Vision Correction. Focus it.
See how short sight (myopia) and long sight (hypermetropia) cause light to focus in front of or behind the retina, and how concave or convex corrective lenses bring the image back into focus.
Eye parameters
Focus diagnosis
📋 The three eye states (Cambridge)
Normal (emmetropic) eye: the cornea + lens system focuses parallel light from infinity exactly onto the retina. Near objects are focused by the lens becoming fatter (accommodation).
Short sight (myopia): the eyeball is too long, or the lens is too powerful. Light from distant objects focuses in front of the retina, giving a blurred image. The far point is closer than infinity.
Long sight (hypermetropia): the eyeball is too short, or the lens is too weak. Light from close objects focuses behind the retina, giving a blurred image. The near point is further than the normal 25 cm.
👓 Correction with lenses
- Myopia → concave (diverging) lens. The lens spreads incoming rays before they reach the eye, so they enter the eye as if from a closer point — moving the focal point back onto the retina.
- Hypermetropia → convex (converging) lens. The lens converges rays before they reach the eye, so the effective focal length is reduced — bringing the focal point forward onto the retina.
- Power is measured in dioptres: P = 1 / f (in m). Negative for diverging lenses, positive for converging.
🎯 Syllabus reference (0625)
- 3.2.4 The eye — recall that the eye contains a converging lens that focuses light onto the retina; identify short sight (myopia) and long sight (hypermetropia); describe their correction using concave and convex lenses respectively.
- Describe accommodation — change in focal length by changing lens curvature.
- Recall that the near point of the normal eye is at approximately 25 cm.