Q18 of 56 Page 1

How are fossils formed? Describe, in brief, two methods of determining the age of fossils.

Fossils are defined as the dead remains of plants and animals that lived in remote past.


Dead plants and animals are decomposed by the microorganisms in presence of oxygen and moisture. The complex substances are broken into simpler substances but sometimes in absence of oxygen and moisture, the dead bodies are not decomposed completely and gets preserved in form of fossils in the rocks.


The methods through which the age of fossils can be determined as follows:


1. Relative method:


Those fossils which are present in closer layer to the surface of Earth are found to be more recent and those which are present in deep layers are found to be older.


2. Carbon dating:


Since the animals and plants contains carbon-14 atoms which are radio-active in nature. After becoming the fossils, the radioactivity of carbon-14 starts decreasing. So, the fossil age can be found by finding the radioactivity present in the fossil and in living objects and then comparing them.


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16

If we cross pure-bred tall (dominant) pea plant with pure-bred dwarf (recessive) pea plant we will get pea plants of F1generation. If we now self-cross the pea plant of F1 generation, then we obtain pea plants of F2 generation.

(a) What do the plants of F1 generation look like?


(b) State the ratio of tall plants to dwarf plants in F2 generation.


(c) State the type of plants not found in F1 generation but appeared in F2 generation, mentioning the reason for the same.

 17

List in tabular form two distinguishing features between acquired traits and inherited traits, with one example of each.

 19

Ethanol is one of the most important industrial chemicals. It is used in medicine, to synthesize many important compounds, it is an excellent solvent.

However, inspite of its benefits it causes many social problems. If a person drinks alcohol regularly, he becomes an alcoholic. Alcohol is non-toxic but it produces physiological effects disturbing brain activity. These persons are also a threat to the lives of others.


(a) Give three reasons in favour and three reasons against 'alcohol-free world'.


(b) 'Alcohol drinking should not be potrayed on media.' Give valid reasons to justify.


(c) As a student what initiative would you take in the concern of "We should condemn drinking alcohol".

 20

With the help of a ray diagram, state what is meant by refraction of light. State Snell's law for refraction of light and also express it mathematically.

The refractive index of air with respect to glass is 2/3 and the refractive index of water with respect to air is 4/3. If the speed of light in glass is 2x108 m/s, find the speed of light in (a) air, (b) water.