Final answer:
Visible light energy triggers photoreceptors in the eyes by causing a shape change in the retinal molecule, initiating visual transduction with subsequent neural signaling.
Step-by-step explanation:
The modality that triggers photoreceptors is light energy. Within the photoreceptors, such as rods and cones, light is absorbed by the molecule retinal. Retinal, which is bound to proteins called opsins, undergoes a shape change when it absorbs photons. This shape change initiates a series of events starting with the activation of opsins, which in turn activate a G protein. The G protein alters the membrane potential of the photoreceptor cell, resulting in the release of less neurotransmitter into the outer synaptic layer of the retina.
This process is known as visual transduction. The bleaching of retinal occurs when the molecule changes shape and cannot respond to light until it is converted back to its 11-cis-retinal shape. A bright flash of light can lead to the bleaching of many photopigments and result in a negative afterimage. Eventually, a series of enzymatic changes reverse the photoisomerization, restoring the retina's ability to respond to light.