Need Help with questions on Photons Please?

Question

1) According to the photon model, light carries its energy in packets called quanta or photons. Why then don't we see a series of flashes when we look at things?

2) Most black-and-white photographic film (with the exception of some special-purpose films) is less sensitive to red light than blue light and has almost no sensitivity to infrared. How can these properties be understood on the basis of photons?

3) Galaxies tend to be strong emitters of Lyman-alpha photons (from the n=2 to n=1 transition in atomic hydrogen). What can you infer from this observation about the temperature in this environment? Explain. (Lyman alpha photons have a wavelength of 122 nm and it's the longest wavelength line in the Lyman series of UV lines in the H spectrum)

4) The emission of a photon by an isolated atom is a recoil process in which momentum is conserved. Thus hf = (energy initial - energy final) should include a recoil kinetic energy for the atom. Why is this energy negligible in that equation?

Answer 1

Question 1) im no smart nerd but I think that q1 is because the packets of energy move extremely fast

Answer 2

1) Because these photons have a very low energy, and to make up for that, there are a lot of them. For example - a typical light bulb will emit more than a million million million photons every second. With that many photons, your vision pretty much smooths out the quantised nature of light.

2) Easy - blue light has a higher energy per photon than red light, and much more than infrared.

4) The momentum of a photon is h/wavelength = hf/c. The energy of a photon is hf.

Since momentum is conserved, an atom will recoil with a momentum of hf/c if it emits a photon of energy hf.

The energy of this recoil is p²/2m = h²f²/(2mc²). *

The ratio of this energy to the energy of the photon is therefore:

hf/(2mc²)

Lyman series photons have a maximum energy of about 13.6 eV, the ground state of the Hydrogen atom (hence this is the maximum of all the hydrogen series).

Also for Hydrogen, mc² is about a billion electron-volts (and since Hydrogen is the lightest atom, this term only increases with the mass of the atom).

So the contribution from the recoiling atom is negligible.

* KE is usually given as 1/2mv², but p²/2m is a better way to express it here.