CBSE Guess > Papers > Question Papers > Class X > 2008 >Science > Science Theory

Giving reasons, explain the following :
(a) Element A is a non-metal.
(b) Atom of element C has a larger size than atom of element A.
(c) Element B has a valency of 1.

Ans. (a) Element A is a non-metal because it belongs to Group VII therefore it has 7 electrons in its outermost shell. It needs one electron to acquire nearest inert gas configuration and form a monovalent – veion.

(b) Atom of element C has a larger size than that of element A because C belongs to the 3rd period therefore it has three shells whereas A belongs to the 2nd period thus it has two shells.

(c) Element B has a valency of 1 because it belongs to Group I and has one valence electron. To complete its outermost shell or to acquire nearest noble gas configuration it loses one electron and forms a monovalent + ve ion.

Q. 13. Study the given diagram and answer the questions that follow it :

(i) Which defect of vision is represented in this case? Give reason for your answer.
(ii) What could be the two causes of this defect?
(iii) With the help of a diagram show how this defect can be corrected by the use of a suitable lens. 3

Sol.

1. In the given diagram, the defect of vision is Hypermetropia because in this defect of vision a person can not see the nearby objects clearly. as the image of nearby objects can not form on the retina but beyond the retina. Therefore this defect is also called far-sightedness.
2. Causes of Hypermetropia
   (a) due to low converging power of eye-lens (because of its large focal length)
   (b) due to eye-ball being too short.
3. This defect is corrected by using spectacles containing convex lenses.
   When a convex lens of suitable power is placed in front of the hypermetropic eye then the diverging rays of light coming from the nearby object are first converged by this convex lens. Due to this, the convex lens forms a virtual image of the nearby object at a point near to the hypermetropic eye. Then the hypermetropic eye can easily focus the image formed by convex lens on the retina.

Q.14. A 2 cm high candle flame is placed at a distance of 80 cm from a white screen. On placing a convex lens exactly at the mid point of the candle and the screen, a distinct image of the flame is seen on the screen. What is the focal length of the lens and the size of the candle flame image formed? Draw a ray diagram to show the formation of the image in this case.

Science 2008 Question Papers Class X
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