# How does a photon impart it's colour to the retina in the instant that it makes contact? [closed]

Considering the speed of light, any number of photons carrying the same colour frequency and hitting the retina at the back of an eyeball would be at varying phases in their respective cycles, but they can all impart their colour frequency. This baffles me. Am I right in saying that a photon emitted from an electron is "pulsing" at an energy level and frequency which determines it's colour? What drives the oscillation anyway? I can't help thinking that we are talking about ideas and not hard science. Is there any proof to these photon theories?

Am I right in saying that a photon emitted from an electron is "pulsing" at an energy level and frequency which determines it's colour?

No, you are not right; photons do not pulse in real time and space.

Photons are quantum mechanical entities, elementary particles in the standard model of particle physics. (Their complex wavefunction has oscillations but that is another story). They just have energy= $$hν$$ where $$ν$$ is the frequency the classical electromagnetic light has, which is built up by the superposition of zillions of photons, and h the Planck constant.

The cones of the retina, are composed of atoms and molecules,and the atoms and molecules absorb the photon in an energy level which will signal to the biological set up of the retina that light is coming in.

It gets even more complicated, because the biological computation path generates color perception: i.e. an electromagnetic frequency corresponds to the color of the spectrum according to frequency, but the perception of color is wider, frequencies add up to the same color perception:

• Comments are not for extended discussion; this conversation has been moved to chat. – Chris May 30 at 12:37

To add to Anna V's answer: the mechanism by which photons are detected in the eye is indeed based on their energy, which is $$h\nu$$, where $$\nu$$ is frequency.

What happens is that there are proteins in the eye called Photopsins, which use retinal (which is a form of Vitamin A). When this absorbs a photon of the correct energy, it changes configuration, and it's this change in configuration that the eye ultimately detects.

And the important thing is that the system depends on the energy of the incoming photon: if the energy is in the right range, and so the frequency is in the right range, then the configuration change happens, while if it's not, it doesn't.

Needless to say, like almost anything that has had billions of years to evolve, the actual mechanism of detection is intricate, to put it mildly.

Concerning your question 'is there any proof to these photon theories?': there is very good evidence for them indeed, because we can detect individual photons (indeed some animals, possibly including humans, have eyes which can detect invididual photons). See this question and this answer (also by Anna V).

• Thankyou so much tfb for this fantastic answer! It makes good sense that an energy level can be detected chemically and it explains how a stream of photons can produce the same colour. We are not therefore concerned with frequency at the retina (I infer). The question of colour frequency is still a mystery to me though but I'll work on it from the other helper! Thanks tfb! – Matt Skeptic May 23 at 7:50
• @MattSkeptic: the energy of a photon corresponds to the frequency of the classical wave, by $E = h\nu$ where $\nu$ is frequency. – tfb May 23 at 9:44
• Now I'm battling with redshift and energy level. If the wavelength of a receding star has a lengthened wavelength, its energy level is reduced - correct? But light is emitted at constant speed and wavelength so we are taking about detection with a device or retina at the point of reception. I can perceive lengthening of a wavelength where the carrier is arriving more slowly (doppler), but since emission the wavelength has not actually changed within the photon and its arrival speed is constant. Are stars receding rapidly not visible due to photon energy drop being unrecognised by the retina? – Matt Skeptic May 24 at 23:50
• @MattSkeptic: stars receding rapidly enough would indeed not be visible to our eyes. The most distant galaxy we know about, GN-z11 has a value of $z$ of about $11$ meaning that wavelengths are multiplied by about $12$: blue light from this galaxy is shifted to mid-infrared by the time it reaches us, so I doubt this would be visible to our eyes even if it were bright enough. – tfb May 25 at 12:29
• Many Thanks tbf that's interesting too! I'm still battling with how redshift can ocurr while the speed of light is constant. I'll post the question properly to better illustrate my problem in understanding it. I know I tend to ramble. Ishould behave myself! – Matt Skeptic May 26 at 2:29