4answers - Homework #4 ANSWERS 1. Electromagnetic radiation...

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Homework #4 ANSWERS 1. Electromagnetic radiation cannot typically pass through conductive metal mesh in which the holes are smaller than the wavelength of the radiation. Which is the largest mesh that would effectively block out radiation at 5 GHz? a. 1 m b. 10 cm c. 1 mm d. 100 μ m e. 10 nm ANS: C In order to block the radiation, you need a mesh which has a smaller hole than the wavelength of the radiation. The speed of light is equal to the product of its wavelength and frequency (c = !" ). The speed of light in a vacuum is 3.0 x 10 8 m/s and 5 GHz = 5 x 10 9 Hz. Dividing the speed of light by the frequency gives us the wavelength: ! = c/ " = 3.0 x 10 8 / 5 x 10 9 = 0.06 m = 6 cm = 60 mm. Therefore, 10 cm would be too big to block the radiation. The next smallest hole is 1 mm and this is sufficiently small to block the radiation. The answer is (c). 2. Femtosecond lasers can produce very short pulses of light. How long would it take a short pulse of light to travel across a 3 m table? a. 10 –15 s b. 10 –12 s c. 10 –8 s d. 10 –4 s e. 10 s ANS: C The speed of light in a vacuum is 3.0 x 10 8 m/s. Speed = distance/time and so the time required to travel 3 meters is 3/ 3.0 x 10 8 = 1 x 10 -8 s. 3. What is the frequency of x-ray radiation with a wavelength of 1 nm? a. 1 # 10 –9 s –1 b. 3 # 10 –9 s –1 c. 1 # 10 9 s –1 d. 3 # 10 15 s –1 e. 3 # 10 17 s –1 ANS: E X-rays are a form of electromagnetic radiation for which c = !" . In order to calculate the frequency, we divide the speed of light by the wavelength. Remember to use the right units. Convert 1nm into meters by recognizing that 1 nm = 10 –9 m. " = c/ ! = 3.0 x 10 8 / 1 x 10 –9 = 3 x 10 17 Hz. 4. The index of refraction is the ratio of the speed of light in a vacuum to that in the medium. If it takes a pulse of 500 nm light 0.15 ns to travel through a 3 cm block of glass, what is the index of refraction of the glass? a. 0.0015
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b. 1.5 c. 10 d. 30 e. 500 ANS: B The first thing we need to do is determine the speed of light in the block of glass by using the correct units. Distance = 3 cm = 3 x 10 –2 m and time = 0.15 ns = 0.15 x 10 –9 s. Speed of light in medium = ( 3 x 10 –2 )/(0.15 x 10 –9 ) = 20 x 10 7 = 2 x 10 8 m/s. Index of refraction = Speed of light in vacuum/Speed of light in medium = ( 3 x 10 8 )/( 2 x 10 8 ) = 1.5 5. Increasing the temperature of a black-body will cause a. the total intensity of emitted radiation to increase b. the peak of the emission spectrum to shift to longer wavelengths c. the peak of the emission spectrum to shift to shorter wavelengths d. both a & b e. both a & c ANS: E An increase in temperature of a black-body will cause the total intensity of the emitted radiation to increase, and the peak of the emission spectrum to shift to shorter wavelengths. See pages 120-121 of the text.
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This note was uploaded on 04/30/2008 for the course CHEM 1411 taught by Professor Parkin during the Spring '08 term at Columbia.

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4answers - Homework #4 ANSWERS 1. Electromagnetic radiation...

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