Physics 2021 JAMB Past Questions - Question 26: If caesium has a work function of 1.9ev, calculate its threshold wave-length (assume 1ev = 1.6 × 10-¹), h = 6.6 × 10-³Js, C = 3 × 10m1s)

Correct Answer

B

Explanation

To calculate the threshold wavelength for caesium, we can use the formula:

λ = h / (E × C)

Where:
λ = threshold wavelength
h = Planck's constant = 6.6 × 10^-34 Js
E = work function = 1.9 eV = 1.9 × 1.6 × 10^-19 J
C = speed of light = 3 × 10^8 m/s

First, convert the work function from electron volts to joules:
E = 1.9 × 1.6 × 10^-19 J = 3.04 × 10^-19 J

Now, we can calculate the threshold wavelength:
λ = 6.6 × 10^-34 Js / (3.04 × 10^-19 J × 3 × 10^8 m/s)
λ ≈ 6.63 × 10^-7 m

So, the threshold wavelength for caesium is approximately 663 nanometers.