Case Study:

During a routine eye checkup, Riya noticed something strange. As soon as the doctor shined a bright torch into her eyes, her pupils quickly became smaller. A few seconds later, when the light was removed, her pupils became larger again. The doctor explained that this automatic adjustment helps protect the eyes from excess light and also improves clear vision.

The human eye works like a camera. The coloured part of the eye, called the iris, controls the size of the pupil. In bright light, the iris contracts and makes the pupil smaller so that less light enters the eye. In dim light, the iris relaxes and enlarges the pupil to allow more light inside.

To get a better handle on the anatomy,  you can read our deep dive into how the human eye works to understand vision in even simpler terms.

During the same test, Riya was asked to look first at a nearby chart placed 25 cm away and then at a distant object outside the window. She noticed that her eyes adjusted automatically without any conscious effort. This happens because the eye lens changes its thickness with the help of ciliary muscles. This ability of the eye to focus on nearby and distant objects is called accommodation.

When this process doesn't work perfectly,  we encounter common eye defects - here is how spectacles help correct them.

The doctor also measured the pupil size during the test. In dim light, the pupil diameter was about 7 mm, while under bright light it reduced to nearly 2 mm. Such quick responses help the eye maintain proper vision and protect sensitive parts of the retina from damage.

CASE-BASED QUESTIONS

 MCQ 

Q1. Which part of the eye controls the size of the pupil?

A. Retina
B. Cornea
C. Iris
D. Optic nerve

Q2. Why did Riya’s pupils become smaller under bright light?

A. To improve hearing
B. To reduce the amount of light entering the eye
C. To increase blood flow
D. To enlarge the retina

 Assertion - Reason 

Q3. Assertion (A): The human eye can focus on both nearby and distant objects.
Reason (R): The eye lens changes its focal length with the help of ciliary muscles.

A. Both A and R are true, and R is the correct explanation of A
B. Both A and R are true, but R is not the correct explanation of A
C. A is true, but R is false
D. A is false, but R is true

 Application-Based 

Q4. While reading a book in dim light, why does the pupil become larger?

Q5. Why was Riya asked to look at both a nearby chart and a distant object during the eye examination?

 Data/Logic-Based 

Q6. The pupil diameter changed from 7 mm in dim light to 2 mm in bright light. What does this observation show about the functioning of the eye?

ANSWER KEY WITH EXPLANATION

A1. C. Iris
Explanation: The iris regulates the size of the pupil and controls the amount of light entering the eye.

A2. B. To reduce the amount of light entering the eye
Explanation: In bright light, the pupil contracts to protect the retina from excess light.

A3. A. Both A and R are true, and R is the correct explanation of A
Explanation: Accommodation occurs because ciliary muscles change the curvature and focal length of the eye lens.

A4. The pupil enlarges in dim light so that more light can enter the eye. This helps the retina receive enough light for clearer vision.

A5. Why was Riya asked to look at both a nearby chart and a distant object during the eye examination?

A6. This shows that the iris automatically controls pupil size according to light intensity. It protects the retina from excessive light and helps maintain proper vision.

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HOTS EXTENSION QUESTIONS

Q1. A person suddenly walks from bright sunlight into a dark movie theatre. Predict how the pupil and iris will respond during the first few seconds.

Q2. Why might a person experience difficulty in focusing quickly between a mobile phone screen and a distant classroom board?

 Still have questions about the human eye?   Head over to our discussion forum to ask an expert, or challenge yourself with more interactive quizzes.

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