Final answer:
Tuan experiences a condition called color blindness, specifically red-green color blindness, which means he lacks functioning cones for red light. This prevents him from distinguishing between raw red meat and cooked meat, as they both appear brownish to him.
Step-by-step explanation:
Tuan experiences a condition known as color blindness because he lacks functioning cones for red light. Cones are the photoreceptor cells in our eyes that are responsible for color vision. There are three types of cones: those that are sensitive to red light, green light, and blue light. When these cones are functioning properly, they allow us to perceive the full spectrum of colors. However, when an individual lacks functioning cones for a specific color, they are unable to differentiate between different shades of that color.
In Tuan's case, since he lacks functioning cones for red light, he sees both raw red meat and cooked meat as a brownish color. This is because the red color of the meat is not properly detected by his cones, so it appears as a brownish color to him. This condition is known as red-green color blindness, which is the most common form of color deficiency.
To better understand this, let's look at an example with dogs. Dogs have only two types of cones, which allow them to see only shades of blue and yellow. Dogs are red-green color-blind because they lack the cone cells sensitive to red and green light. Similarly, Tuan's lack of functioning cones for red light prevents him from distinguishing the true red color of the meat.