Science textbook solutions for Class 7 Chapter 17 Effects of Light

Important points to remember :

1. Eclipses often occur in the solar system. As  seen from the earth, when a planet or a star passes behind  the  moon, that  state  is called  a ‘occultation’. 
2. It is a common phenomenon that occurrs  with the sun,  the moon or other stars.  
3. For  example, in 2016 the star called  ‘Rohini’  was hidden behind the  moon.
4. An eclipse  is a natural phenomenon. Many  superstitions connected  to eclipses  are prevalent in the society.

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 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.

Question 2: Give reasons.
a) Space beyond the earth’s atmosphere appears dark.
Answer : 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) We are able to read while sitting in the shade.
Answer : 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 the solar eclipse with naked eyes.
Answer : 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.

Question 3: Give some example of scattering of light that we come across in day-to-day life.
Answer :
Scattering of light :-
1) Such as sound ; light ;
Some areas where scattering and scattering theory are significant include radar sensing, medical ultrasound, semiconductor waferinspection, polymerization process monitoring, acoustic tiling, free-space communications and computer-generated imagery.

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.

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.

Question 6: Answer the following questions in your own words.
a) What is meant by scattering of light ?
Answer : Scattering occurs when light or other energy waves pass through an imperfect medium (such as air filled with particles of some sort) and are deflected from a straight path. The light is deflected off of its straight path and scatters in many directions.

b) Does the shadow really vanish in the zero shadow condition?
Answer : When the vertical object is placed in the sun around noontime, the shadow will get shorter and disappeared. It reappears in about a minute’s time. It can also be said that the shadow appears exactly under the object. Hence the shadow will not vanish.

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

Question 7: Discuss and write.


a) Write a science based paragraph on ‘What if the sun did not rise?’
Answer : If sun does not rise it implies → Earth has stop rotating. Earth rotates on its axis from west towards east at about 1675 km/hr. Every thing on the earth will be thrown east wards at a huge speed resulting in death, destruction. The atmosphere will create winds as disastrous as Atomic Blasts

b) What efforts will you make to remove the misconceptions about eclipses?
Answer : During a total solar eclipse so much of the sun is covered that a person may be tempted to stare at it directly. It is possible to suffer serious and permanent eye damage by looking at any type of solar eclipse and there is no treatment. Children are especially at risk due to more light reaching the retina than adults.

c) Various eclipses and the conditions during the period.
Answer : Up to three eclipses may occur during an eclipse season, a one- or two-month period that happens twice a year, around the time when the Sun is near the nodes of the Moon’s orbit. An eclipse does not occur every month, because one month after an eclipse the relative geometry of the Sun, Moon, and Earth has changed.

Question 8: Explain the difference:


a) Point sources and extended sources of light.
Answer : Point source is particularly localised emission or source of light at a point which decreases in proportiom to the inverse square of distance from the source while extended source are collection of such point sources of light together.

The umbra is the center part or the darkest part of the shadow. The penumbra is an extended part of the shadow that is slightly lighter than the umbra where some or all of the light sources are obscured.

b) Umbra and penumbra.
Answer : The umbra is a central cone of darkness which tapers away from the Earth or Moon, whilst the penumbra is an outer cone of partial shadow which diverges instead of tapering. The Moon’s shadow cast on the Earth appears as a solar eclipse.

# let’s recall 
1. What is a shadow ? How is a shadow formed ?
Answer : Shadows are made by blocking light. Light rays travel from a source in straight lines. If an opaque (solid) object gets in the way, it stops light rays from traveling through it. This results in an area of darkness appearing behind the object.

2. Time  periods of lunar and solar eclipse. 
Answer : Successive solar or lunar eclipses may occur 1, 5 or 6 synodic months apart. Semester. Half a lunar year. Eclipses will repeat exactly one semester apart at alternating nodes in a cycle that lasts for 8 eclipses.

3. Various  eclipses in the past  and  relevant interesting information about  them. 
Answer : Facts about solar eclipses Each year there are between 2 and 5 solar eclipses. The total solar eclipse, when the Moon completely obscures the Sun and leaves only the faint solar corona, is known as a Totality. Total solar eclipses are rare, happening only once every 18 months.

4. Eclipses and transits which will occur in the near future.
Answer : Like an eclipse, a transit occurs when one object appears to pass in front of another object. But in a transit, the apparent size of the first object is not large enough to cast the second into complete shadow. Instead a much smaller dark shadow makes its way across the face of the further planet or star.

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