1
JEE Main 2021 (Online) 26th August Morning Shift
MCQ (Single Correct Answer)
+4
-1
A particular hydrogen like ion emits radiation of frequency 2.92 $$\times$$ 1015 Hz when it makes transition from n = 3 to n = 1. The frequency in Hz of radiation emitted in transition from n = 2 to n = 1 will be :
A
0.44 $$\times$$ 1015
B
6.57 $$\times$$ 1015
C
4.38 $$\times$$ 1015
D
2.46 $$\times$$ 1015
2
JEE Main 2021 (Online) 27th July Evening Shift
MCQ (Single Correct Answer)
+4
-1
Out of Syllabus
Consider the following statements :

A. Atoms of each element emit characteristics spectrum.

B. According to Bohr's Postulate, an electron in a hydrogen atom, revolves in a certain stationary orbit.

C. The density of nuclear matter depends on the size of the nucleus.

D. A free neutron is stable but a free proton decay is possible.

E. Radioactivity is an indication of the instability of nuclei.

Choose the correct answer from the options given below :
A
A, B, C, D and E
B
A, B and E only
C
B and D only
D
A, C and E only
3
JEE Main 2021 (Online) 27th July Morning Shift
MCQ (Single Correct Answer)
+4
-1
Out of Syllabus
If 'f' denotes the ratio of the number of nuclei decayed (Nd) to the number of nuclei at t = 0 (N0) then for a collection of radioactive nuclei, the rate of change of 'f' with respect to time is given as :

[$$\lambda$$ is the radioactive decay constant]
A
$$-$$ $$\lambda$$ (1 $$-$$ e$$-$$$$\lambda$$t)
B
$$\lambda$$ (1 $$-$$ e$$-$$$$\lambda$$t)
C
$$\lambda$$e$$-$$$$\lambda$$t
D
$$-$$ $$\lambda$$e$$-$$$$\lambda$$t
4
JEE Main 2021 (Online) 25th July Morning Shift
MCQ (Single Correct Answer)
+4
-1
Out of Syllabus
Some nuclei of a radioactive material are undergoing radioactive decay. The time gap between the instances when a quarter of the nuclei have decayed and when half of the nuclei have decayed is given as :

(where $$\lambda$$ is the decay constant)
A
$${1 \over 2}{{\ln 2} \over \lambda }$$
B
$${{\ln 2} \over \lambda }$$
C
$${{2\ln 2} \over \lambda }$$
D
$${{\ln {3 \over 2}} \over \lambda }$$
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