Rod cells in the retina of the eye detect light using a photopigment called rhodopsin. 1.8 ev is the lowest photon energy that can trigger a response in rhodopsin.
1. what is the maximum wavelength of electromagnetic radiation that can cause a transition? (in nm)
3. in what part of the spectrum is this? (visible light, infrared, xray, or ultraviolet)

1. 686 nm

The lowest photon energy is

Let's convert this energy into Joules first

The energy of the photon is given by

where

h is the Planck constant

c is the speed of light

is the wavelength

Re-arranging the equation for , we find the maximum wavelength of the photon that can cause a transition:

2. Visible light

The photon of this light is in the visible light part of the spectrum.

In fact, the range of wavelengths of the visible part of the spectrum is

[380 nm - 750 nm]

In particular, we have that the wavelengths in the range

[640 nm - 750 nm]

corresponds to the red light part of the spectrum: since 686 nm falls withing this range, this photon is a red light photon.

answer: 31, 5556. 952 seconds