Final Answer:
Red eye in photographs is caused by both fixating light (reflection off blood vessels) and testing light (pre-flash affecting pupil constriction), making both (a) and (b) contributing factors.
Thus the correct option is (c) both a and b.
Step-by-step explanation:
The phenomenon of red eye in photographs is attributed to both fixating light (a) and testing light (b). When a camera flash is used, the light reflects off the retina's blood vessels, causing the eyes to appear red in the photograph. This occurs because the light from the flash penetrates the eye, reflects off the blood vessels, and returns to the camera lens. The fixating light (a) refers to the reflection from the blood vessels, giving the eyes a reddish appearance.
On the other hand, testing light (b) involves the camera's autofocus system emitting a pre-flash of light to measure the distance to the subject. This pre-flash can cause the pupils to contract, reducing the amount of light entering the eye. When the actual flash occurs, the pupils may still be constricted, leading to an increased reflection of light off the blood vessels in the retina. Therefore, the combination of fixating light and testing light contributes to the red eye effect in photographs.
In summary, the red eye phenomenon is not solely attributed to fixating light or testing light but results from the interplay of both factors. Understanding this interaction helps in mitigating the red eye effect by using techniques such as pre-flashes to reduce pupil constriction or employing red-eye reduction features in modern cameras.
Therefore, the correct option is (c) both a and b.