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Sight defects

  You are expected to know about the three common states of eyesight:-
  • The Normal Eye: The near point is the closest distance at which an object can be clearly focused by the eye. In the "normal eye" this distance is about 25 cm. The far point is the farthest distance at which an object can be clearly focused. In the "normal eye" this is infinity. Much of the population does not have "normal eyes" in this sense.
  • Myopia: Nearsightedness or myopia refers to an eye that can only focus on objects that are close. Distant objects are not seen clearly (ie the far point is not infinity). It can be caused by an eyeball that is too long or a cornea that has too much curvature. It is corrected by a diverging lens (concave)
  • Hypermetropia: Farsightedness or hypermetropia refers to an eye that can only focus on objects that are at a distance. Close objects are not seen clearly (ie the near point is somewhat greater than the "normal" 25 cm). It can be caused by an eyeball that is too short or a cornea that has too little curvature It is corrected by a converging lens (convex).

 
You are expected to be able to draw simple diagrams to represent the passage of light through the eye
  • Make sure that your diagrams clearly show which way the curved surfaces make the light bend!
  • It is good practice to continue the path of a ray through the boundary with a dotted line, draw in a normal to the surface and then draw in the refracted ray
  • The diagrams below do NOT do this - the computer drawn images would have been too cluttered.. but you should practice drawing these diagrams until you can draw them easily to clearly show refraction.

You are expected to know:

  • What happens to rays of light passing through a normal, short-sighted and long-sighted eye.

Viewing distant objects

(NB Refraction is only shown at the air/cornea boundary)

Viewing close-up objects

(NB Refraction is only shown at the air/cornea boundary)

 

Click here to go to sight correction of myopia and hypermetropea

Another common eyesight problem is astigmatism - click here to see the notes on astigmatism