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Name: Roxana
Status: student
Grade: 9-12
Location: N/A
Country: N/A
Date: 5/31/2005

Why am I still able to see if I take off my glasses and look through a small hole that I make with my index finger and thumb?

An object is "out of focus" because the lens of the eye converges some rays behind your retina, some rays in front of the retina and some right on the retina. These different focal depths correspond to light that goes through different regions of the lens (moving outward from the center). If you limit the light to only go through the center of the lens then it has only one focal depth.

Greg Bradburn

Think about what has to happen for you to see some object clearly: light from each spot on the object has to land on one particular spot on your retina, and not on any other spot. If you are looking through a small hole, this is guaranteed, because there is only one path from any given spot on the object to your retina. Light that does not go in exactly the right direction does not make it through the pinhole, and so cannot blur the image you see.

But when the hole you are looking through gets bigger, you need some other way to make light from a spot on the object land all on one spot on your retina. A lens can do that. So, if you are looking through a large hole, you need a good, well focused lens; if you are looking through a small hole, the lens is irrelevant.

Tim Mooney

You have reinvented the pinhole camera! It works because, at least if the hole is infinitesimal, every point in the scene you are looking at illuminates just one point on the retina, To understand this, draw a retina (or a flat screen) and an object (perhaps an arrow) with a tiny hole between them, Drawing straight lines (because light travels in straight lines) from points on the object, through the pinhole and onto the screen will illustrate the principle.

There are disadvantages to pinholes, which is why lenses are popular. First a small hole lets through only a small amount of light whereas you would in general like to have a bright image. Second, light going through holes is diffracted, more so as the hole is made smaller. This means the light ray can be bent as it goes through the pinhole (light does not always travel in straight lines) and makes the image fuzzy and badly focused.

Best, Dick Plano, Professor of Physics emeritus, Rutgers University

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