Lenses

Modeling the normal eye, the refractive defects and their correction using an optical bench

Theory of the method:

Lenses (to download)

Materials:

  • Optical bench (source of light, racks for lenses, white screen, transformer)
  • Set of lenses (test-eye set)

Procedure:

A. Modeling emmetropia (normal eye)

1. Set the optical bench according to the corresponding elements for modeling

  • object = incandescent lamp
  • human lens = lenses from the kit (spherical converging lens with OP higher than 5 diopters)
  • retina = screen
  • final image on the retina = the image of the incandescent lamp on the screen

2. Obtain the image on the screen: 2 options:

  • the image is higher than the object: the lens is situated close to the object
  • the image is smaller than the object (a small bright dot): the lens is closed to the screen

3. Calculate the convergence of the lens used for modeling emmetropic eye

  • Select the dot-like image on the screen
  • Measure the distances on the optical bench: x1 (object – lens) and x2 (lens – screen)
  • Calculate the optical power of the lens using formula: P = 1/x2 + 1/x1
  • Check the calculated value with the diopter from the lens

B. Modeling refractive defects of the human eye and their correction

1. Modeling myopia and its correction

a. Select the dot-like image on the screen – similar to the emmetropic eye model

b. Remove the screen a few cm behind of the initial place (= the image will be formed before the retina); it can be noticed that x1m = x1; x2m > x2; Pm < Pe (x1m = object – lens distance in case of myopia; x2m = lens – screen distance in case of myopia; Pm = optical power in the case of myopia; Pe = optical power in case of emmetropia).

c. Calculate the optical power in the case of myopic eye

d. Calculate the optical power of the lens that should be used to correct the myopia: Pc = Pm – Pe (Pc = optical power of the lens used for correction; Pm = optical power in the case of the myopia; Pe = optical power in the case of an normal eye)

e. Check on the optical bench the correction of the myopia using a lens with corrected optical power (the image on the screen should be again a dot-like bright spot)

2. Modeling hyperopia and its correction

a. Select the dot-like image on the screen – similar to the emmetropic eye model

b. Remove the screen a few cm before the initial place (= the image will be formed behind the retina); it can be notice that: x1h = x1; x2h < x2; Ph > Pe (x1h = object – lens distance in case of hyperopia; x2h = lens – screen distance in case of hyperopia; Ph = optical power in the case of hyperopia; Pe = optical power in case of emmetropia)

c. Calculate the optical power in the case of the hyperopic eye.

d. Calculate the optical power of the lens that should be used to correct the hyperopia: Pc = Ph – Pe (Pc = optical power of the lens used for correction; Ph = optical power in the case of the hyperopia; Pe = optical power in the case of an normal eye)

e. Check on the optical bench the correction of the hyperopia using a lens with calculated corrected optical power (the image on the screen should be again a dot-like bright spot)

3. Modeling astigmatism and its correction

a. Select the dot-like image on the screen – similar to the emmetropic eye model

b. Add one cylindrical lens (= modeling astigmatism)

c. Add one more cylindrical lens, but the opposite sign + correct the optical axis (= astigmatism – correction)

4. Place back in the test-eye set all the used lenses; please, place the lenses according the their diopters

5. State 1 – 3 conclusions