1
JEE Main 2019 (Online) 10th April Evening Slot
+4
-1
Two radioactive substances A and B have decay constants 5$$\lambda$$ and $$\lambda$$ respectively. At t = 0, a sample has the same number of the two nuclei. The time taken for the ratio of the number of nuclei to become $${\left( {{1 \over e}} \right)^2}$$ will be :
A
$${2 \over \lambda }$$
B
$${1 \over {4\lambda }}$$
C
$${1 \over {2\lambda }}$$
D
$${1 \over {\lambda }}$$
2
JEE Main 2019 (Online) 10th April Evening Slot
+4
-1
In Li+ +, electron in first Bohr orbit is excited to a level by a radiation of wavelength $$\lambda$$. When the ion gets deexcited to the ground state in all possible ways (including intermediate emissions), a total of six spectral lines are observed. What is the value of $$\lambda$$?
(Given : H = 6.63 × 10–34 Js; c = 3 × 108 ms –1)
A
10.8 nm
B
12.3 nm
C
9.4 nm
D
11.4 nm
3
JEE Main 2019 (Online) 10th April Morning Slot
+4
-1
Two radioactive materials A and B have decay constants 10$$\lambda$$ and $$\lambda$$, respectively. It initially they have the same number of nuclei, then the ratio of the number of nuclei of A to that of B will be 1/e after a time :
A
1/9$$\lambda$$
B
11/10$$\lambda$$
C
1/10$$\lambda$$
D
1/11$$\lambda$$
4
JEE Main 2019 (Online) 9th April Evening Slot
+4
-1
A He+ ion is in its first excited state. Its ionization energy is :-
A
13.60 eV
B
6.04 eV
C
48.36 eV
D
54.40 eV
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