Two common defects of the eyes are myopia (short sightedness) and…

The idea

The eye focuses light using the cornea and lens so that a sharp image falls on the retina. In myopia (short sightedness) the eyeball is too long or the lens is too powerful, so parallel rays from a distant object converge in front of the retina and the image is blurred. In hyperopia (long sightedness) the eyeball is too short or the lens is too weak, so rays from a near object would converge behind the retina. Both defects mean rays of light do not focus on the retina. For example, a myopic student cannot read a whiteboard clearly, while a hyperopic student struggles to read a book. The defect is identified by comparing where rays converge with the retina's position.

What earns the marks

• State that in a normal eye, light rays from an object are refracted by the cornea and lens to focus precisely on the retina.
• Define myopia as short sightedness, where the eyeball is too long or the lens too powerful, so rays from a distant object focus in front of the retina.
• Define hyperopia as long sightedness, where the eyeball is too short or the lens too weak, so rays from a near object would focus behind the retina.
• Explain that in both defects the image is blurred because the rays of light do not focus on the retina.
• Use ray diagrams or descriptions to compare the focal point with the retina's position for each defect.

Common mistakes and how to put them right

• Confusing the two defects.

  Put it right by linking each name to the distance that is unclear.

• Thinking the lens cannot change shape at all.

  Put it right: The defect concerns the resting focus of the eye, and the correction is about where rays converge, not about the lens being paralysed.

• Stating that light does not enter the eye.

  Put it right: Light does enter, but it focuses at the wrong place, so the image on the retina is blurred.

Exam tips

• Always name the defect and then state whether the focal point is in front of or behind the retina.
• Use the terms myopia and hyperopia alongside the everyday names short sightedness and long sightedness to show precise vocabulary.
• When drawing a ray diagram, label the retina, the lens and the focal point so the examiner can see the comparison.