Question

The figure shows a 100 W light bulb 1 meter away from my finger. If my finger tip has an area of 1 cm2 and if the wavelength of the light from the bulb is λ = 588 nm = 588 × 10−9 m, then show that the number of photons hitting my finger per second is about 1015γ/second.1 Watt of power is 1 Joule/second.Number of photons per second?

Answer 1

We are given the following information

Energy of bulb = 100 W = 100 Joules/second

Area of fingertip = 1 cm² = 0.0001 m²

Wavelength of light = 588×10⁻⁹ m

Number of photons per second = ?

Let us first convert the wavelength into energy

`E=(h\cdot c)/(\lambda)`

Where h is the plank's constant (6.626x10⁻³⁴J.s), c is the speed of light (3×10⁸ m/s) and λ is the wavelength.

`\begin{gathered} E=(6.626*10^(-34)\cdot3*10^8)/(588*10^(-9)) \\ E=3.3806*10^(-19)\; (J)/(s) \end{gathered}`