When a person enters a dark room after being in bright sunlight, why is he or she unable to see objects clearly for a short time?

Introduction / Context:
This question tests basic understanding of human eye physiology and how the eye adjusts to changes in light intensity. In everyday life, people often experience temporary difficulty seeing when moving from a brightly lit environment into a dark room, such as entering a cinema hall from bright sunlight. This adjustment process is called dark adaptation and is controlled by the iris and pupil as well as by chemical changes in the photoreceptor cells of the retina. Knowing the immediate mechanical reason for the temporary loss of clarity is important in general science and biology questions.

Given Data / Assumptions:

• A person is initially in bright light, for example sunlight.
• The person then moves suddenly into a dark or dim room.
• For a short while, he or she cannot see objects clearly.
• The options mention movements of the retina, iris controlled pupil changes and power of accommodation.

Concept / Approach:
The iris is the coloured part of the eye that controls the size of the pupil, the opening through which light enters. In bright light, the iris constricts the pupil to a small size to reduce the amount of light entering. In darkness, the iris dilates the pupil to a larger size to allow more light to reach the retina. This adjustment takes some time. When a person suddenly moves from bright light to darkness, the pupil is still small and cannot immediately dilate to the necessary size. As a result, very little light reaches the retina at first, and the person cannot see clearly until the iris has gradually dilated the pupil and the retina has adapted.

Step-by-Step Solution:
Step 1: In bright light, the iris keeps the pupil constricted so that only a limited amount of light enters the eye. Step 2: When the person suddenly enters a dark room, the light level falls sharply, but the pupil is still small for a brief period. Step 3: Because the pupil is small, very little light reaches the retina, and the visual image is too weak to be perceived clearly. Step 4: The iris then responds by slowly dilating the pupil to let more light enter, a process that takes some time. Step 5: During this adjustment interval, vision is poor, and once the pupil becomes wider and the retina adapts, the person begins to see clearly.

Verification / Alternative check:
The retina does not physically move forward or backward in ordinary light changes. Instead, the main rapid mechanical change is the variation of pupil size controlled by the iris. The power of accommodation refers to the eye changing the curvature of the lens to focus on near or distant objects and is not directly related to adapting to darkness. Therefore, the only option that correctly describes an immediate cause of temporary poor vision is the limited ability of the iris to dilate the pupil instantly, which matches option about the iris and pupil adjustment.

Why Other Options Are Wrong:
The retina of the eye moving forward is not a normal physiological response during light changes and does not explain the delay in vision. The retina moving backward is also incorrect; the retina remains fixed while other parts such as iris and lens adjust. The eye does have power of accommodation in dark conditions; accommodation is related to focusing on objects at different distances, not to the quantity of light entering.

Common Pitfalls:
Students sometimes confuse accommodation with light adaptation and may think that inability to change focus is responsible. Others may not know the exact roles of the iris and retina and may fall for options that sound technical involving retina movements. To avoid such confusion, it is important to remember that the iris and pupil control the amount of light entering the eye, and that dark adaptation is mainly due to delayed dilation of the pupil and chemical adjustments in photoreceptors, not movement of the retina or lack of accommodation.

Final Answer:
When we move from bright light to a dark room, we cannot see clearly for a while because the iris cannot dilate the pupil immediately to allow more light in.