Power of the bulb = 25 W = 25 Js-1
Wave length of light λ = 0.57 μm = 0.57 x 10-6 m
Energy of the photon, ε = hv = hc/λ
= {6.626 x 10-34 Js x 3 x 108 ms-1}/{0.57 x 10-6 m}
= 3.487 x 10-19 J
Therefore, Rate of emission = {25 Js-1}/{3.487 x 10-19 J} = 7.18 x 1019 s-1.