Find the separation between two cones of the same type on the fovea of the eye by using the resolving power of the eye.

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Resolving Power Of the Eye

Objective: Find the separation between two cones of the same type on the fovea of the eye by using the resolving power of the eye.

Introduction: The retina contains two types of light detecting cells: rods and cones. Cones provide the eye’s colour sensitivity, rods, though more sensitive than cones do not detect colour. There is an area on the retina with a much higher density of cones called the fovea. When an object is observed its image is focused on the fovea. The fovea is a 0.3mm diameter area containing on rods and very thin densely packed cones. Cones can be divided into three types; one type detects each of red, green and blue light. The green and red cones are concentrated in the fovea centralis.

To measure the separation between two cones in the eye we can use the resolving power of the eye, for two objects to be resolved optically the viewer must be able to clearly differentiate two distinct bodies. (Rayleigh’s criterion:θ = λ/d)

Critical case shown where objects are just resolved

For two light sources of the same wavelength to be resolved the light must stimulate two cones on either side of one unstimulated cone.

Resolving power due to a circular aperture can be calculated by:

        θ = 1.22λ

                  d

Where:            θ = resolving power of optical instrument

            λ = Wavelength of light

            d = diameter of aperture

The resolving power of the eye will not be as high as this calculated theoretical resolving power as although the optical equipment of the eye may be of this power the eye’s detection facilities give the brain discreet not continuous signals and so the actual resolving power of the eye will never reach this theoretical value even if the optical facilities of the eye are perfect.

As a result we must calculate the separation between two cones using:

        θ = s                Where: s = separation

              r                         r = distance

Plan:

Factors identified affecting the experiment:

  • The separation of the two light sources. I will vary this to produce a range of distances from which the objects can be resolved.
  • The perpendicular distance that the viewer of the light sources can be from them and still completely resolve them. This will vary as a direct result of changes to the separation of the light sources.
  • The size and shape of the light sources. I will control this by shining the light through measured apertures made in black acrylic. I will control the size of the apertures by using a 1mm drill bit to create the holes through which the light sources will be shone, this will also ensure the sources are of a constant shape.
  • The amount of light that is reflected. I will use a black background surrounding the light sources to ensure that only a minimal amount of light is reflected.
  • The brightness of the two light sources must be equal to each other, I will achieve this by connecting the light sources in series to the same power source.
  • The brightness of the light sources must be constant as it is easier to resolve brighter objects. I will control this factor by keeping the voltage of the power source constant.
  • The wavelength of light must be kept constant by using the same equipment (same L.E.D.s) for each repeat of the experiment. This must be  controlled as different colours of light stimulate different cones on the retina and also the wavelength of light affects the calculated value for the resolving power of the eye.
  • The height of the eye relative to the light sources must be constant and perpendicular. This will be controlled by having the light sources at a height of 1 metre and having the viewer use a metre ruler as a guide to the height of their eyes when resolving the light sources.
  •  The size of the eye and the strength of eyesight must be constant throughout the experiment. This will be achieved by using the same observer  throughout the procedure and carrying out the procedure in a single session.
  • The brightness of the environment must be constant. This will be kept constant by carrying the experiment out in a single session. Also the environment will be kept as dark as possible to maximise the resolving power of the eye and so optimise the accuracy of the results.
  • The diameter of the pupil must be kept constant and as large as possible to allow as much light into the eye as possible. This will be kept constant as a direct result of controlling the brightness of the environment. (Variation of 1mm to 10mm possible.)
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Method: Set up the apparatus as shown in the previous diagram, switch on the power pack at 3.00v. Turn off the lights in the room and block out daylight coming from any windows or doors. Allow 2 minutes for the eyes to adjust to this darkness and do not allow any light into the room from this point up until the end of the procedure. The viewer of the lights must then stand on the masking tape and back away from the light sources following the masking tape using a metre ...

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