Lab 5_ Resolving Power of the Human Eye

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Lab: Resolving Power of the Human Eye


In this experiment, you will determine the resolving power of the human eye and
investigate on what factors it depends.


        Humans are a very clever lot. In our never-ending attempt to adapt to the world,
we sometimes encounter physical obstacles that might hinder the adaptive scheme. We
need to fell a tree to build a shelter, but our bare hands are inadequate for the purpose.
No matter, we extend the capabilities of our hands and make an axe. The tree is down.
We perceive a need to hurl an object a great distance and we invent catapults and
ICBM’s. We make extensions for our eyes (telescopes) and we see into the great dark
between the stars. Our struggle to adapt to the world has usually involved extending the
human body and its senses, and in doing so we have given ourselves greater control over
our environment and its influences.
        But just what can our bodies do? How fast can humans run, how faint a sound
can we hear? In this lab experiment, you will examine the resolving power of the human
eye; that is, for two point-like objects separated by a particular distance, how far away
can you be and still see the two objects as separate and distinguishable.
        We will express the resolving power of the human eye in terms of an angle. As
shown in the figure below, for two point-like objects a distance d apart, we need to
determine the largest distance D from the two objects so that they are still distinguishable
as separate. Once d and D are known the angle θ in the figure can be determined. We
will refer to θ as the resolving power of the human eye.
        When the triangle is long and thin as in the figure below, the angle θ is given to a
good approximation by θ = d/D. The angle θ in this relation is in radians. Recall that
there are 2π radians in a circle of 360˚, so if you want to convert an angle in radians to
one in degrees, you need to multiply by 360/2π ≈ 57.29.

        In science, and often in everyday affairs, a well-phrased question is frequently
half the answer. Unfortunately the question “What is the resolving power of the human
eye?” is not well-phrased. Does the angle θ depend on the experimental arrangement? If
it does, then a unique answer cannot be provided. In this experiment you will investigate
the parameters influencing the resolving power and attempt, nonetheless, to answer the
central question.

The Experiment:

Unlike other experiments you will be doing this semester, you will not be given step-by-
step instructions; only some general guidelines and ideas will be offered.

   1. Your point-like objects should be separated by about 3-5 mm. The point-like
      objects themselves should be no larger than about ½ mm in diameter; that is, they
      must be small when compared to their separation.
   2. Does it matter that the point-like objects are black dots on white paper or white
      dots on black paper?
   3. Does it matter whether you start far away and approach the point-like objects or
      stand nearby and then move away?
   4. Do you want to make a single measurement to answer a question, or do you want
      to make multiple measurements and average?
   5. How do your results compare with your partner’s and with others?

Available for your use are various supplies to help you. You may also want to discuss
with your instructor other ideas for factors influencing the resolving power of the eye.


   1. Write a short account of what factors influence the resolving power of the human
      eye and how they influence it.
   2. How would you answer the question: “What is the resolving power of the human
   3. It is claimed that an eagle can spot a mouse at 3,000 ft (≈ 1,000 m), although it’s
      not certain just what the eagle is detecting. Estimate the resolving power of the
      eagle’s eye and compare it to your own.
   4. Most people cannot focus their eyes on objects closer than about 10 cm (≈ 4
      inches) from the eye. Using the results of your investigation, what is the smallest
      object that you can see?

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