A Laser light of wavelength 660 nm is used to weld Retina detachment. If a Laser pulse of width 60 ms and power 0.5 kW is used, the approximate number of photons in the pulse are: [Take Planck's constant h = 6.62 × 10⁻³⁴ Js]

Power due to laser = Δn/Δt * hc/λ
Δn → Number of photons per pulse
Δt = 60 ms
0.5 × 10³ W = Δn/(60 × 10⁻³ s) * (6.62 × 10⁻³⁴ Js * 3 × 10⁸ m/s) / (660 × 10⁻⁹ m)
0.5 × 10³ W = Δn/(60 × 10⁻³) * 2.99 × 10⁻¹⁹ J
0.5 × 10³ W = Δn * 5 × 10¹⁵ s⁻¹ * 2.99 × 10⁻¹⁹ J
0.5 × 10³ W = Δn * 1.5 × 10⁻³
0.99 × 10²⁰ = Δn
Δn ≈ 10²⁰