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CBSE Class 8 Answered

A very smooth, distempered wall reflects the light falling on it, Why doesn’t a person standing in front of it, sees his or her image in it? A transparent object/ material allow the light falling on it, to pass through it. How does it make itself visible? What happens when a ray of light is incident normally on the surface of a plane mirror? The ray of light falling on a rough surface follow the laws of reflection and yet no image in obtained due to reflection from a rough surface. Justify the phenomenon. What will happen if the light sensitive cells or photoreceptors are completely missing from the retina of a person’s eye? The normal rate of flashing the till pictures in front of human eyes to project a movie film at normal speed is 24 stills per second. T o show a slow motion film, the rate of flashing stills should be more than or less than 16 stills per second? Give reason for your answer. Causing of presbyopia, a defect of eye? The Moon is seen in different shapes every night throughout a month. The life of the Sun will come to an end after roughly 5, 00,000 years from now. Why are stars visible only at night   Why is Pluto not considered a planet of our solar system after August 2006 Why does the tail of a comet always point away from the SUN? How do meteorites help in getting information about space?
Asked by polygonpatna | 17 Feb, 2018, 01:58: AM
Expert Answer
A very smooth, distempered wall reflects the light falling on it, Why doesn’t a person standing in front of it, sees his or her image in it?
 
reflection from distempered wall is diffused reflection where as reflection from plane mirror is regular reflection. in regular reflection pattern of incinent light rays will not be changed after reflection. Parallel rays will be reflected as parallel ray only. But in diffused reflection incident parallel ray will be reflected in different direction so image formation is not there in diffused reflection.
 
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A transparent object/ material allow the light falling on it, to pass through it. How does it make itself visible?
 
in transparent material, there is a partial transmission of light and partial reflection of light. Hence due to partial reflection of light we are able to see the transparent objects.
 
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What happens when a ray of light is incident normally on the surface of a plane mirror?
 
it will be reflected in the same direction normally
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The ray of light falling on a rough surface follow the laws of reflection and yet no image in obtained due to reflection from a rough surface.
Justify the phenomenon
 
This is known as scattering of light. when parallel beam of light rays incident on rough surface makes different angles of incidence for individual ray due to surface roughness. Though the reflected ray obeys the law of reflection, angle of reflection will be different for different light rays. Hence incident light is scattered in different direction without forming image.
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What will happen if the light sensitive cells or photoreceptors are completely missing from the retina of a person’s eye?
 
The person's eye will not see any object
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Causing of presbyopia, a defect of eye?
For some old age person, due to ciliary muscles becoming weak and teh eye-lens becoming inflexible (or rigid), the eye loses its power of accomodation. Presbyopia is that defect due to which an old person cannot see the nearby objects clearly due to loss of power of accomodatin  of eye.
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The Moon is seen in different shapes every night throughout a month
 
This is due to atmospheric refraction.  Atmospheric refraction is due to density difference in different regions of atmosphere. The moon shape varies due to atmospheric refraction. Atmospheric condition will not remain same throughout the month. hence moon is seen in different shapes every night throughout a month.
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Why does the tail of a comet always point away from the SUN?

The streams of dust and gas thus released form a huge, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun's radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun.
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