A rod's dark current in darkness is about

PHYSICS · Multi-part · Fermi University Physics · Optics & Photonics · Biophysics & Medical Physics · Bioelectricity, the senses and the physics of the body

Forces, Machines and Signals, Ch Vision and photoreception: counting photons

Problem

20\,\mathrm{pA}

. Absorption of a single

505\,\mathrm{nm}

photon by rhodopsin reduces this current by about

1.2\,\mathrm{pA}

for approximately

0.8\,\mathrm{s}

before the current recovers (treat the response as a rectangular pulse of that height and duration for the estimate). ((a)) Estimate the number of elementary charges kept from crossing the rod's membrane by this one photon absorption (the gain of the single-photon response). ((b)) Express the energy of the absorbed

505\,\mathrm{nm}

photon in units of

k_BT

at body temperature,

310\,\mathrm{K}

. State the assumptions used in each part.

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