would lead to faster evaporation of water and there would be no rainfall. As a result, plant as well as animal life would come to an end.
 

We will make people understand and accept the fact that an eclipse is a natural phenomenon like rain, spectrum, seasons. For which, we will take following steps :

• We will provide the scientific information about eclipses through various Social media such as Newspapers, T.V., radio,etc. These mediums should discuss why science do not support these misconceptions.

• If an eclipse is going to take place in the near future, we will create awareness about it through advertisements.
• We will give the experience of viewing an eclipse through special goggles and telescopes.

• At the school level, discussion should be held in classrooms regarding these misconceptions.
• We will arrange a tour to observe an eclipse taking place in the region far away from us.
• We will show people how an eclipse takes place by making models of the sun, the moon and the earth.

We will convince people that the intake of food during the period of eclipse does not cause any ill effect on health.

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: It is visible from within the part of the earth where the umbra of the moon falls. The solar disc is completely covered by the moon.  Darkness spreads on the part of the earth where the shadow of the moon falls.

An annular solar eclipse : It is seen/visible from the part of the earth where a very small portion of the umbra of the moon falls. It is seen that the pat of solar disc, except the peripheral ring, is completely covered by the moon. The edge of solar disc appears like an illuminated ring.

• 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.