Vision Defects and Correction

This lesson covers vision defects (short-sightedness and long-sightedness) and how they are corrected using lenses, as required by the Edexcel GCSE Physics specification (1PH0), Topic 5: Light and the Electromagnetic Spectrum. You need to understand what causes each defect, where the image forms, and which type of lens is used to correct it.

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Short-Sightedness (Myopia)

What Is Short-Sightedness?

A short-sighted person can see near objects clearly but distant objects appear blurred.

What Causes It?

Short-sightedness is caused by one (or both) of the following:

1. The eyeball is too long — the distance from the lens to the retina is greater than normal.
2. The lens is too powerful (too curved) — it refracts light too strongly.

Where Does the Image Form?

In a short-sighted eye, light from a distant object is focused in front of the retina rather than on it. By the time the light reaches the retina, it has spread out again, producing a blurred image.

How Is It Corrected?

Short-sightedness is corrected by placing a diverging (concave) lens in front of the eye.

• The diverging lens spreads the light rays out slightly before they enter the eye.
• This means the eye's own lens does not over-converge the light.
• The light is now focused on the retina, producing a clear image.

flowchart TD
    A["Short-sighted eye<br/>(uncorrected)"] --> B["Distant object"]
    B --> C["Light focuses IN FRONT<br/>of the retina"]
    C --> D["Image is BLURRED"]

    E["Short-sighted eye<br/>(corrected)"] --> F["DIVERGING lens placed<br/>in front of eye"]
    F --> G["Light is spread out<br/>before entering the eye"]
    G --> H["Eye focuses light<br/>ON the retina"]
    H --> I["Image is CLEAR"]

    style A fill:#e74c3c,color:#fff
    style D fill:#e74c3c,color:#fff
    style E fill:#27ae60,color:#fff
    style I fill:#27ae60,color:#fff

Exam Tip: Short-sightedness → image in front of retina → corrected with a diverging (concave) lens. Use the mnemonic: "Short = in front = diverging".

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Long-Sightedness (Hyperopia)

What Is Long-Sightedness?

A long-sighted person can see distant objects clearly but near objects appear blurred.

What Causes It?

Long-sightedness is caused by one (or both) of the following:

1. The eyeball is too short — the distance from the lens to the retina is less than normal.
2. The lens is too weak (too flat) — it does not refract light strongly enough.

Where Does the Image Form?

In a long-sighted eye, light from a near object would be focused behind the retina (if the retina were not in the way). The light has not converged enough by the time it reaches the retina, producing a blurred image.

How Is It Corrected?

Long-sightedness is corrected by placing a converging (convex) lens in front of the eye.

• The converging lens partially converges the light rays before they enter the eye.
• This helps the eye's own lens to bring the light to a focus on the retina.
• The light is now focused on the retina, producing a clear image.

flowchart TD
    A["Long-sighted eye<br/>(uncorrected)"] --> B["Near object"]
    B --> C["Light focuses BEHIND<br/>the retina"]
    C --> D["Image is BLURRED"]

    E["Long-sighted eye<br/>(corrected)"] --> F["CONVERGING lens placed<br/>in front of eye"]
    F --> G["Light is partially converged<br/>before entering the eye"]
    G --> H["Eye focuses light<br/>ON the retina"]
    H --> I["Image is CLEAR"]

    style A fill:#e74c3c,color:#fff
    style D fill:#e74c3c,color:#fff
    style E fill:#27ae60,color:#fff
    style I fill:#27ae60,color:#fff

Exam Tip: Long-sightedness → image behind retina → corrected with a converging (convex) lens. Use the mnemonic: "Long = behind = converging".

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Comparison of Short-Sightedness and Long-Sightedness

Feature	Short-Sightedness (Myopia)	Long-Sightedness (Hyperopia)
Can see clearly	Near objects	Distant objects
Cannot see clearly	Distant objects	Near objects
Cause (eye shape)	Eyeball too long	Eyeball too short
Cause (lens)	Lens too powerful / too curved	Lens too weak / too flat
Image forms	In front of retina	Behind retina
Corrected by	Diverging (concave) lens	Converging (convex) lens
Lens power	Negative (−)	Positive (+)

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Ray Diagrams for Vision Defects

In the exam, you may be asked to draw ray diagrams showing:

1. The uncorrected defect — show where the image forms (in front of or behind the retina).
2. The corrected eye — show the corrective lens in front of the eye and the image now forming on the retina.

Drawing Tips

• Draw the eye as a simple circle or oval.
• Draw the retina as a line at the back of the eye.
• For the uncorrected eye, show the rays converging before the retina (short-sighted) or after the retina (long-sighted).
• For the corrected eye, draw the appropriate lens in front of the eye and show the rays now converging on the retina.

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Other Methods of Vision Correction

Contact Lenses

Contact lenses work in the same way as spectacle lenses (converging for long-sightedness, diverging for short-sightedness) but are placed directly on the surface of the eye (on the cornea). They are thinner and lighter than spectacle lenses.

Laser Eye Surgery

Laser surgery can reshape the cornea to correct vision defects:

• For short-sightedness: the laser makes the cornea flatter (less curved), reducing its refracting power.
• For long-sightedness: the laser makes the cornea more curved, increasing its refracting power.

Method	Advantages	Disadvantages
Spectacles	Non-invasive, easy to use, can be changed	Can be inconvenient, may break
Contact lenses	Less visible, good for sport	Risk of infection, need daily care
Laser surgery	Permanent correction, no lenses needed	Irreversible, risks of complications, expensive

Exam Tip: When comparing methods of vision correction, you need to weigh up the benefits and risks of each. The exam often asks for advantages and disadvantages — learn at least two of each for spectacles, contact lenses, and laser surgery.

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Worked Example — Choosing the Correct Lens

A patient cannot see distant objects clearly but can read a book without difficulty. Name the eye defect and state which type of lens is needed.

Answer:

• The patient can see near objects clearly but not distant objects → this is short-sightedness (myopia).
• Short-sightedness is corrected with a diverging (concave) lens.

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Worked Example — Calculating Lens Power (Higher)

A short-sighted patient is prescribed a lens with a focal length of −50 cm. Calculate the power of the lens.

Step 1: Convert to metres: f = −50 cm = −0.50 m

Step 2: Calculate power:

$P = \frac{1}{f} = \frac{1}{-0.50} = -2.0 \text{ D}$P=f1​=−0.501​=−2.0 D

The power of the lens is −2.0 D (negative, confirming it is a diverging lens).

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Summary