When a person enters a dark room after being in bright sunlight, they are not able to see objects clearly for a short time. What is the main reason for this temporary inability to see?

Introduction / Context:
This question deals with the adaptation of the human eye to changes in light intensity. Our eyes can operate over a wide range of brightness levels, from bright sunlight to dim starlight. However, the adjustment to new light levels is not instantaneous. When moving from a bright environment into a dark one, people often experience a brief period where they cannot see clearly. Understanding the role of the iris and pupil in this process is essential for basic biology and physics of vision.

Given Data / Assumptions:

• The person moves suddenly from bright sunlight into a dark room.
• Initially, they are unable to see objects clearly in the dark room.
• The options mention the retina moving, the iris and the power of accommodation.
• We assume a normal healthy eye without disease.

Concept / Approach:
The iris is the coloured part of the eye surrounding the pupil. It controls the size of the pupil and hence the amount of light entering the eye. In bright light, the pupil becomes small (constriction) to protect the retina from excess light. In dim light, the pupil must widen (dilate) to allow more light to enter. When you move suddenly from bright to dark, the iris cannot immediately dilate the pupil to its new required size. Until the pupil has widened sufficiently and chemical changes in the photoreceptor cells occur, vision is poor. This delay in dilation and adaptation is the main reason for the temporary loss of clear vision.

Step-by-Step Solution:
Step 1: In bright sunlight, the pupil is constricted to a small size to reduce the amount of light reaching the retina. Step 2: When the person suddenly enters a dark room, the available light is much less, but the pupil is still small for a brief period. Step 3: Because the pupil is small, very little light reaches the retina, so the image formed is too dim to be perceived clearly. Step 4: The iris muscles gradually respond to the reduced light by dilating the pupil, increasing its size. Step 5: As the pupil widens and photochemical adaptation in rods occurs, more light reaches the retina and sensitivity improves, restoring clear vision. Step 6: Therefore, the temporary inability to see clearly is due to the iris not being able to widen the pupil instantly.

Verification / Alternative check:
This effect can be felt in daily life. When entering a dark cinema hall from a sunny street, you initially struggle to see the steps and seats, but after a short time your eyes adapt and you can see quite well. Conversely, when you come out into bright light from a dark room, you may be dazzled until the pupils constrict. In both cases, the iris and pupil size adjustment takes finite time. The retina does not move forwards or backwards; its position is fixed on the back of the eye. These observations support the explanation based on iris and pupil dilation.

Why Other Options Are Wrong:
The retina of the eye moving forward or backward is incorrect; the retina is a fixed light sensitive layer and does not move in normal eye function. The eye having no power of accommodation would affect the ability to focus on near or far objects, not the ability to see in low light. The key process here is the time taken by the iris and associated muscles to dilate the pupil and for photoreceptors to adapt, not a change in retinal position.

Common Pitfalls:
Students may confuse accommodation, which is the adjustment of the lens to focus on objects at different distances, with adaptation to different light levels, which mainly involves changes in pupil size and retinal sensitivity. It is important to remember that accommodation affects focus (near versus far), while pupil dilation and photochemical changes affect brightness sensitivity. In this question, the issue is brightness, so the iris and pupil are the central elements.

Final Answer:
The temporary inability to see clearly in a dark room after bright light occurs because the iris cannot immediately dilate the pupil to let in enough light.