1
JEE Main 2019 (Online) 12th April Evening Slot
+4
-1
The electron in a hydrogen atom first jumps from the third excited state to the second excited state and subsequently to the first excited state. The ratio of the respective wavelengths, $${{{\lambda _1}} \over {{\lambda _2}}}$$, of the photons emitted in this process is :
A
$${{22} \over 5}$$
B
$${7 \over 5}$$
C
$${9 \over 7}$$
D
$${{20} \over 7}$$
2
JEE Main 2019 (Online) 12th April Evening Slot
+4
-1
Half lives of two radioactive nuclei A and B are 10 minutes and 20 minutes, respectively, If initially a sample has equal number of nuclei, then after 60 minutes, the ratio of decayed numbers of nuclei A and B will be :
A
9 : 8
B
1 : 8
C
8 : 1
D
3 : 8
3
JEE Main 2019 (Online) 12th April Morning Slot
+4
-1
An excited He+ ion emits two photons in succession, with wavelengths 108.5 nm and 30.4 nm, in making a transition to ground state. The quantum number n, corresponding to its initial excited state is (for photon of wavelength $$\lambda$$, energy $$E = {{1240\,eV} \over {\lambda (in\,nm)}}$$) :
A
n = 4
B
n = 7
C
n = 5
D
n = 6
4
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
$${1 \over {\lambda }}$$
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