1
JEE Main 2021 (Online) 26th February Evening Shift
+4
-1
A radioactive sample is undergoing $$\alpha$$ decay. At any time t1, its activity is A and another time t2, the activity is $${A \over 5}$$. What is the average life time for the sample?
A
$${{\ln 5} \over {{t_2} - {t_1}}}$$
B
$${{\ln ({t_2} + {t_1})} \over 2}$$
C
$${{{t_1} - {t_2}} \over {\ln 5}}$$
D
$${{{t_2} - {t_1}} \over {\ln 5}}$$
2
JEE Main 2021 (Online) 26th February Morning Shift
+4
-1
If $$\lambda$$1 and $$\lambda$$2 are the wavelengths of the third member of Lyman and first member of the Paschen series respectively, then the value of $$\lambda$$1 : $$\lambda$$2 is :
A
7 : 135
B
7 : 108
C
1 : 9
D
1 : 3
3
JEE Main 2021 (Online) 25th February Evening Shift
+4
-1
An electron of mass me and a proton of mass mp = 1836 me are moving with the same speed. The ratio of their de Broglie wavelength $${{{}^\lambda electron} \over {{}^\lambda proton}}$$ will be :
A
1
B
1836
C
$${1 \over {1836}}$$
D
918
4
JEE Main 2021 (Online) 25th February Morning Shift
+4
-1
Two radioactive substances X and Y originally have N1 and N2 nuclei respectively. Half life of X is half of the half life of Y. After three half lives of Y, number of nuclei of both are equal. The ratio $${{{N_1}} \over {{N_2}}}$$ will be equal to :
A
$${1 \over 8}$$
B
$${3 \over 1}$$
C
$${1 \over 3}$$
D
$${8 \over 1}$$
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