Question

In an experiment on photoelectric effect, the stopping potential is measured for monochromatic light beams corresponding to different wavelengths. The data collected are as follows:
Wavelength (nm): 350 400 450 500 550
Stopping potential (V): 1.45 1.00 0.66 0.38 0.16
Plot the stopping potential against inverse of wavelength (1/λ) on a graph paper and find (a) Planck's constant (b) the work function of the emitter and (c) the threshold wavelength.

Answer 1

(a)



When λ = 350, V_s = 1.45
and when λ = 400, V_s = 1
$$\begin{gathered} \therefore \frac{hc}{350}=w+1.45...\left(1\right) \\ \mathrm{and}\frac{hc}{400}=w+1...\left(2\right) \end{gathered}$$
Subtracting (2) from (1) and solving to get the value of h, we get:

h = 4.2 × 10⁻¹⁵ eV-s

(b) Now, work function,
$$\begin{gathered} w=\frac{12240}{350}-1.45=2.15e\mathrm{v} \\ \left(c\right)w=\frac{nc}{\lambda } \\ \Rightarrow {\lambda }_{\mathrm{threshold}}=\frac{hc}{w} \\ \Rightarrow {\lambda }_{\mathrm{threshold}}=\frac{1240}{1.15} \\ \Rightarrow {\lambda }_{\mathrm{threshold}}=576.8\mathrm{nm} \end{gathered}$$