Physics — Modern Physics for the JEE Main Exam — Indian candidates

Common failure modes

These are the patterns that cause most candidates to lose marks on this topic. Recognising them in advance is half the work.

• !Confusing threshold frequency with work function (φ = hν₀, not hν₀/e)
• !Applying Einstein's photoelectric equation without subtracting the work function
• !Misidentifying the nuclear binding energy trend — iron-56 is the most stable, not the lightest or heaviest nucleus
• !Forgetting de Broglie wavelength depends on momentum, not speed, when the particle is relativistic
• !Mixing up NPN and PNP transistor biasing configurations

Study tips

• 1Memorise Planck's constant (h = 6.626 × 10⁻³⁴ J·s) and the eV–joule conversion (1 eV = 1.6 × 10⁻¹⁹ J) — both are used in almost every modern-physics calculation.
• 2Drill Bohr model energy levels for hydrogen: Eₙ = −13.6/n² eV. JEE Main frequently asks about energy transitions.
• 3Practice half-life problems with the formula N(t) = N₀(1/2)^(t/T½) — variants appear every year.
• 4For semiconductors, understand intrinsic vs extrinsic (n-type, p-type) and the forward-bias vs reverse-bias I-V curve.
• 5Work through at least one paper-year of photoelectric problems to internalise the Einstein equation and the stopping-potential concept.
• 6For candidates in India, JEE Main test windows are typically denser in the spring; book test centres in metro cities (Delhi, Mumbai, Bengaluru, Chennai, Kolkata) early to secure preferred dates.

Sample JEE Main Physics — Modern Physics questions

These sample items mirror the format and difficulty of real JEE Main questions. Practice with thousands more on the free Koydo question bank.

1. 1

   In the photoelectric effect, the stopping potential depends on:

   • AIntensity of incident light only
   • BFrequency of incident light onlyCorrect
   • CBoth intensity and frequency
   • DNeither intensity nor frequency
   Why this answer?

   Illustrative JEE-style: Einstein's photoelectric equation gives KE_max = hν − φ = eV_stop. The stopping potential V_stop depends only on the frequency ν of the incident light, not on its intensity. Higher intensity increases the number of photoelectrons but not their maximum energy.

2. 2

   A radioactive sample has a half-life of 5 years. After 15 years, the fraction remaining is:

   • A1/8Correct
   • B1/4
   • C1/3
   • D1/16
   Why this answer?

   Illustrative JEE-style: 15 years = 3 half-lives. The fraction remaining = (1/2)³ = 1/8.

3. 3

   The de Broglie wavelength of an electron accelerated through a potential difference V is proportional to:

   • AV
   • B1/√VCorrect
   • C√V
   • D1/V
   Why this answer?

   Illustrative JEE-style: KE = eV, so momentum p = √(2meV). de Broglie wavelength λ = h/p = h/√(2meV) ∝ 1/√V.

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