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Chapter 20: Q5P (page 525)
What is the role of a filter in a grating monochromator?
The role of a filter in a grating monochromator is to eliminate different wavelengths at the same angle as the desired wavelengths.
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Consider a planar waveguide used for attenuated total reflection measurement of a film coated on one surface of the waveguide. For a given angle of incidence, the sensitivity of attenuated total reflectance increases as the thickness of the waveguide decreases. Explain why. (The waveguide can be less thanthick.)
The exitance (power per unit area per unit wavelength) from a blackbody (Box 20-1) is given by the Planck distribution: where is wavelength, Tis temperature K), his Planck’s constant, Cis the speed of light, and kis Boltzmann’s constant. The area under each curve between two wavelengths in the blackbody graph in Box 20-1is equal to the power per unit areaemitted between those two wavelengths. We find the area by integrating the Planck function between wavelengthsand
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For a narrow wavelength range, the value of is nearly constant and the power emitted is simply the product .
(a) Evaluate and at
(b) Calculate the power emitted per square meter atin the intervalby evaluating the product
(c) Repeat part (b) for the interval
(d) The quantityis the relative exitance at the two wavelengths. Compare the relative exitance at these two wave-lengths atwith the relative exitance at 100K. What does your answer mean?
Consider a reflection grating operating with an incident angle of 408 in Figure 20-7. (a) How many lines per centimeter should be etched in the grating if the first-order diffraction angle for 600 nm (visible) light is to be 2308? (b) Answer the same question for 1 000 cm21 (infrared) light.
Explain how a laser generates light. List important properties of laser light.
(a) The true absorbance of a sample is 1.500, but stray light reaches the detector. Find the apparent transmittance and apparent absorbance of the sample. (b) How much stray light can be tolerated if the absorbance error is not to exceedat a true absorbance of? (c) A research-quality spectrophotometer has a stray light level of ,at. What will be the maximum absorbance error for a sample with a true absorbance of 2? Of 3?
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