Exam Cheat Sheet

PH-01 NEET Exam Cheat Sheet

CORE EQUATIONS

Equation	Formula	Notes
Einstein's equation	KE_max = hν − φ	In J or eV; use same units
Stopping potential	e $V_{0}$ = hν − φ → $V_{0}$ = KE_max/e	$V_{0}$ in volts = KE_max in eV
Photon energy (fast)	E = 1240/λ(nm) eV	hc = 1240 eV·nm (memorize!)
Threshold wavelength	λ_{0} = 1240/φ(eV) nm	Derived from hc/φ
de Broglie (general)	λ = h/p = h/(mv)	p in kg·m/s
Electron through V	λ = 1.227/√V nm	ONLY for electrons!
From KE	λ = h/√(2m·KE)	Any particle
Mass ratio at same V	λ_{1}/λ_{2} = √(m_{2}/m_{1})	Same charge assumed

KEY FACTS (answer triggers)

$V_{0}$ vs intensity → UNCHANGED ( $V_{0}$ = hν − φ/e, no intensity term)
KE_max vs intensity → UNCHANGED
Photocurrent vs intensity → INCREASES (∝ intensity)
Slope of KE vs ν → h (same for ALL metals)
Slope of $V_{0}$ vs ν → h/e (same for ALL metals)
Davisson-Germer: 54 V, 50°, Ni crystal, 1927
λ_e/λ_p at same V → √(mp/me) ≈ 43
λ_e/λ_α at same V → ≈ 121

UNIT CHECKS

$[h] = \text{J·s} = \text{kg·m}^2\text{·s}^{-1} = [ML^2$T^{-1}$]$ $[\phi] = \text{J or eV} = [ML^2$T^{-2}$]$ $[V_0] = \text{V} = \text{J/C} = [ML^2$T^{-3}$ A^{-1}$][\lambda] = \text{m} = [L]$$

DECISION TREE FOR NUMERICALS

Given λ? → E = 1240/λ(nm) eV → KE_max = E − φ → $V_{0}$ = KE_max (in V)
Given ν? → E = hν → KE_max = hν − φ → $V_{0}$ = KE_max/e
Given V (accelerating) for electron? → λ = 1.227/√V nm
Given V for other particle? → λ = h/√(2mqV) [use specific m and q]
Given two λ_{1}, λ_{2} and $V_{01}$, $V_{02}$? → Subtract equations to find h, then find φ

CONSTANTS TO MEMORISE

h	e	m_e	m_p	hc
$6.63 \times 10^{-34}$ J·s	$1.6 \times 10^{-19}$ C	$9.1 \times 10^{-31}$ kg	$1.67 \times 10^{-27}$ kg	1240 eV·nm