SPM Physics: AHA! Photoelectric Effect – Find W First!

 

In photoelectric effect problems, the work function (W) is often hidden. The exam is not testing whether you can memorize formulas, but whether you can spot W before applying the threshold wavelength equation.

Step 1: Find the Work Function

Use the photoelectric equation:

$$hf=W+E_k$$

Given:

$\mathrm{hf}=2.00eV$

,

$E_k=0.10eV$

$$W=2.00-0.10=1.90eV$$

Convert to SI:

$$W=(1.90)(1.60\times {10}^{-19})=3.04\times {10}^{-19}J$$

Step 2: Find the Threshold Wavelength

$${\lambda }_0=\frac{hc}{W}$$

$${\lambda }_0=\frac{(6.63\times {10}^{-34})(3.00\times {10}^8)}{3.04\times {10}^{-19}}=6.54\times {10}^{-7}m$$

$${\lambda }_0=654nm$$

Answer: 654 nm (Option C)

Exam trap: Don’t use photon energy directly in

${\lambda }_0=hc\mathrm{/}W$

. Always calculate W first.

AHA! W is hidden, insight reveals it. Not by sight, but by INSIGHT!

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