20. What is the photon energy emitted from the transition from n=2 to n=1 in the hydrogen atom? (A) 10.2 meV. (B) 10.2 eV. (C) 10.2 keV. (D) 10.2 MeV.

4. A point-like bug of mass m lies on the rim of a uniform disk of mass M that can rotate freely about its center. Initially the bug and disk rotate together with an angular velocity ω₀. Then the bug walks half way to the center of the disk. What is ratio of the new kinetic energy of the system to its initial kinetic energy? (A) (4m+2M)/(m+2M) (B) (m+2M)/(4m+2M) (C) (2m+4M)/(2m+M) (D) (2m+M)/(2m+4M)

20. What is the photon energy emitted from the transition from n=2 to n=1 in the hydrogen atom?
(A) 10.2 meV.
(B) 10.2 eV.
(C) 10.2 keV.
(D) 10.2 MeV.

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