Physics — Optics & Modern Physics for the NEET 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.

• !Using the wrong sign convention in the mirror equation — NEET uses the New Cartesian sign convention (object always to the left, distances measured from the pole)
• !Confusing converging (concave) mirrors with converging (convex) lenses — their focal-length sign conventions differ
• !Forgetting that interference requires coherent sources — two independent sources cannot produce a stable interference pattern
• !Misapplying Einstein's photoelectric equation — using ν (frequency) where λ (wavelength) or hc/λ is needed
• !Forgetting the nuclear binding energy per nucleon trend — peaks around iron-56, not at the lightest or heaviest nuclei

Study tips

• 1Memorise the mirror and lens equations in the New Cartesian sign convention: 1/v + 1/u = 1/f (mirrors: v image, u object, f focal length; same form for thin lenses).
• 2For wave optics, know the conditions for constructive and destructive interference: path difference = nλ (constructive) and (n + ½)λ (destructive).
• 3Drill Young's double slit experiment: fringe width β = λD/d. NEET tests direct substitution into this formula.
• 4For Bohr model: memorise En = −13.6/n² eV for hydrogen. Energy-level transition questions are near-guaranteed.
• 5Semiconductor: know the difference between n-type (donor impurities, majority carriers are electrons) and p-type (acceptor impurities, majority carriers are holes). Zener diode as voltage regulator and solar cell are NCERT topics.

Sample NEET Physics — Optics & Modern Physics questions

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

1. 1

   An object is placed 30 cm in front of a concave mirror of focal length 10 cm. The image distance is:

   • A15 cm
   • B−15 cmCorrect
   • C20 cm
   • D−20 cm
   Why this answer?

   Using the mirror equation 1/v + 1/u = 1/f with New Cartesian convention: u = −30 cm (object in front of mirror), f = −10 cm (concave). 1/v = 1/f − 1/u = −1/10 − (−1/30) = −1/10 + 1/30 = −3/30 + 1/30 = −2/30. v = −15 cm. The image is 15 cm in front of the mirror (real).

2. 2

   In Young's double-slit experiment, fringe width is directly proportional to:

   • ADistance between the slits
   • BWavelength of lightCorrect
   • CSquare of the wavelength
   • DSquare root of the screen distance
   Why this answer?

   Fringe width β = λD/d. It is directly proportional to wavelength λ (for fixed D and d). Increasing wavelength increases fringe width; decreasing wavelength (or using blue light instead of red) decreases fringe width.

3. 3

   The energy of a photon of frequency ν is given by:

   • Ahν²
   • BhνCorrect
   • Ch/ν
   • Dν/h
   Why this answer?

   Planck's relation: E = hν, where h = 6.626 × 10⁻³⁴ J·s and ν is the frequency of the photon. Equivalently, E = hc/λ. This is the fundamental energy-frequency relationship in quantum mechanics.