Lesson 3: Physics: Electrodynamics and Optics

Electrodynamics and optics together form a large, formula-driven block of NEET Physics. The strategy is the same as for mechanics: identify the circuit or the ray diagram, write the governing law, and check signs and units before choosing an option.

This lesson covers electrostatics, current electricity, magnetism, electromagnetic induction and alternating current, then ray optics and wave optics. Keep the sign conventions for lenses and mirrors straight — most optics errors are sign errors, not physics errors.

Remember the standard NCERT conventions used here: for lenses, the object distance u is negative when the object is on the incident-light side, and magnification m = v/u. State your convention at the start of a calculation and stick to it.

Electrostatics and current electricity

Coulomb's law gives the force between point charges; electric field is force per unit charge, and potential is work per unit charge. A capacitor stores charge Q = CV and energy ½CV². In circuits, Ohm's law V = IR, resistors in series add directly, and resistors in parallel combine as 1/R = 1/R₁ + 1/R₂.

Worked example — Resistance, power, parallel combination and capacitor charge

A 12 V supply drives 3 A through a resistor: R = V/I = 12/3 = 4 Ω.

A 12 V supply driving 2 A delivers power P = VI = 12 × 2 = 24 W (also I²R = 4 × 6 = 24 W for a 6 Ω resistor at 2 A).

Resistors of 6 Ω and 3 Ω in parallel: 1/R = 1/6 + 1/3 = 1/2, so R = 2 Ω.

A 20 μF capacitor at 12 V stores Q = CV = 20 × 10⁻⁶ × 12 = 240 × 10⁻⁶ C = 240 μC, and energy ½ × 20 × 10⁻⁶ × 144 = 1.44 × 10⁻³ J.

For series resistors 4 Ω, 6 Ω and 10 Ω, the total is simply 4 + 6 + 10 = 20 Ω. In a series circuit the current is the same everywhere; in a parallel circuit the voltage is the same across each branch.

Magnetism, induction and alternating current

A moving charge in a magnetic field feels F = qvB sin θ, and a current-carrying wire feels F = BIL sin θ. Faraday's law says the induced emf equals the rate of change of magnetic flux, and Lenz's law fixes its direction: the induced current opposes the change that caused it.

In alternating current, the rms value of a sinusoidal current is I₀/√2, and resonance in a series LCR circuit occurs when inductive and capacitive reactances are equal, so the impedance is at its minimum and the current at its maximum. Electromagnetic waves travel at c = 3 × 10⁸ m/s in vacuum, with c = fλ.

Worked example — Frequency from wavelength

Light of wavelength 600 nm has frequency f = c/λ = 3 × 10⁸ / (600 × 10⁻⁹) = 5 × 10¹⁴ Hz.

Ray optics and wave optics

Snell's law: n₁ sin θ₁ = n₂ sin θ₂. For a lens, 1/f = 1/v − 1/u. Total internal reflection occurs when light travels from a denser to a rarer medium at an angle above the critical angle. In wave optics, Young's double-slit fringe width is β = λD/d, and a single-slit diffraction minimum occurs where the path difference condition is met.

Worked example — Lens image and refractive index

A convex lens of focal length f = 10 cm forms an image of an object at u = −30 cm: 1/v = 1/f + 1/u = 1/10 − 1/30 = 2/30, so v = 15 cm (a real image on the opposite side).

Magnification m = v/u = 15/(−30) = −0.5: the image is inverted and half the object's size.

Light passing from air into a medium with angle of incidence 60° and angle of refraction 30° gives n = sin 60°/sin 30° = (√3/2)/(1/2) = √3 ≈ 1.73.

A lens of focal length 0.25 m has power P = 1/f = 1/0.25 = 4 D; combined powers add, so +2 D and −1 D together give +1 D.

Common pitfalls

Do not mix series and parallel rules: series adds resistances and shares current; parallel shares voltage and combines as reciprocals.

Do not guess lens signs: write u with its sign, apply 1/f = 1/v − 1/u, and read magnification from m = v/u — a negative m means inverted.

NEET Preparation

Electrostatics and current electricity

Magnetism, induction and alternating current

Ray optics and wave optics

Common pitfalls

Further study

📝 Quiz