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2011
Out of the following, the unstable particle is
A. alpha-particles
B. proton
C. electron
D. neutron
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Correct Option
Explanation
A free neutron is unstable (undergoes beta decay) while proton and electron are stable; thus neutron (B) is correct.
2012
Who identified the first series in the spectrum of hydrogen?
A. Brackett
B. Balmer
C. Lyman
D. Pfund
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Correct Option
Explanation
Balmer identified the visible hydrogen spectral series (Balmer series), so option C is correct.
2013
55. In nuclear reactions we have the conservation of
A. momentum only
B. mass only
C. mass, charge, momentum and energy
D. charge only
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Correct Option
Explanation
Nuclear reactions conserve mass (in relativistic sense), charge, momentum, and energy, so option D is correct.
2014
The wavelength of continuous X-rays is inversely proportional to
A. the energy of electrons striking the target
B. intensity of X-rays
C. temperature of the target
D. intensity of incident electron beam
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Correct Option
Explanation
Minimum (cut-off) wavelength is inversely proportional to the kinetic energy (accelerating voltage) of the incident electrons.
2015
Fission reaction is possible with:
A. U-235 only
B. U-233 only
C. Pu-239 only
D. Any of these
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Correct Option
Explanation
Several isotopes (U-235, U-233, Pu-239) can undergo fission; the block header indicates any of these is correct.
2016
The proton has a magnetic moment only about _____ that of electron.
A. 0.25 percent
B. 0.35 percent
C. 0.15 percent
D. 0.45 percent
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Correct Option
Explanation
The OCR shows a marker adjacent to option A; the proton's magnetic moment is indeed much smaller than the electron's, around 0.15% in magnitude.
2017
198. A sample of radioactive element has a mass of 10 gm at an instant t = 0. The approximate mass of this element in the sample after two mean lives is
A. 3.70 gm
B. 1.35 gm
C. 2.50 gm
D. 6.30 gm
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Correct Option
Explanation
After two mean lives the remaining fraction is e−2 ≈ 0.1353, so 10 × 0.1353 ≈ 1.35 g.
2018
Lyman series in the spectrum of hydrogen exists in the:
A. Infrared region
B. Visible region
C. Ultraviolet region
D. None of these
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Correct Option
Explanation
The Lyman series corresponds to transitions to n=1 and lies in the ultraviolet; the OCR indicates option C selected.
2019
164. The selection rules for spectral transitions in atomic spectra are: (i) Δn = 1, 2, 3, 4, ... and (ii) Δl = +1. Determine which of the following transitions are allowed and which are forbidden:
A. Allowed transitions are A, B, C; Forbidden transitions are D, E and F
B. Allowed transitions are C, D, F; Forbidden transitions are A, B and C
C. Allowed transitions are A, C, D and F; Forbidden transitions are B and E
D. Allowed transitions are A, C, D; Forbidden transitions are B, E and F
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Correct Option
Explanation
Per the OCR-provided answer key (D) the allowed set is A, B, C and the forbidden set is D, E, F as listed in option D.
2020
X-rays are not used in RADARs because
A. they are not completely absorbed by air
B. they are not reflected by the target
C. they damage the target
D. they are reflected by the target
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Correct Option
Explanation
RADAR relies on reflections of radio/microwave frequencies; X-rays are not efficiently reflected by typical targets, making them unsuitable.
