All Free Physics MCQs with Answers

231. The wavelength of the photon emitted in a transition depends on

• A. the difference in energy between the two levels
• B. the sum of the two energy levels
• C. the number of electrons in the atom only
• D. the temperature of the gas

Explanation: The photon carries away exactly the energy difference, and since energy is hc over wavelength, a larger gap gives a shorter wavelength. This is why transitions to the ground state produce ultraviolet while transitions between high levels produce infrared. Temperature affects how many atoms are excited, not the wavelengths available.

232. A limitation of the Bohr model is that it

• A. cannot explain the hydrogen spectrum
• B. works well only for hydrogen and other one electron systems
• C. assumes energy is continuous
• D. does not use quantisation

Explanation: The model reproduces the hydrogen spectrum beautifully but fails for atoms with more than one electron, and it cannot account for the relative intensities of lines or their splitting in a magnetic field. It also mixes classical orbits with quantum postulates in a way that is not self consistent. Quantum mechanics replaced the orbits with probability distributions and resolved these failures.

233. In a laser, light is produced by

• A. spontaneous emission only
• B. stimulated emission, in which a passing photon triggers an identical photon from an excited atom
• C. heating a filament
• D. the photoelectric effect

Explanation: The triggered photon matches the original in frequency, phase and direction, so the beam is coherent and highly directional, which is what distinguishes laser light from that of a lamp. Achieving it requires a population inversion, with more atoms excited than in the ground state, maintained by pumping. Metastable states are needed so that atoms remain excited long enough.

234. Laser light differs from ordinary light mainly in that it is

• A. faster
• B. monochromatic, coherent and highly directional
• C. always more intense at every wavelength
• D. invisible

Explanation: A single transition supplies essentially one wavelength, stimulated emission keeps the waves in phase, and the resonant cavity makes the beam nearly parallel, which is why it can be focused to a very small spot. All light travels at the same speed in a vacuum, so speed is never the difference. These properties are what make lasers useful in surgery, fibre optics and CD players.

235. The characteristic X ray lines of an element are produced when

• A. an outer electron is excited
• B. an inner shell electron is knocked out and an outer electron falls in to fill the vacancy
• C. the nucleus decays
• D. electrons are slowed by the target

Explanation: The energy gaps between inner shells are large, of the order of thousands of electron volts, so the photons emitted lie in the X ray region and their wavelengths are characteristic of the element. The continuous background, by contrast, comes from electrons being decelerated in the target. Moseley used these characteristic lines to establish atomic number as the ordering principle of the periodic table.

236. The energy levels of the hydrogen atom vary with the principal quantum number n as

• A. proportional to n
• B. proportional to n squared
• C. inversely proportional to n squared
• D. independent of n

Explanation: The energy is minus 13.6 divided by n squared electron volts, so the levels crowd closer together as n rises and converge on zero at ionisation. This convergence is directly visible in a spectrum, where the lines of a series bunch towards the series limit. It also explains why transitions between high levels emit low energy infrared photons.

237. Spectral analysis can identify the elements present in a distant star because

• A. every element has its own unique pattern of spectral lines
• B. all stars contain the same elements
• C. starlight is white
• D. the star's temperature is known in advance

Explanation: The line pattern is fixed by the element's own energy levels, so it acts as a fingerprint that survives the journey across space. Helium was in fact discovered in the Sun's spectrum before it was found on Earth, which is how it got its name. Line positions also reveal motion through the Doppler shift.

238. An electron in a hydrogen atom falls from n equals 3 to n equals 2. The energy of the emitted photon is about

• A. 1.89 eV
• B. 13.6 eV
• C. 10.2 eV
• D. 3.4 eV

Explanation: The n equals 3 level is at minus 1.51 eV and n equals 2 at minus 3.4 eV, so the difference is 1.89 eV, a red photon and the first line of the Balmer series. The value 10.2 eV belongs to the n equals 2 to n equals 1 transition, in the ultraviolet. Always subtract the two level energies rather than quoting either one alone.

239. A gas discharge tube glows when a high voltage is applied because

• A. the gas burns
• B. accelerated electrons collide with gas atoms, exciting them, and the atoms then emit light as they return to lower levels
• C. the glass becomes hot
• D. the gas is compressed

Explanation: Electrons accelerated by the field transfer energy to the atoms in collisions, lifting electrons to higher levels, and the light appears when they fall back. Neon gives its familiar red because of its particular set of energy levels, while sodium vapour gives yellow. Nothing is burnt, and the tube can run indefinitely because the gas is not consumed.

240. The series limit of a spectral series corresponds to a transition from

• A. the next level up
• B. n equals infinity, that is from a free electron
• C. the ground state
• D. a level below the final one

Explanation: As the starting level rises the lines crowd together and converge on a shortest wavelength, which corresponds to an electron falling from infinity, that is from the point of ionisation. Measuring the Lyman series limit therefore gives the ionisation energy of hydrogen directly. Beyond the limit the spectrum becomes continuous.