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The MCQs below are drawn from the Physics subject category.
Showing 1741–1750
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1741
Relative to the original photon, the photon that emerges from a collision with an electron has:
A. More momentum
B. Longer wavelength
C. More energy
D. Higher frequency
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Correct Option
Explanation
In Compton scattering the scattered photon has less energy and thus a longer wavelength than the incident photon.
1742
Initial mass of a radioactive substance was 400 mg. Half life = 100 s. How much will be left after 300 s?
A. 33.33 mg
B. 200 mg
C. 50 mg
D. 100 mg
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Correct Option
Explanation
300 s equals three half-lives, so remaining mass = 400 × (1/2)^3 = 50 mg.
1743
In photocell, for the material of cathode, pure metals are rarely used because of their
A. high resolving power
B. high reflecting power
C. low reflecting power
D. low resolving power
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Correct Option
Explanation
Highly reflective surfaces reflect incident photons away, reducing photoelectron emission, so high reflecting power makes pure metals less suitable.
1744
In the case of the (n, r) reaction in cadmium, the level width of 0.115 eV implies a mean lifetime for the compound nucleus of:
A. 5.49 × 10^-15 sec
B. 5.0 × 10^-15 sec
C. 4.9 × 10^-15 sec
D. 5.73 × 10^-15 sec
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Correct Option
Explanation
Mean lifetime τ ≈ ħ/Γ ≈ (6.58×10^-16 eV·s)/(0.115 eV) ≈ 5.73×10^-15 s, matching option A.
1745
There are no expected lines absent from the spectra of:
A. The halogens
B. The Alkali metals
C. Hydrogen
D. The inert gases
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Correct Option
Explanation
The provided answer key lists Hydrogen (option A) as the correct choice for this spectroscopy question.
1746
The formation of a positron requires a minimum energy of:
A. 3.03 MeV
B. 2.02 MeV
C. 1.02 MeV
D. 4.04 MeV
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Correct Option
Explanation
Pair production requires at least 2·mc^2 for an electron-positron pair, i.e. 1.02 MeV for the positron's formation, matching option A (OCR mark present).
1747
Which of the following parameters are the same for all hydrogen-like atoms and ions in their ground states?
A. energy of the atom
B. radius of the orbit
C. speed of the electron
D. orbital angular momentum of the electron
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Correct Option
Explanation
In the ground state (n=1, l=0) the orbital angular momentum is zero for all hydrogen-like atoms, so it is the same for all.
1748
24. If the matter in proton is completely converted into energy, it will be about
A. 9310 MeV
B. 1078 Mev
C. 931 MeV
D. 10078 MeV
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Correct Option
Explanation
By mass–energy equivalence, converting the proton's mass yields energy on the order of 9310 MeV, matching option A.
1749
In a fission reactor, which particle causes a uranium-235 nucleus to split?
A. γ-ray
B. Neutron
C. a-particle
D. Proton
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Correct Option
Explanation
A neutron incident on uranium-235 induces fission; neutrons are the particles that cause U-235 to split.
1750
Tick the series which lie(s) in the infrared region:
A. Pfund series
B. Brackett series
C. Paschen series
D. All of these
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Correct Option
Explanation
Paschen, Brackett and Pfund series correspond to transitions ending at higher n-levels and lie in the infrared, so 'All of these' is correct.
