NCERT Class 8 Science — Chapter 4: Light

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When a person moves from a dimly lit environment to a brightly lit environment, how does the pupil of the eye adapt?

Official previous-year question

Verified 1 Aug 2026.

Options

AIt maintains a constant diameter.
BIt expands to allow more light.
CIt changes its pigmentation.
DIt contracts to restrict light entry.

Did you get this right?

Solution

The correct option is

It contracts to restrict light entry.

Explanation

The human eye functions as a natural optical instrument. The pupil acts as an aperture through which light enters the eye, while the iris functions as a diaphragm that regulates the size of the pupil based on the intensity of the external light.

Mechanism of Adaptation

In Bright Light: When exposed to high-intensity light, the circular muscles of the iris contract. This action causes the pupil to constrict (decrease in diameter). This restriction reduces the amount of light entering the eye, protecting the retina from damage and improving image sharpness.
In Dim Light: Conversely, in low-light conditions, the radial muscles of the iris contract, causing the pupil to expand (dilate) to allow more light to enter for better visibility.

Key Takeaway

The iris automatically adjusts the diameter of the pupil to regulate light exposure: the pupil contracts in bright light and expands in dim light.

Your note

More from Chapter 4: Light

In the context of the reflection of light, what does the term 'angle of incidence' refer to?

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With reference to image formation by a plane mirror, consider the following statements: 1. The image formed cannot be obtained on a screen. 2. The image appears at a greater distance behind the mirror than the object is in front. 3. The image undergoes lateral inversion. Which of the statements given above are correct?

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Which one of the following terms describes the phenomenon where the left of an object appears on the right in a mirror image?

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With reference to reflection from different surfaces, consider the following statements: 1. Regular reflection occurs when parallel rays fall on an irregular surface. 2. Diffused reflection occurs when parallel rays fall on an irregular surface and are reflected in different directions. 3. Diffused reflection is caused by irregularities in the reflecting surface. Which of the statements given above is/are correct?

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What is the primary optical mechanism that enables the visibility of non-luminous objects?

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The correct option is

It contracts to restrict light entry.

Explanation

Mechanism of Adaptation

In Bright Light: When exposed to high-intensity light, the circular muscles of the iris contract. This action causes the pupil to constrict (decrease in diameter). This restriction reduces the amount of light entering the eye, protecting the retina from damage and improving image sharpness.
In Dim Light: Conversely, in low-light conditions, the radial muscles of the iris contract, causing the pupil to expand (dilate) to allow more light to enter for better visibility.

Key Takeaway

The iris automatically adjusts the diameter of the pupil to regulate light exposure: the pupil contracts in bright light and expands in dim light.