# img086 - PHY2054 Spring 2007 6 A laser beam of wavelength...

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Unformatted text preview: PHY2054 Spring 2007 6. A laser beam of wavelength 632.8 nm has intensity 13.3 W/mz. The amplitudes of the associ- ated electric and magnetic ﬁelds, in V/m and pT, respectively are: @ 100, 0.33 (2) 70.7, 0.24 (3) 10,000, 33.4 (4) 56.5, 0.19 (5) none of these The intensity is relate to the peakE and B ﬁelds by I = EmBmx / 2;:0 = Es,“ /2p0c. We solve this for Em yielding E”, = 100 V/m. Using B = E /c yields BM, = 0.33 ,uT 7. The laser beam in the above question has an area of 0.2 cm2. What force (in pico N) will this beam exert on a 1 cm2 mirror from which it is totall reﬂected? @ 1.77 (2) 8.86 (3) 0.89 (4) 0.77 (5) none of these The total force ﬁJr a beam of intensity I is F = 214/0, where A is the area of the beam, 0 is the speed of light and the factor of 2 accounts jbr reﬂection. The totaljhrce is then calculated to be I . 77 pN. 8. A band of frequencies for many cellular telephones is centered near 800 MHz. At this fre- quency, what is the photon energy in eV (not joules)? 3.3 x 10*S (2) 4.2 x 10‘5 (3) 8.9 x 10'3 (4) 8.0 x 10‘ (5) 3.8 x 10‘1 The energy of each photon of ﬁ'equency f is hf where h = Planck ’s constant = 6.6 x 10"“ J-sec. This relation yields a photon energy of 5. 26 X 10'” .1 Converting to eVyields 3.3 x 10'6 eV. ...
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