OCR General Certificate of Secondary Education (GCSE) Biology Practice Exam

Which part of the eye is primarily responsible for light adjustment?

1. Pupil

2. Cornea

3. Lens

4. Iris

The correct answer is: Iris

The iris is the part of the eye that is primarily responsible for light adjustment. It is a ring of muscle that surrounds the pupil, the opening that allows light to enter the eye. When there is bright light, the iris contracts to make the pupil smaller, reducing the amount of light that enters. Conversely, in low-light conditions, the iris relaxes, causing the pupil to expand and allow more light to enter. This process is vital for protecting the retina from excessive light exposure and enhancing vision in various lighting conditions. The pupil's size is regulated by the iris, but it does not have the capability to adjust light levels on its own. The cornea primarily focuses light onto the retina, and the lens also helps in focusing by changing shape but is not directly involved in adjusting light levels. Hence, it is the iris that plays the crucial role in adjusting light entering the eye.