Question

The work functions of cesium (Cs) and lithium (Li) metals are 1.9 eV and 2.5 eV, respectively. If we incident a light of wavelength 550 nm on these two metal surfaces, then photo-electric effect is possible for the case of:

• A. Li only
• B. Cs only
• C. Neither Cs nor Li
• D. Both Cs and Li

Hint:

For the photoelectric effect to occur, the energy of the incoming light must be greater than the work function of the material.

Answer 1

The Correct Option is B

The energy of the incident photon is: \[ E = \frac{1240}{\lambda} = \frac{1240}{550} \approx 2.25 \, \text{eV} \] For the photoelectric effect to occur, the energy of the photon must be greater than the work function of the metal. 

For Cs (work function = 1.9 eV): 

Since \( 2.25 \, \text{eV} > 1.9 \, \text{eV} \), photoelectric effect is possible for Cs. 

For Li (work function = 2.5 eV): 

Since \( 2.25 \, \text{eV} <2.5 \, \text{eV} \), photoelectric effect is not possible for Li. 

Thus, the answer is \( \boxed{\text{Cs only}} \).