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1168 Chapter 26 | Vision and Optical Instruments
images, alluded to by Essential Knowledge 6.E.4, can be magnified, shrunk, or inverted, depending upon the lens arrangement.
When a new medium is not fully transparent, the incident light may be reflected or absorbed, and some light may be transmitted. This idea, referenced in Essential Knowledge 6.E.1, is utilized in the construction of telescopes. By relying on the law of reflection and the idea that reflective surfaces can be used to form images, telescopes can be constructed using mirrors to distort the path of light. This distortion allows the person using the telescope to see objects at great distance. While household telescopes utilize wavelengths in the visible light range, telescopes like the Chandra X-ray Observatory and Square Kilometre Array are capable of collecting wavelengths of considerably different size. Essential Knowledge 6.E.2, 6.E.4, and 6.F.1 are all addressed within this telescope discussion.
While ray tracing may easily predict the images formed by lenses and mirrors, only the wave model can be used to describe observations of color. This concept, covered in Section 26.3, underlines Essential Knowledge 6.F.4, the idea that different models of light are appropriate at different scales. The understanding and utilization of both the particle and wave models of light, as described in Enduring Understanding 6.F, is critical to success throughout this chapter.
Big Idea 6 Waves can transfer energy and momentum from one location to another without the permanent transfer of mass and serve as a mathematical model for the description of other phenomena.
Enduring Understanding 6.E The direction of propagation of a wave such as light may be changed when the wave encounters an interface between two media.
Essential Knowledge 6.E.1 When light travels from one medium to another, some of the light is transmitted, some is reflected, and some is absorbed.
Essential Knowledge 6.E.2 When light hits a smooth reflecting surface at an angle, it reflects at the same angle on the other side of the line perpendicular to the surface (specular reflection); and this law of reflection accounts for the size and location of images seen in plane mirrors.
Essential Knowledge 6.E.3 When light travels across a boundary from one transparent material to another, the speed of propagation changes. At a non-normal incident angle, the path of the light ray bends closer to the perpendicular in the optically slower substance. This is called refraction.
Essential Knowledge 6.E.4 The reflection of light from surfaces can be used to form images.
Essential Knowledge 6.E.5 The refraction of light as it travels from one transparent medium to another can be used to form images.
Enduring Understanding 6.F Electromagnetic radiation can be modeled as waves or as fundamental particles.
Essential Knowledge 6.F.1 Types of electromagnetic radiation are characterized by their wavelengths, and certain ranges of wavelength have been given specific names. These include (in order of increasing wavelength spanning a range from picometers to kilometers) gamma rays, x-rays, ultraviolet, visible light, infrared, microwaves, and radio waves.
Essential Knowledge 6.F.4 The nature of light requires that different models of light are most appropriate at different scales.
26.1 Physics of the Eye
Learning Objectives
By the end of this section, you will be able to:
• Explain the image formation by the eye.
• Explain why peripheral images lack detail and color.
• Define refractive indices.
• Analyze the accommodation of the eye for distant and near vision.
The information presented in this section supports the following AP® learning objectives and science practices:
• 6.E.5.1 The student is able to use quantitative and qualitative representations and models to analyze situations and solve problems about image formation occurring due to the refraction of light through thin lenses. (S.P. 1.4, 2.2)
The eye is perhaps the most interesting of all optical instruments. The eye is remarkable in how it forms images and in the richness of detail and color it can detect. However, our eyes commonly need some correction, to reach what is called “normal” vision, but should be called ideal rather than normal. Image formation by our eyes and common vision correction are easy to analyze with the optics discussed in Geometric Optics.
Figure 26.2 shows the basic anatomy of the eye. The cornea and lens form a system that, to a good approximation, acts as a single thin lens. For clear vision, a real image must be projected onto the light-sensitive retina, which lies at a fixed distance from the lens. The lens of the eye adjusts its power to produce an image on the retina for objects at different distances. The center of the image falls on the fovea, which has the greatest density of light receptors and the greatest acuity (sharpness) in the visual field. The variable opening (or pupil) of the eye along with chemical adaptation allows the eye to detect light intensities from the
lowest observable to times greater (without damage). This is an incredible range of detection. Our eyes perform a vast
number of functions, such as sense direction, movement, sophisticated colors, and distance. Processing of visual nerve impulses begins with interconnections in the retina and continues in the brain. The optic nerve conveys signals received by the eye to the brain.
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