The whole chapter in one sitting: a cartoon movie, one live experiment for each of the five lessons, the formula sheet, the traps NEET sets, eight quick questions, and your path through the lessons.
Light frees electrons only if its colour (frequency) is high enough, because it arrives in packets called photons. And electrons, the most particle-like things we know, make wave patterns. Both radiation and matter have a dual nature.
Around 1900 physics had a clean picture: light is a wave and the electron is a particle. Then two surprises broke it. Light falling on a metal knocks out electrons as if it were made of tiny bullets (the photoelectric effect), and electrons fired through a crystal spread out like ripples (matter waves). This chapter is the story of both surprises, and the start of quantum physics.
NEET questions here are short numericals and graph readings. They are some of the fastest marks in the paper, if you know which quantity depends on frequency and which on intensity.
"One line to remember the whole chapter: light can act like a particle, and a particle can act like a wave. Every formula in the next ten minutes is just that sentence written in symbols."
Free electrons roam inside a metal, but the surface holds them back. The minimum energy needed to pull one out is the work function φ₀, measured in electron volts (1 eV = 1.602 × 10⁻¹⁹ J). Pick how you give the energy and slide it past φ₀.
φ₀ belongs to the metal and its surface, not to the radiation. Three ways to pay it: heat (thermionic), a very strong electric field (field emission), or light (photoelectric emission). The lesson also covers cathode rays, Thomson's e/m and why charge is quantised.
Light falls on a sodium plate in a vacuum tube; a microammeter reads the photocurrent while we change the collector's voltage. Change the intensity and the frequency, and pin curves to compare. Watch the two numbers that matter: the saturation current and the stopping potential V₀.
Intensity decides how many electrons (saturation current ∝ intensity). Frequency decides how fast (V₀ and Kmax = eV₀ grow with ν). Below the threshold frequency ν₀: no current at all, however bright, and above it emission is instant (about 10⁻⁹ s).
The wave theory cannot explain a threshold or the missing time lag. Einstein's answer: light comes in packets of energy hν. One packet gives all its energy to one electron; φ₀ pays the exit fee and the rest is kinetic energy. Slide the wavelength and switch the metal.
Kmax = hν − φ₀ = eV₀ The V₀–ν graph is a straight line of slope h/e, the same for every metal; it cuts the ν-axis at ν₀ = φ₀/h. Millikan used it to measure h.
A photon has energy E = hν = hc/λ and momentum p = h/λ, but zero rest mass. A lamp of power P sends out N = Pλ/(hc) photons every second. Change the colour and the power, and watch the size and the number of packets.
Shortcut: E (eV) = 1240 / λ (nm). Same power, longer wavelength: more photons, each weaker. Light is a wave when it travels (interference, diffraction) and a stream of photons when it gives energy to matter.
De Broglie (1924) turned the idea round: if waves can act as particles, particles should act as waves, with λ = h/p = h/(mv). Choose a particle, give it energy, and see where its wavelength lands on the ruler of sizes.
Electron through V volts: λ = 12.27 / √V Å. Heavier or faster means shorter λ. Same V: λ ∝ 1/√(mq); same K: λ ∝ 1/√m; same p: same λ. A cricket ball's λ (~10⁻³⁴ m) is far too small to ever notice.
Energy gained by one electron through 1 V. K = qV.
Balanced fields: eE = evB. e/m = 1.76 × 10¹¹ C/kg; charge is quantised, q = ne.
V₀ in volts = Kmax in eV. Depends on ν and the metal, never on intensity.
= h(ν − ν₀) = hc(1/λ − 1/λ₀).
λ₀ (nm) = 1240 / φ₀ (eV). Emission only if ν > ν₀ (λ < λ₀).
Slope h/e for all metals; Kmax–ν slope is h. Intercept on ν-axis = ν₀.
E (eV) = 1240 / λ (nm). Zero rest mass, speed c, neutral.
Power divided by the energy of one photon.
Charge q through V: K = qV, λ = h/√(2mqV).
100 V → 1.227 Å. Answer is in ångström (10⁻¹⁰ m), not nm.
"Two numbers save time in every numerical: hc = 1240 eV nm, and 12.27 for electrons. Use them and most questions take less than a minute."
| The trap | The fix |
|---|---|
| "Brighter light gives faster electrons" | Intensity changes only the number of electrons (the current). Kmax and V₀ depend on frequency. |
| "Red light will work if it is bright enough, or if we wait" | Below ν₀ there is no emission at any intensity, and above ν₀ there is no time lag. |
| Slope of the V₀–ν graph = h | Slope of V₀–ν is h/e. Slope of the Kmax–ν graph is h. Same slope for every metal. |
| Mixing eV and joule | Put φ₀, hν and K in the same unit; 1 eV = 1.602 × 10⁻¹⁹ J. |
| "Work function depends on the light" | φ₀ is a property of the metal and its surface only. |
| 12.27/√V read as nanometres | It gives ångström: 100 V → 1.227 Å = 0.1227 nm. |
| Mass in grams in λ = h/(mv) | Convert to kg first, or λ is off by 1000. |
| Comparing λ of proton and alpha | First write what is common: same V → λ ∝ 1/√(mq); same K → λ ∝ 1/√m; same p → equal λ. |
"Before you calculate, ask one question: is the problem changing the brightness or the colour? Half the traps disappear."
Take h = 6.63 × 10⁻³⁴ J s, c = 3 × 10⁸ m/s, hc = 1240 eV nm. One question from each idea, NEET style.
Score: 0 / 8
Each lesson is a full masterclass of about an hour: a cartoon movie, live experiments, worked problems, 12 MCQs and an exam corner. Do them in order; each one uses the last.
Values used: h = 6.63 × 10⁻³⁴ J s, e = 1.602 × 10⁻¹⁹ C, c = 3 × 10⁸ m/s, hc = 1240 eV nm, mₑ = 9.11 × 10⁻³¹ kg, mₚ = 1.673 × 10⁻²⁷ kg, mα = 6.64 × 10⁻²⁷ kg. Work functions from NCERT Table 11.1. NEET weightage is the usual range in recent papers.
Follow the four steps: understand it, remember it, try it, then prove it with past papers.