Case Study:

Aarav, a Class 10 student from Lucknow, recently noticed that he could not clearly read the words written on the classroom board while sitting at the back. However, he could read books and notebooks easily when they were close to his eyes. His parents took him to an eye specialist, who recommended spectacles.

The doctor explained that Aarav was suffering from myopia (short-sightedness). In this condition, the eye lens focuses light from distant objects in front of the retina, causing distant objects to appear blurred. This may happen due to excessive curvature of the eye lens or elongation of the eyeball. 

To correct this problem, Aarav was prescribed spectacles with a concave lens of power -2.0 diopters. A concave lens diverges the incoming parallel light rays slightly before they enter the eye so that the image finally forms exactly on the retina. 

Meanwhile, Aarav’s grandmother uses spectacles for reading books and newspapers. Her doctor explained that she has hypermetropia (long-sightedness), where nearby objects appear blurred because the image forms behind the retina. In such cases, a convex lens is used to converge the incoming light rays and bring the image onto the retina. 

Spectacles therefore work based on the refraction of light through lenses, which bend light rays so that the eye can form clear images on the retina. By selecting lenses of appropriate focal length and power, opticians help millions of people in India correct vision problems and perform daily activities like studying, driving, and reading safely.

Questions

Section A – MCQs

1. Aarav cannot see the classroom board clearly but can read his notebook easily. Which type of lens will correct his vision?

A. Convex lens
B. Concave lens
C. Cylindrical lens
D. Plane glass lens

2. A person suffering from hypermetropia will have difficulty in:

A. Seeing distant mountains clearly
B. Seeing nearby objects clearly
C. Seeing both near and distant objects
D. Seeing objects in bright light

3. A concave lens used in spectacles primarily works by:

A. Converging parallel light rays
B. Reflecting light rays
C. Diverging light rays before entering the eye
D. Increasing brightness of the image

4. A spectacle lens has power +2 D. The lens used is most likely:

A. Convex lens
B. Concave lens
C. Plane mirror
D. Concave mirror

Section B - Short Answer Questions

1. Explain why a concave lens is used to correct myopia.

2. Why does a hypermetropic person require a convex lens for clear vision?

3. Aarav’s spectacles have power -2 D. Calculate the focal length of the lens.

Section C - Long Answer Question

1.A student can see objects clearly only up to 100 cm, but distant objects appear blurred.

Analyze the situation and answer the following:

• Identify the defect of vision.
• Which type of lens should be used to correct the defect?
• Explain using ray behaviour how the lens helps in forming the image on the retina.

Answer Key

MCQ Answers

1. B - Concave lens
2. B - Seeing nearby objects clearly
3. C - Diverging light rays before entering the eye
4. A - Convex lens

Short Answer Solutions

1. In myopia, the image of distant objects forms in front of the retina. A concave lens diverges the incoming rays, shifting the image formation backward so that it falls on the retina.

2. In hypermetropia, the image of nearby objects forms behind the retina. A convex lens converges the light rays, bringing the image forward onto the retina.

3. Power formula:

P = 1 / f
f = 1/ P

Given ( P = -2D )

f = 1/ -2 = -0.5 m

Focal length = –0.5 m (–50 cm)

Long Answer Solution

• Defect: Myopia (short-sightedness).
• Correcting Lens: Concave lens.

Explanation:

1. In myopia, light rays from distant objects converge before reaching the retina.
2. A concave lens diverges the incoming light rays slightly.
3. These rays then enter the eye and are focused by the eye lens onto the retina.
4. Thus, the image of distant objects becomes clear.