In this study, quanta refer to which of the following?

• A. Photons absorbed by rods
• B. Photons incident on the cornea
• C. Photons absorbed by cones
• D. Photons scattered in the vitreous

Answer: (B)

Quanta are the individual photons that carry light energy. In vision studies, researchers quantify a stimulus by how many photons reach the eye, i.e., the number of photons entering the eye. The photons incident on the cornea represent the amount of light that has actually entered the optical system at its entry point, making them the proper measure of quanta for a stimulus. Photons absorbed by rods or cones or scattered in the vitreous happen after light has already entered and interacted with ocular tissues, so they are downstream effects, not the value used to describe the incoming light.