# Vision and Optical Instruments

## Connection for AP® Courses

Seeing faces and objects we love and cherish—one’s favorite teddy bear, a picture on the wall, or the sun rising over the mountains—is a delight. Intricate images help us understand nature and are invaluable for developing techniques and technologies in order to improve the quality of life. The image of a red blood cell that almost fills the cross-sectional area of a tiny capillary makes us wonder how blood makes it through and does not get stuck. We are able to see bacteria and viruses and understand their structure. It is the knowledge of physics that provides the fundamental understanding and the models required to develop new techniques and instruments. Therefore, physics is called an enabling science—it enables development and advancement in other areas. It is through optics and imaging that physics enables advancement in major areas of biosciences.
This chapter builds an understanding of vision and optical instruments on the idea that waves can transfer energy and momentum without the transfer of matter. In support of Big Idea 6, the way light waves travel is addressed using both conceptual and mathematical models. Throughout this unit, the direction of this travel is manipulated through the use of instruments like microscopes and telescopes, in support of Enduring Understanding 6.E. 
When light enters a new transparent medium, like the crystalline lens of your eye or the glass lens of a microscope, it is bent either away or toward the line perpendicular to the boundary surface. This process is called “refraction,” as outlined in Essential Knowledge 6.E.3. In both the eye and the microscope, lenses use refraction in order to redirect light and form images. These 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.