Q1.
Assertion (A): We cannot hear the sound of a bell ringing in a closed jar after most of the air is pumped out. Reason (R): Sound requires a medium to travel. Choose the correct statement: (i) Both A and R are true, but R is not the correct explanation of A. (ii) Both A and R are true, and R is the correct explanation of A. (iii) A is true, but R is false. (iv) A is false, but R is true.
Answer
(ii) Both A and R are true, and R is the correct explanation of A.
- A is true: in the bell jar experiment, once most of the air has been pumped out the bell is still seen ringing but is practically inaudible.
- R is true: sound is a mechanical wave and needs a material medium to propagate.
- R explains A: pumping out the air removes the very particles that were relaying the compressions and rarefactions. With almost no medium left, the wave cannot reach the jar wall and then your ear — so the sound is not heard. The reason is not merely a true fact placed beside A; it is the cause of A.
How to attack an assertion–reason question: check three things in order — (1) Is A true on its own? (2) Is R true on its own? (3) Does R answer the question 'why is A true?' Only if all three are yes do you choose 'R is the correct explanation'.