Reply To: A Christmas Photonic Teaser

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Steve Holmes

Hi Alan,

The simple answer is that the photons are “absorbed” by the process which stimulates the receptors in the retina. This process requires energy, which is supplied by the photons – which are basically just little packets of energy. In a similar way, photons incident upon a green leaf are absorbed by the chlorophyll molecules in the leaf to excite them to the higher energy level required to begin the cascade of processes involved in photosynthesis.

The reason the photons are able to travel through the eye without being absorbed is that quantum theory says they must be entirely absorbed, or not at all. For something to absorb a photon thus requires the absorber to have two energy levels separated by exactly the amount of energy contained within the photon. This is not true for the “interior substance” of the eye but is true for the retinal cells – something which evolution has honed throughout aeons. Moreover, because light of different colours corresponds to photons of different energies, discrete sets of retinal cells have evolved to respond to different energies/colours – hence our ability to see in colour. “Colour” is merely an artifact of our perception though – much like the false-colour renderings of the images from Hubble and JWST. Given this, different eyes see colour differently. Even within the human population increased sensitivity to both infra-red and ultra-violet light is not unknown. For example, evolution has given Schrodinger’s Cat greater sensitivity to low light, which is important for hunting at dusk, but a lesser sensitivity in the red. Assuming said Cat is actually alive, that is. Oh, sorry, I wasn’t supposed to mention said feline, was I? I promise no mention of singularities or higher-order infinities though.

Hope that is both non-PhD understandable and generally helpful!