the eye of the reptile!

The energy of the single photon is given by:

$E_{0}=\frac{hc}{\lambda}$

where $\lambda$ is wavelength of the photon.

Let $n$ be the minimum number of photons required to trigger the receptor. Then the total energy of these photons is $n\times E_{0}$ .

Using the given values in the problem, we get: $\boxed{n = 20000}$

To account for all the confusions. We need to know that each photon is recognized by its frequency. Meaning that each photon is different from other based on its frequency. (Talking in terms of energy). So if wavelength of the photon is given. We used the well known relation:

$f = \frac{c}{lmbda}$

When frequency of one photon is known. We calculate the energy associated with one photon:

$E = hf$

and divide total energy by energy of one photon to get number of photons. Which gives us the answer $\boxed{20000}$

The number of photons is given by dividing the total energy with the energy of each photon.The total energy is 3*10^-14J.The energy of each photon is given by E=h.c/lambda=1.5 10^-18.n=20000

Calculate the amount of energy emitted by a single photon of a given wavelength, 132.6nm. ( remember to convert to meter) Thus, E= h * f where f= c/lambda. Rewriting, E= h * c/lambda 6.63E-34 * ( 3E8/1.326E-7)Joules = 1.5E18 Joules If 3E-14 Joules are required to trip the signal, then we need (3E-14/1.5E18) photons or 20,000.