Outline the function of rods and cones, limited to: (a) greater sensitivity of rods for night vision (b) three different kinds of cones, absorbing light of different colours, for colour vision

The functions of rods and cones in the human retina are essential for vision, particularly under varying lighting conditions. Here’s an outline of their specific roles:

(a) Greater Sensitivity of Rods for Night Vision

• Sensitivity to Light: Rods are highly sensitive photoreceptors that can respond to very low levels of light, making them crucial for night vision (scotopic vision). They require only a fraction of the light needed by cones to become activated—about 500 to 1,000 times less light than cones. This sensitivity allows rods to detect shapes and movements in dim lighting, enabling vision in dark environments.
• Distribution: Rods are predominantly located in the peripheral regions of the retina, which enhances peripheral vision in low-light conditions. Their high sensitivity enables humans to navigate and perceive their surroundings even when light is minimal.
• Color Perception: While rods excel in low-light conditions, they do not contribute to color vision. Instead, they provide a monochromatic view, primarily detecting shades of gray. This limitation is why night vision appears almost entirely black and white.

(b) Three Different Kinds of Cones for Color Vision

• Types of Cones: There are three types of cones in the human retina, each sensitive to different wavelengths of light:
  • S-cones (Short-wavelength cones): Sensitive to blue light (approximately 420 nm).
  • M-cones (Medium-wavelength cones): Sensitive to green light (approximately 534 nm).
  • L-cones (Long-wavelength cones): Sensitive to red light (approximately 564 nm).
• Color Perception: The combination of signals from these three types of cones allows the brain to perceive a wide range of colors. For example, when both M-cones and L-cones are stimulated, the brain interprets this as yellow.
• Distribution: Cones are concentrated mainly in the fovea, the central part of the retina, where visual acuity is highest. This central concentration enables detailed color vision and sharp focus under bright lighting conditions.