This colorful burst of “spaghetti” is really a bundle of optical fibers. These are hair-thin threads of glass that transmit laser light that has been encoded with messages. Optical fibers are a crucial component of modern communications. The use of light in devices such as these is possible because of optics.
Optics and Optical Instruments
Optics is the study of visible light and the ways it can be used to extend human vision and do other tasks. Knowledge of light was needed for the invention of optical instruments such as microscopes, telescopes, and cameras, in addition to optical fibers. These instruments use mirrors and lenses to reflect and refract light and form images.
Q: What is an image?
A: An image is a copy of an object created by the reflection or refraction of visible light.
A light microscope is an instrument that uses lenses to make enlarged images of objects that are too small for the unaided eye to see. A common type of light microscope is a compound microscope, like the one shown in the Figure below. A compound microscope has at least two convex lenses: one or more objective lenses and one or more eyepiece lenses. The objective lenses are close to the object being viewed. They form an enlarged image of the object inside the microscope. The eyepiece lenses are close to the viewer’s eyes. They form an enlarged image of the first image. The magnifications of all the lenses are multiplied together to yield the overall magnification of the microscope. Some light microscopes can magnify objects more than 1000 times! For more on light microscopes and the images they create, watch the video at this URL: http://www.youtube.com/watch?v=Xo7mr90GYLA
Q: How has the microscope advanced scientific knowledge?
A: The microscope has revealed secrets of the natural world like no other single invention. The microscope let scientists see entire new worlds, leading to many discoveries—especially in biology and medicine—that could not have been made without it. Some examples include the discovery of cells and the identification of bacteria and other single-celled organisms. With the development of more powerful microscopes, viruses were discovered and even atoms finally became visible. These discoveries changed our ideas about the human body and the nature of life itself.
Like microscopes, telescopes use convex lenses to make enlarged images. However, telescopes make enlarged images of objects—such as distant stars—that only appear tiny because they are very far away. There are two basic types of telescopes: reflecting telescopes and refracting telescopes. The two types are compared in the Figure below. They differ in how they collect light, but both use convex lenses to form enlarged images. You can learn more about telescopes and how they evolved in the video at this URL:
A camera is an optical instrument that forms and records an image of an object. The image may be recorded on film or it may be detected by an electronic sensor that stores the image digitally. Regardless of how the image is recorded, all cameras form images in the same basic way, as shown in the Figure below.
- Light passes through the lens at the front of the camera and enters the camera through an opening called the aperture.
- As light passes through the lens, it forms a reduced real image. The image focuses on film (or a sensor) at the back of the camera. The lens may be moved back and forth to bring the image into focus.
- The shutter controls the amount of light that actually strikes the film (or sensor). It stays open longer in dim light to let more light in.
For a series of animations showing how a camera works, go to this URL: http://www.shortcourses.com/guide/guide1-3.html
Did you ever see a laser cat toy like the one in the Figure below? A laser is a device that produces a very focused beam of visible light of just one wavelength and color. Waves of laser light are synchronized so the crests and troughs of the waves line up. The diagram below shows why a beam of laser light is so focused compared with ordinary light from a flashlight.
The following Figure below provides a closer look at the tube where laser light is created. Electrons in a material such as a ruby crystal are stimulated to radiate photons of light of one wavelength. At each end of the tube is a concave mirror. The photons of light reflect back and forth in the tube off these mirrors. This focuses the light. The mirror at one end of the tube is partly transparent. A constant stream of photons passes through the transparent part, forming the laser beam. You can see an animation showing how a laser works at this URL: http://www.youtube.com/watch?v=gUbBzEXlEho
Besides entertaining a cat, laser light has many other uses. One use is carrying communication signals in optical fibers. Sounds or pictures are encoded in pulses of laser light, which are then sent through an optical fiber. All of the light reflects off the inside of the fiber, so none of it escapes. As a result, the signal remains strong even over long distances. More than one signal can travel through an optical fiber at the same time, as you can see in the Figure below. Optical fibers are used to carry telephone, cable TV, and Internet signals.
The optical fiber in the diagram is much larger than a real optical fiber, which is only about as wide as a human hair.
Q: When lasers were invented in 1960, they were called "a solution looking for a problem.” Since then, they have been put to thousands of different uses. Can you name other ways that lasers are used?
A: The first widespread use of lasers was the supermarket barcode scanner, introduced in 1974. The compact disc (CD) player was the first laser-equipped device commonly used by consumers, starting in 1982. The CD player was quickly followed by the laser printer. Some other uses of lasers include bloodless surgery, cutting and welding of metals, guiding missiles, thermometers, laser light shows, and acne treatments.
- Optics is the study of visible light and the ways it can be used to extend human vision and do other tasks. Optical instruments are based on optics. They use mirrors and lenses to reflect and refract light and form images.
- The light microscope and telescope use convex lenses and mirrors to make enlarged images of very tiny or distant objects. A camera uses a convex lens to make a reduced image of an object.
- A laser is a device that produces a very focused beam of visible light of just one wavelength and color. Pulses of laser light carry communication signals through optical fibers.
laser: Device that produces a very focused beam of light of just one wavelength and color.
optics: Study of visible light and the ways it can be used to extend human vision and do other tasks.
At the following URL, practice using a telescope with the telescope simulator. Select an object to view, and then try different combinations of aperture and eyepiece size. For each combination, adjust the focus until the image is sharp and clear. Make a data table to record 10 different combinations of aperture and eyepiece size and the resulting magnification. Draw one conclusion based on the data in your completed table.
- Define optics.
- Explain how a compound microscope uses convex lenses to make an enlarged image of a tiny object.
- Compare and contrast reflecting and refracting telescopes.
- Write a paragraph explaining how a camera works. Assume you are explaining it to a much younger student. Draw a sketch as a visual aid.
- Explain how reflection is involved in the production of laser light.
- How are optical fibers used?