Question Video: Determining the Near Point of the Eye | Nagwa Question Video: Determining the Near Point of the Eye | Nagwa

# Question Video: Determining the Near Point of the Eye Science • Third Year of Preparatory School

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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 6 meters away? [A] 6 centimeters [B] 100 centimeters [C] 25 centimeters [D] 60 centimeters

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### 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.

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