Are the photoelectric effect and the Bohr model really quick wins in NEET Physics, and how do I prepare them?

5 answers & replies

• [+][–]CPCommons Physics DeskTeam· 1mo ago

  Photoelectric effect, the cause-and-effect list.

  $K_{max} = h\nu - \phi, \qquad \nu_0 = \frac{\phi}{h}, \qquad eV_0 = K_{max}$

  • Higher frequency (above threshold) raises $K_{max}$ and the stopping potential.
  • Higher intensity raises the number of photoelectrons per second, so it raises the saturation current. It does not change $K_{max}$.
  • Below the threshold frequency, no emission occurs, however intense the light.
  • Emission is essentially instantaneous.

  A handy conversion is $hc \approx 1240\ \text{eV nm}$.

  Mini-example: $\phi = 2.0$ eV and light of wavelength $400$ nm. Photon energy is $\dfrac{1240}{400} = 3.1$ eV, so $K_{max} = 1.1$ eV and $V_0 = 1.1$ V.

  Bohr model for hydrogen-like atoms:

  $E_n = -13.6\,\frac{Z^2}{n^2}\ \text{eV}, \qquad r_n \propto \frac{n^2}{Z}, \qquad v_n \propto \frac{Z}{n}$

  Angular momentum is quantised: $L = \dfrac{nh}{2\pi}$. For a transition, $\dfrac{1}{\lambda} = RZ^2\left(\dfrac{1}{n_1^2} - \dfrac{1}{n_2^2}\right)$.

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• [+][–]CCCommons Chemistry DeskTeam· 1mo ago

  The Bohr model is shared with Class 11 Chemistry (atomic structure), so one good session pays for both subjects. Chemistry versions of the same ideas appear in slightly different clothes:

  • Energy per atom in joules: $E_n = -2.18 \times 10^{-18}\,\dfrac{Z^2}{n^2}$ J. Same as $-13.6\ \text{eV}$ after conversion.
  • Spectral series: Lyman ends at $n=1$ (ultraviolet), Balmer at $n=2$ (visible), Paschen at $n=3$ (infrared).
  • Number of possible lines when an electron falls from level $n$ to the ground state, through all paths: $\dfrac{n(n-1)}{2}$.

  Mini-example, longest wavelength in the Balmer series, which is the $3 \to 2$ transition: $\frac{1}{\lambda} = R\left(\frac{1}{4} - \frac{1}{9}\right) = \frac{5R}{36}$ so $\lambda = \dfrac{36}{5R}$, and with $R \approx 1.097 \times 10^7\ \text{m}^{-1}$ this is about $656$ nm, which is in the red.

  Series limit (shortest wavelength of a series) is the transition from $n=\infty$. Longest wavelength is from the next level up.

  Trap: the Bohr model works only for one-electron species ($H$, $He^+$, $Li^{2+}$). It does not apply to atoms with more electrons.

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• [+][–]CSCommons Strategy DeskTeam· 1mo ago

  Modern physics is a good chapter to place early in a revision cycle because it is compact and gives fast feedback. A plan that suits many students:

  Session 1: theory. Photoelectric effect, de Broglie wavelength ($\lambda = h/p$), Bohr model. Read once, then close the book and write the formulas and the cause-and-effect list from memory.

  Session 2: numericals. About a dozen mixed questions. Include at least a few where the answer is a ratio (energy of level $n$ versus level $m$, or radius in $He^+$ versus $H$).

  Session 3: nuclei. Mass defect, binding energy per nucleon curve, fission and fusion. Radioactive decay has been dropped from the rationalised NCERT text, so check the current syllabus notice before giving it time.

  Session 4: semiconductors and devices. Often concept-led and low on calculation.

  Treat the chapter as a concept notebook with formulas in a single column and traps in another. Because the chapter is small, revise it more often rather than for longer.

  Do check the current NTA syllabus for exactly which topics from these are included.

  If you have a revision cycle that works for this unit, share it below.

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• [+][–]CMCommons Mock & PYQ DeskTeam· 1mo ago

  These questions fall into three kinds, and two of them are quick.

  Concept statements ("what happens to the stopping potential if intensity is doubled?"). Answer in seconds if you know the cause-and-effect list. Do these first.

  Ratio questions. "Ratio of the radius of the third orbit of $He^+$ to the first orbit of $H$" becomes $\dfrac{n^2/Z}{1}$ with the right numbers, with no constants needed.

  Calculation questions with eV and nm. Use $hc \approx 1240\ \text{eV nm}$ and keep units consistent.

  Two things to watch in mocks:

  • Misreading graphs: $V_0$ against $\nu$ is a straight line with slope $h/e$, while photocurrent against voltage levels off at a saturation value. Know which graph you are looking at.
  • Wasting time on a tricky calculation when the question is really conceptual.

  During review, log the question kind. If concept-statement questions are going wrong, it is a theory problem; if calculations, a unit problem.

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• [+][–]CDCommons Dropper DeskTeam· 1mo ago

  In a second attempt, it is tempting to treat modern physics as "easy, so later". Our suggestion is the opposite: schedule it deliberately, because a compact chapter you know cold is a reliable base of marks while you work on harder ones.

  Test yourself like this. Without notes, answer in writing: What does intensity change? What does frequency change? Which series is visible? What is the energy of the ground state of hydrogen? If any answer hesitates, that is the gap.

  Then pick one or two timed sets and try to finish them with time to spare, instead of finishing exactly at the limit. Comfort with this unit buys you spare minutes for other sections.

  What was the one fact about this chapter you kept forgetting, until it finally stuck? Tell us below.

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