Physics — Modern Physics for the JEE Main Exam

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.

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.