Optics Demonstration with Student Eyeglasses Using the Inquiry Method
A student's face is viewed from two angles to observe the distortion in the outline of her face caused by the left and right lenses. The student's left lens is a converging lens for farsightedness that causes the left outline of her face viewed through the lens to extend beyond the actual outline of her face. The slight prescription in her right lens is not apparent. The shapes created by projected light through both lenses reveal different prescriptions along perpendicular axes (astigmatism), resulting in projection shapes that do not exactly coincide with the shapes of the lenses.
Diverging lenses for nearsightedness cause the outline of the student's face on both sides to be drawn inward. Since the effect is slightly more pronounced viewing the student's left eye, we conclude that the left lens has a stronger correction. The projection of the student's lenses also reveals a slightly stronger prescription for the student's left eye since the size of the left lens projection is slightly larger and more diffuse than the right. Comparing this student's rectangular lenses to another student's oval lenses, it is apparent that the rectangular lenses are much stronger since the bright projected areas are so much larger than the actual lenses for the rectangular lenses. Note that the shadows cast by all lenses shown in the bottom image are minified since we are observing the shadow images through the diverging lenses. The fact that the shadows observed in the rectangular lenses are more strongly minified compared to the oval lenses provides us with further evidence that these lenses are more powerful.
Light is projected through bifocal-type lenses. All four lens components are converging for farsightedness. The reading lens components have shorter focal lengths (higher power) than the lens components used for distance vision. The screen distance is less than the reading focal lengths in the top photo and approximately equal to the reading focal lengths in the middle photo. The bottom photo illustrates that when the screen distance is increased beyond the reading focal lengths, the projection is inverted and the size of the bright projection increases since rays diverge after passing through the focal point.
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