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.