Why do a cat's eyes glow red? - briefly
A cat's eyes appear to glow red due to the reflection of light off the blood vessels in the retina, which occurs when light passes through the tapetum lucidum and reflects back.
Why do a cat's eyes glow red? - in detail
The phenomenon of a cat's eyes glowing red is primarily attributed to a unique anatomical feature called the tapetum lucidum. This reflective layer, located behind the retina, enhances a cat's night vision by reflecting light back through the retina, allowing photoreceptor cells a second chance to detect it. When light enters a cat's eyes in low-light conditions, it passes through the retina and strikes the tapetum lucidum. This layer then reflects the light, often producing a visible glow. The color of this glow can vary based on factors such as the angle of the light source, the cat's eye color, and the specific composition of the tapetum lucidum. In some cases, the glow appears red due to the presence of blood vessels in the choroid layer, which lies beneath the tapetum lucidum. When light reflects off these vessels, it can create a reddish hue. Additionally, certain lighting conditions or camera flashes can amplify this effect, making the red glow more pronounced. This red glow is less common than the typical green or yellow glow associated with cats but is still a natural occurrence tied to the interplay of light and the eye's structure. Understanding this mechanism highlights the evolutionary adaptations that enable cats to thrive in low-light environments.