Test: Class 8 General Science NCERT Based - 4 - UPSC MCQ

Iron is used to strengthen the bridges and automated vehicles. But, iron has a tendency of corrosion and rusting. Therefore, a layer of zinc is electroplated on the iron.

Hence, R is not the explanation for A; rather A is the explanation for R. The process of electroplating is used in a variety of everyday products. For example, some parts of the car, taps in the bathroom, gas burner, rim of the wheel have a layer of chromium on them. This gives them a shiny appearance and prevents corrosion.

During the development of a thunderstorm, the air currents move upward while the water droplets move downward.

These vigorous movements cause separation of charges. By a process, the positive charges collect near the upper edges of the clouds and the negative charges accumulate near the lower edges.

There is accumulation of positive charges near the ground also. When the magnitude of the accumulated charges becomes very large, the air which is normally a poor conductor of electricity, is no longer able to resist their flow. Negative and positive charges meet, producing streaks of bright light and sound. We see streaks as lightning .

The process of generation of lightning is called electric discharge. This can occur between two or more clouds; or between the clouds and earth.

Lightning Conductor is a device used to protect buildings from the effect of lightning. A metallic rod, taller than the building, is installed in the walls of the building during its construction. One end of the rod is kept out in the air and the other is buried deep in the ground. The rod provides an easy route for the transfer of electric charge to the ground.

Earthquakes arise due to the movement of plates. The boundaries of the plates are the weak zones where earthquakes are more likely to occur.

The weak zones are also known as seismic or fault zones.In India, the areas most threatened are Kashmir, Western and Central Himalayas, the whole of North-East, Rann of Kutch, Rajasthan and the Indo – Gangetic Plain. Some areas of South India also fall in the danger zone.

The power of an earthquake is expressed in terms of a magnitude on a scale called Richter scale. The Richter scale is not linear. It is logarithmic.

As it is logarithmic, each measurement from 1 to 10, marked on the Richter scale denotes a ten times amplitude and 32 times the energy on the seismograph. In fact, an increase of 2 in magnitude means 1000 times more destructive energy.

To be able to see an object, it is necessary for light from the object to reach the eyes. This light can reach the eyes through emission or reflection.Objects which emit light of their own are called luminous objects, while objects which do not emit light are called non luminous.

The phenomenon of splitting of white light into its constituent colours is called dispersion. Rainbow is a natural phenomenon caused by dispersion of light. Dispersion is not responsible for the sight of objects.

Retina contains several nerve cells. Sensations felt by the nerve cells are then transmitted to the brain through the optic nerve.

There are two kinds of cells (i) cones, which are sensitive to bright light and (ii) rods, which are sensitive to dim light. At the junction of the optic nerve and the retina, there are no sensory cells, so no vision is possible at that spot. This is called the blind spot.

The impression of an image does not vanish immediately from the retina. It persists there for about 1/16th of a second. So, if still images of a moving object are flashed on the eye at a rate faster than 16 per second, then the eye perceives this object as moving.

Eyes of a crab are quite small but they enable the crab to look all around him. Butterflies have large eyes that seem to be made up of thousands of little eyes. It can see not only the front and the sides but back as well.

The owl has a large cornea and a large pupil to allow more light in its eye. Owls can see very well at night but not during the day. Also, it has on its retina a large number of rods and only a few cones. The day birds on the other hand, have more cones and fewer rods.

The moon completes one rotation on its axis as it completes one revolution around the Earth. The day of the appearance of the full disc of the moon is called full moon day, but the time period between two consecutive full moons is a little more than 29 days. In many calendars, this is the period of a month.

The day on which the whole disc of the moon is visible is known as the full moon day. Thereafter, every night the size of the bright part of the moon appears to become thinner and thinner.

On the fifteenth day the moon is not visible. This day is known as the new moon day. The next day, only a small portion of the moon appears in the sky. This is known as the crescent moon.

Then again the moon grows larger every day. On the fifteenth day once again we get a full view of the moon. On July 21, 1969 the American astronaut Neil Armstrong landed on the moon for the first time followed by Edwin Aldrin.

Stars are present in the sky during the daytime as well, but cannot be seen due to the glare of the sun. Due to the rotation of the earth from east to west, the stars appear to be rising from the east and setting at west.

The distance between stars is expressed in light years. Hence, statement 2 is correct. It is the distance travelled by light in one year. The speed of light is about 300,000 km per second. Thus, the distance of the Sun from the Earth may be said to be about 8 light minutes.

Constellations were devised by ancient people to be able to recognise stars in the sky. The shapes of constellations resemble objects familiar to those people.

One of the most famous constellations is Ursa Major, which is also known as the Big Dipper, the Great Bear or the Saptarshi.Orion is one of the most magnificent constellations in the sky.

It also has seven or eight bright stars. Orion is also called the Hunter.

Cassiopeia looks like distorted W or M.Some constellations like the Saptarishi are not visible from some places of the Southern Hemisphere. All the stars which make up a constellation are not at the same distance. They are just in the same line of sight in the sky.

When viewed from outer space, the light reflected from the water and land on the Earth, makes it appear blue green. Earth has only one satellite, the moon.Mars appears to be red in colour, hence the name, red planet.

Mars has two small natural satellites. Saturn appears to be yellowish in color. The rings around Saturn make it the most unique in the solar system. Its density is least amongst all the planets of the solar system, even less than water.

There is a large gap in between the orbits of Mars and Jupiter. This gap is occupied by a large number of small objects that revolve around the Sun. These are called asteroids.Comets revolve around the Sun in highly elliptical orbits. Their period of revolution round the Sun is usually very long.

A comet appears generally as a bright head with a long tail. The length of the tail grows in size as it approaches the sun. The tail of a comet is always directed away from the sun.

At night, when the sky is clear and the moon is not there, bright streaks of light in the sky can be seen. These are commonly known as shooting stars, although they are not stars. They are called meteors.

A meteor is usually a small object that occasionally enters the earth’s atmosphere. At that time, it had a very high speed. The friction due to the atmosphere heats it up. It glows and evaporates quickly. That is why the bright steak lasts for a very short time.

Pesticides and weedicides used for the protection of crops, dissolve in water and are washed into water bodies from the fields. They also seep into the ground to pollute groundwater.

Excessive quantities of chemicals which get washed from the fields, act as nutrients for algae to flourish. Once these algae die, they serve as food for decomposers like bacteria.

A lot of oxygen in the water body gets used up. This results in a decrease in the oxygen level which may kill aquatic organisms. Hot water can also be a pollutant.

This is usually water from power plants and industries. It is released into the rivers. It raises the temperature of the waterbody, adversely affecting the animals and plants thriving in it.