Receptor cells in the eye that work best in dim light and enable you to see black, white, and gray.

• A. Cone (cells)
• B. Rod (cells)
• C. Pupil
• D. Rod (cells)

Answer: (D)

The eye uses two types of photoreceptor cells to sense light: rods and cones. Rods are extremely sensitive to light, so they work best in dim conditions and allow you to see in grayscale—black, white, and gray—rather than in color. They are packed mainly around the periphery of the retina and have a lot of photopigment, which boosts sensitivity but sacrifices sharp detail and color. Cones, on the other hand, require brighter light and provide color vision and fine detail, which is why day vision is colorful and crisp. The pupil isn’t a receptor; it’s the opening that lets light into the eye and changes size to regulate how much light enters. So, in low light, rod cells are the ones doing the work to help you see in grayscale.