Question

(b) A Blu-ray laser has a power of 5 milliwatts (1 watt = 1 J s−1). How many photons of light are produced by the laser in 1 hour? Energy=(Power)x(time) 5x10-3 J s-1x 1 x 3600s=18.0 J

Answer 1

Answer:

The number of photons of light produced by the laser in 1 hour is

1 Photon / hour

Explanation:

Number of photons of light produced is given by

$Number of photons = \frac{Power}{Energy}$

From the question,

Power = 5 mW (milliwatts) = 5 × 10⁻³ W

Since 1 Watt = 1 Js⁻¹

Then, 5 × 10⁻³ W = 5 × 10⁻³ Js⁻¹

For the Energy,

As given from the question

Energy=(Power)x(time)

Time = 1 hour = (1 × 60 × 60) s = 3600 s

∴ Energy = 5 × 10⁻³ Js⁻¹ x 1 x 3600s

Energy =18.0 J

Now for the Number of photons produced,

$Number of photons = \frac{Power}{Energy}$

Power = 5 × 10⁻³ Js⁻¹ = 0.005 Js⁻¹

$Number of photons = \frac{0.005}{18}$

Number of photons = 2.78 × 10⁻⁴ Photons / sec

This is the number of photons produced in 1 second.

For the number of photons produced in 1 hour, we will multiply the result by 3600

(NOTE: 1 sec = $\frac{1}{3600}$ hour)

Number of photons = 2.78 × 10⁻⁴ Photons / sec

= 2.78 × 10⁻⁴ × 3600 Photons / hour

= 1.0008 Photons / hour

≅ 1 Photon / hour

Hence, the number of photons of light produced by the laser in 1 hour is

1 Photon / hour