11.4 The Optics of Other Devices

projection head Activity 11.4.1 Optics of an Overhead Projector focus knob Overhead projectors (Figure 1), like many optical systems, consist of three sys- tems that work together: a mechanical system, an electronic system, and an optical system. Their function is to project an enlarged image from a transparent film onto a distant screen. In this activity, you will see how the different optical optical components of the projector work together. stage Materials overhead projector appropriate screwdrivers projector case Procedure Figure 1 1. Before turning on the overhead projector, open the optical stage to see An overhead projector inside the projector case. Sketch the arrangement of optical components by considering what a cross-section of the projector would look like. Note the arrangement of any bulbs, mirrors, or lenses that you find in the projector case. Add the optics of the projection head to your sketch. 2. Turn on the projector to project an image of a letter onto a screen nearby. Make adjustments to focus the image. 3. Use the focus knob to move the projection head upward. How does this affect the image? Refocus the image. 4. Use the focus knob to move the projection head downward. How does this affect the image? Refocus the image. 5. Move the projector farther from the screen. How does this affect the image? Analysis (a) Draw a ray diagram, with at least three different rays, showing how light travels from the bulb to the screen. (b) In table form, describe the structure and function of each optical compo- nent of the overhead projector. (c) In what ways do you think an overhead projector is similar to a micro- scope? In what ways is it different?

Slide or Movie Projector Figure 2 shows the arrangement of lenses and mirrors in a typical slide or movie projector. The source of light is usually a tungsten filament lamp or a quartz iodide lamp that produces the bright light necessary to illuminate a slide or movie. Two converging lenses refract the light so that the object to be projected is uniformly illuminated. The object is placed a distance of one to two focal lengths in front of the projection lens, resulting in a large, real image.

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converging mirror object light source (slide or movie)

Figure 2

image In a slide or movie projector the image is inverted, so the slide or film must be inverted vertically and horizontally when placed in the projector. The projection lens is mounted in a condenser projection lens sliding tube or geared mount so that it can be screen moved to focus the image.

Large-Screen Projection Systems While attending Expo 67 in Montreal, three Canadian filmmakers—Graeme Ferguson, Roman Kroitor, and Robert Kerr—were inspired by the effects created by the multi-projector systems being used. Their goal was to develop a cine- matographic projection system that would give the audience a sensation of reality with the size and clarity of its images. That same year, they founded IMAX Corporation in Mississauga, Ontario. The first permanent giant-screen theatre opened at Ontario Place in Toronto in 1971. To capture a large field of view, special wide-angle lenses had to be designed for a new camera system. The lens illustrated in Figure 3 has a field of view of 150°. To obtain enough clarity in the projected image, each film frame (the storage medium) had to be much larger than the regular 35-mm and conven- tional 70-mm film formats that are still the standard for motion pictures.

front element moves for focus

filter

Figure 3

As shown in Figure 4, conventional 70-mm film runs vertically in conven- tional 70-mm cameras, and each frame is only five perforations long. (The perforations are the holes on each side of the film that keep the film on track as it runs through the camera.) In IMAX cameras, the film width is still 70 mm, but each frame is 15 perforations long, and the film runs horizontally through the camera. Since the film runs horizontally and the film frame is longer, each film frame used in IMAX photography is much larger than the film frames of con- ventional motion picture photography.

Optical Instruments 403 Figure 4 Standard film formats compared with IMAX film (far right). Such a large film format To project images from this film onto a huge screen, a powerful light source increased the weight of a film roll to the is needed. Typical lamps for large-screen theatres are rated at an average of extent that a new method of moving the film 15 000 W. A typical lens exposed to light of such high intensity would absorb so across the projection lens was needed. During much heat that it could explode or crack; therefore, a special cooling system is projection, the film is fed through the pro- used to remove some of the heat generated by these lamps. IMAX projection jector horizontally instead of vertically. To lenses are specially designed to withstand enormous amounts of heat. eliminate vibrations in the film as it moves, it is held firmly against the rear element of the lens by a vacuum. Practice

Understanding Concepts 1. (a) What is the main function of all projectors? (b) Do all projectors produce the same type of image? Explain.

Applying Inquiry Skills 2. (a) Describe two different sets of measurements you could make to calculate the magnification of the overhead projector used in your classroom. (b) Estimate the magnification of the overhead projector when it is used at its typical location. (c) With your teacher’s approval, apply your suggestions in (a) above to check your estimate.

SUMMARY The Optics of Other Devices

• Different arrangements of optical components can be designed to manipu- late light in many ways. • Overhead projectors, slide and movie projectors, and large screen projec- tion systems are examples of complicated optical devices.

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Section 11.4 Questions

Understanding Concepts 1. State the four characteristics of the image produced by a movie projector. Does the image of a slide projector have the same characteristics? 2. Explain why the lens in an IMAX system must be a wide-angle lens.

Making Connections 3. OMNIMAX theatres, such as the one in the Ontario Science Centre in Toronto, use advanced optical technology. (a) What does the prefix “omni” mean? How does the meaning relate to an OMNIMAX movie presentation? (b) Use the Internet to find out about OMNIMAX theatres, film, and movie projectors. Describe how the features compare with IMAX features. Follow the links for Nelson Physics 11, 11.4. GO TO www.science.nelson.com

11.5 Construction of Optical Instruments By now you have seen ray diagrams for many optical instruments. You have also studied overhead projectors, cameras, and microscopes. Using two lenses, you observed how we can magnify objects that we can’t see because they are either too small or too far away. In this section you will use your knowledge to construct and refine an optical instrument of your choice. But first we look at how instru- ments control the position of lenses and the amount of light. Controlling the Position of Lenses Recall that an object positioned between the focus and the optical centre of a converging lens produces a virtual image. If the lens were moved farther away, the object would be located at a distance longer than the focal length, producing a real, inverted, and smaller image. For this reason, there are many different methods of moving lenses to allow ease of focusing. Figure 1 In inexpensive plastic microscopes and hand lenses (Figure 1), a lens is Simple hand lenses use a threaded eyepiece mounted into a hollow, threaded tube that is connected to a base. The object dis- to adjust the position of the lens. tance can be varied by twisting the lens clockwise or counterclockwise to move the lens up or down. The simplest way to control the relative positions of two lenses is to mount the lenses in a sliding tube (Figure 2), like the slide of a trombone.

Figure 2 In an alignment telescope, object and image distances can be adjusted with a sliding tube.

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