Video Transcript
Which of the following is the
approximate minimum distance at which an eye with normal vision can focus light from
an object just as well as it can focus light from an object that is approximately
six meters away? Is it option (A) six centimeters,
option (B) 100 centimeters, option (C) 25 centimeters, or is it option (D) 60
centimeters?
This question is asking about the
near limit of a normal human eye. This limit is the closest distance
an object can be so that it is just as clear to see as another object that is
further away, in this case six meters away.
When an object is clear to see, the
light from the object is focused correctly. Light that is focused converges or
comes together at a point. An object will appear blurry when
the light rays do not converge. When an object is blurry, this is
called being out of focus. We can notice this near limit when
bringing objects closer to our eyes. They will start to blur at a
certain distance that is quite close to our eyes. This is the near limit. In this case, the pencil in the
first image is further away and is not blurry. It must be beyond the near
limit. In the second image, the pencil is
being held close to the eye. It is out of focus, or blurry, and
so must be at the near limit or closer.
We only clearly see an object if
light coming from it enters the eye and is focused on the retina. This is because the retina is the
part of the eye that is sensitive to light. To focus light coming from an
object, the eye contains a lens. A lens will focus light so that it
converges at a point on the retina. If that object is moved closer to
the eye so that it is at the near limit, the light rays from it no longer focus at
the retina. They instead meet the retina at
different points. When this happens, the object is
not clear and is blurred. If the object is moved even closer
to the eye, the distance between the object and the eye will be smaller than the
near limit. The object will become more blurry
than the object which was at the near limit as the light rays coming meet the retina
at two points which are a greater distance apart.
We will have to rely on our
experiences of the near limit to find the correct answer. 100 centimeters, option (B), is
equivalent to one meter. This cannot be the correct answer
as our experience tells us that objects only start to appear blurry much closer to
our eyes. Option (D), 60 centimeters, can be
pictured as two 30-centimeter rulers. This distance also cannot be the
correct answer, as our experience tells us that objects appear out of focus much
closer to our eyes. The correct answer is option (C),
25 centimeters. The near limit of a normal human
eye is 25 centimeters.
