Desarrollo de los Premios Nacionales a la Calidad

This activity allows students to continue their hands-on work with infrared radiation by building a simple transmitter and receiver. This activity provides a great opportunity for students to build significant connections between their work with circuits in Chapter 1 and the use of electromagnetic radiation in communication

technologies in this chapter. Most of the materials are things that are familiar to students from their work earlier in this lesson or in Chapter 1.

The light-emitting diode (LED) and the capacitor are exceptions to the previous statement. This should not be a problem because students are given sufficient background information about these two components in the handout “Building an Infrared Transmitter and Receiver.” These components can be obtained from a local electronic supplies store. If you look up “Electronic Equipment & Supplies—Retail” in the Yellow Pages, you should be able to find a supplier. The best sources for inexpensive electronic supplies are typically found in industrial parks, not in shopping malls.

Part B: Testing the Infrared Transmitter and Receiver Procedure and Observations

The transmitter and receiver work best when the lens (top curved surface) of the LED is aimed directly at the flat surface of the photovoltaic cell. This allows the photovoltaic cell to capture the maximum amount of infrared radiation. This reasoning also explains why it is best to have the photovoltaic cell as close as possible to the LED. Depending upon the strength of the 1.5-V cell and the sensitivity of the photovoltaic cell, most students will be able to hear faint transmissions up to a distance of 0.5 m away. Although the infrared radiation does have some penetrating ability, as demonstrated by the signal’s ability to penetrate one sheet of tissue, it does have its limitations. At a distance of 8 cm, six sheets of tissue are able to completely absorb the energy, thereby breaking the communication link to the receiver.

analysis

1. The short range of this communication technology, combined with the need to have an uninterrupted “line of sight” between the transmitter and receiver, explains many of the strengths and weaknesses of this new communication technology. The weakness is that unless the transmitter and receiver are in the same room and are quite close together, this technology will not work—this technology would not be suitable for mobile communication or for communication between people separated by walls in the same building. Surprisingly, this weakness could also be considered a strength because it would be very difficult to eavesdrop or to electronically “bug” a conversation between two people using this technology.

2. At the time this textbook was published, infrared communication technologies were relatively new. The following lists represent the strengths and weaknesses of this technology in the early stages of its development:

• Strengths

– It is ideally suited to applications where connecting cables would be a nuisance or a hazard, such as when you want to send information from a laptop or a handheld computer to a printer. Other wireless applications would include beaming business cards between two handheld computers or beaming images from a digital camera to a computer.

– New applications are being found in hospitals, laboratories, airports, and aircraft because this technology causes none of the interference problems that are common among devices that use radio waves for communication.

– Applications where information must be kept secure are finding infrared communication technologies to be valuable because there are none of the “leakage” problems that can occur with radio-wave communications. Science 30 © 2007 Alber ta Education (www .education.go v.ab .ca). Third-par ty cop

yright credits are listed on the attached cop

• Weaknesses

– In bright sunlight, infrared receivers can become saturated with infrared radiation from the Sun. Under these circumstances, incoming messages get lost in all the background radiation.

– Two devices must be facing each other, with an uninterrupted line of sight.

– The range is limited, with a maximum distance between devices being about 30 cm for handheld computers or about 5 m for leading-edge systems.

– Although window glass is transparent to infrared radiation, at large angles enough reflection occurs that the transmission becomes too weak to be useable.

– Infrared radiation can be absorbed by snow or rain, so this technology may not work well outdoors.

Practice, page 428

16. Visible light is produced by hot sources, such as the filaments of light bulbs, the flames of candles, and the surface of the Sun.

17. A photon of red light has many things in common with a photon of violet light. Both photons consist of electric and magnetic fields that travel at right angles to one another in an electromagnetic wave. The fact that both photons are a type of EMR means that they travel at the same speed, 3.0 ¥ 108 _{m/s, and that they}

share all the properties common to electromagnetic waves. Both photons can be detected by the human eye and, therefore, both are a part of the visible spectrum. The main differences have to do with wavelength, frequency, and energy. Photons of violet light have a shorter wavelength but a higher frequency and a larger energy content than do photons of red light.

18. The photons from the red and violet ends of the spectrum are absorbed by chlorophyl molecules during photosynthesis. The leaves of most plants look green because photons of green light are not absorbed during photosynthesis and, thus, are reflected or passed through the leaves to people’s eyes.

19. A chlorophyl molecule is like an antenna because it is able to absorb light energy in the form of photons and convert that energy into the energy of moving electrons. The leaves of a plant turn toward a light source to expose the maximum surface area and, therefore, the maximum number of chlorophyl molecules to the incoming light photons. This allows the leaves to absorb as much energy as possible.

Practice, page 430

20. UV photons are more hazardous to living tissue than photons of visible light because UV photons have more energy.

21. a. A sunblock with homosalate is able to absorb the energy of UVB radiation before it penetrates the skin.

b. A sunblock with zinc oxide is able to absorb the energy of UVA radiation before it penetrates the skin.

c. UVC rays are blocked about 50 km above Earth’s surface by a region of the atmosphere called the ozone layer, so there is no need to block these rays with sunblock. If the ozone layer continues to deteriorate, shielding from UVC could become a need.

Science 30 © 2007 Alber ta Education (www .education.go v.ab .ca). Third-par ty cop

yright credits are listed on the attached cop

Practice, page 432

22. The tiny screws appear white in the image because the X-rays are unable to penetrate this material, leaving a shadow on the photographic film.

23. The teeth on the lower left of the X-ray have dark areas inside, indicating that the X-rays were able to penetrate these areas and develop the photographic film. Since dental X-rays are not able to penetrate dense materials, like bone and the hard exterior of teeth, teeth must have soft tissues inside them.

24. f c = ¥ = ¥ = 7 1 10 3 00 10 18 8 . . ? Hz m/s l c f c f = = = ¥ ¥ = ¥ - l l 3 00 10 7 1 10 4 2 10 8 18 11 . . . m/s Hz m The wavelength of the X-ray photons is 4.2 ¥ 10- 11 _m.

25. The doctor would ask the woman if there is a chance that she could be pregnant because the rapidly dividing cells of an unborn child would be very susceptible to damage from X-ray radiation.