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A single optical element might be useful for a bathroom mirror or magnifying glass, but most technologies that use optics include more than one optical device and are called compound optics. The refracting telescope and microscope are historically important optical instruments that produce inverted and magnified images. In their basic design, both use two lenses; the image formed by the first lens acts as the object for the second lens. Modern binoculars use multiple lenses, but they also take advantage of internal reflections within a pair of prisms so that the image is upright. The human eye uses a lens with variable shape (or focal length) to focus on objects either nearby or far away. Within the human eye, rods and cones detect the intensity and colors of light, respectively. 
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rods, cones
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Review problems and questions |
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- Do the lenses in a camera produce a real or a virtual image?
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The images produced in a camera must be real, because they must be projected onto film or a CCD detector. 
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- Why is your vision sharpest when the pupils of your eyes are contracted or very narrow?
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The same principles of a pinhole camera apply here: The narrower the aperture or opening for light to enter, the sharper the image formed because light from one point on the object has only one narrow path it can take to the image location. A narrow opening also restricts the light rays to be close to the optical axis, which minimizes potential optical aberrations of the eye's lens.
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- Identify the part of the human eye that causes each of the following vision problems and what is used to correct for it.
- farsightedness
- nearsightedness
- color blindness
- astigmatism
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- The lens in the eye causes the vision problem, which can be corrected using by a converging lens (eyeglasses or contact lenses).
- The lens in the eye causes the vision problem, which can be corrected by using a diverging lens (eyeglasses or contact lenses).
- The cones in the eye cause the vision problem, which cannot be corrected (although devices exist, such as mobile phone applications, to assist with everyday living).
- The cornea in the eye causes the vision problem, which can be corrected by using a cylindrical lens (eyeglasses or contact lenses).
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- Imagine using the optical telescope you constructed in Investigation 21D on page 622 to create an image on a screen of the letter “F” printed on a piece of paper. Now cover up the top half of the entrance (or objective) lens for the telescope.
- Will you continue to see the entire letter “F” or only part of it?
- Will the image magnification be changed?
- Will the brightness of the image change?
- Explain your answers for parts a through c.
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- The entire letter “F” will still be imaged.
- The image magnification will be unchanged.
- The brightness of the image will be reduced (by half).
- A lens redirects light rays to form an image, and the image is formed where the light rays meet. Light rays from one point on the object spread out in all directions, and so may enter the lens at many different points, but will all be redirected to converge at one particular point at the image—such as at the top of the letter “F”. By covering up part of the lens, you are not covering up part of the image; instead, you are reducing the number of light rays from the object that pass through the lens to form each and every part of the image.
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- A microscope usually has a rotating set of objective lenses of different focal lengths. A student was using the objective lens with a 10 mm focal length and found that the images were magnified 40 times. What will happen to the magnification if the student switches to an objective lens with a shorter, 4 mm focal length?
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The new magnification will be 100 times. The microscope will now focus at a distance that is closer by a factor of 2.5 than that of the first objective. Since the object distance is shorter by this factor, while the image distance is basically the same (since the microscope’s height is unchanged), the magnification will be 2.5 times higher from equation (21.4), m = −di / do.
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