Physics questions NEED HELP

Question

1.Making estimates. The human eye can detect as few as 9760 photons per second entering the pupil. About how much energy is this, per second? (Assume the photons have a wavelength of 300 nm.)

 

2.How many visible photons would be needed to have enough energy to lift a 0.6 newton weight through 0.2 meter? (Assume a visible photon has a frequency of 5 ✕ 1014 Hz.)

Answer 1

(1)

 

For each photon,

 

E = hc/λ = (6.626 x 10^-34 Js)(3 x 10⁸ m/s) / (300 x 10^-9 m) = 6.626 x 10^-19 J

 

Multiply this by the number of photons per second.

 

In 1 second,

 

E = (9760 photons)(6.626 x 10^-19 J/photon) ≅ 6.467 x 10^-15 J

 

(2)

 

Calculate the work W, and divide by the energy of a single photon.

 

W = Fd = (0.6 N)(0.2 m) = 0.12 J

 

E = hf = (6.626 x 10^-34 Js)(5 x 10¹⁴ Hz) = 3.313 x 10^-19 J

 

N = number of photons

 

N = W / E = (0.12 J) / (3.313 x 10^-19 J) ≅ 3.622 x 10¹⁷