Refraction, Eyes, and Optical Instruments

[Bernard NebelKeymaster]

Begin by having students conduct the exercises described in the text that demonstrate that light rays are bent as they enter or leave water (or other transparent materials), a phenomenon called REFRACTION. (Emphasize how it is distinct from reflection). Likewise, have them experiment with a pocket magnifier lens and observe how it will cast images as well as magnify.  Emphasize that both the magnifying capacity and the image casting ability of a lens is based on refraction. Explanations as to why refraction occurs and how this results in casing images can can be explored as you wish but usually waits for high school or college level physics courses. At this level, it is sufficient to have students become familiar with the fact that these things happen as they do.

In experimentation with a pocket magnifier casting an image, have kids take note of how they must carefully adjust the distance between the lens and screen or between their eye and lens to get a clear image. This adjustment of distance is known as FOCUS, i.e. they must adjust the focus to get a clear image. Yes, in adjusting the focus of their microscopes, cameras, or telescopes, they are simply adjusting the distance between component parts.

Eyes, Cameras, Microscopes

The optics of eyes and cameras is the same as using the lens of a pocket magnifier to cast an image of a distant object on a piece of paper.

Type into your browser: diagram of eye/ The lens at the front of the eye casts an image of what is being looked at on the retina (the “screen”) at the back of the eye. The retina is comprised of light and color sensitive receptor (rod and cone nerve) cells. Each one will send impulses to the brain according to the intensity and color of light falling on it. Consider that each rod or cone cell acts as a single pixel in a TV screen. In the brain the impulses from all rod and cone cells (“pixels”) are integrated into the image we see.

Kids with normal vision often ask a kid with glasses, “What do things look like without your glasses.” The answer is: Out of focus. The lens of the eye is casting an image that is a bit in front of or behind the retina. Therefore, what is seen is a blurry, out-of-focus image.  Fortunately, the problem is easily corrected with glasses, i.e., a lens that, in conjunction with the eye lens, cast the image exactly on the retina.

In a camera, the lens in front similarly casts a image on a screen that is comprised of pixels, each one responding to the intensity and color of light falling on it, and these are transmitted to make the picture.

In a compound microscope there are two lenses at work. The objective lens being very close to the specimen casts an enlarged image of the specimen to a point up the tube of the microscope. (Note that an image can exist in a plane in space whether or not there is screen present.) The eye piece is a magnifier lens focused on that enlarged image. Thus, the total magnification achieved by a compound microscope is the power of the objective lens times the power of the eyepiece.

Making images without light 

With modern technology it is possible to make images without light. Sonograms are a well known example. See:

Of course our eyes still depend on light. Thus, it is a matter of changing  the sound waves into images of light and dark.

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