Short Answer Type(i) Draw a ray diagram to show how the image of a distant object is formed by a refracting astronomical telescope when in normal use.
(ii) State one advantage of a reflecting astronomical telescope over a refracting astronomical telescope.
(i) What is meant by the term activity of a radioactive substance.
(ii) A radioactive sample has a half life of 1.4 x 1016 seconds and a mass number of 238. Calculate the number of disintegration/s of 1g of this sample.
(i) The activity of a radio active substance is defined as the rate of decay or the number of disintegration per second.
If we denote the activity by A
A = 
As N = N0. e-λt, therefore, the activity A at any time is given by
A = λ N0 e-λt ...(i)
and if the activity at t = 0 is A0, then
A0 = λN0 ...(ii)
From (i) and (ii)
A = Ao e-
, which shows that activity fall exponentially with time.
The unit of activity is a curie.
If 3.7 x 1010 disintegrations take place per second, the activity of a radioactive substance is said to be 1 curie.
ii)
Given,
T1/2 = 1.4 x 1016 sec.
Mass number = 238
As, 
Which of the radiations, α-rays, β-rays, or γ-rays are :
(i) easily absorbed by matter ?
(ii) similar to X-rays ?
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