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Homework1 - MSE 110 Problem Set 1, Problem 1 Photons...

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Unformatted text preview: MSE 110 Problem Set 1, Problem 1 Photons emitted by a red He-Ne laser have energy E=3.14510-19 Joules. What is the wavelength of these photons? (5 pts) [Answer in nm] Ans: c h h E = = where E=3.14510-19 J, c = 3 10 8 m /sec, and h = 6.626 10-34 Jsec Therefore, nm m J m s J 632 10 632 10 145 . 3 sec 10 3 10 626 . 6 9 19 8 34 = = = Problem 2 A green laser pointer emits photons of energy E=2.35eV. What is the wavelength of these photons? (5 pts) [Answer in nm] Ans: An eV is the energy gained by an electron accelerated by a potential of 1 V. Therefore it is equal to the charge of the electron multiplied by 1 volt: E = 1 V1.610-19 C. Hence 1 eV=1.610-19 Joules. The energy of a green photon in units of Joules is therefore: E = 2.35 1.610-19 J = 3.7610-19 J And its wavelength is nm m J m s J 6 . 528 10 6 . 528 10 76 . 3 sec 10 3 10 626 . 6 9 19 8 34 = = = Problem 3 CO 2 lasers are very intense lasers used industrially to cut and weld metal parts. It is a dangerous laser because it emits a beam invisible to the human eye in the infrared region at a wavelength =10600 nm. What is the energy of these photons in eV. [Answer in eV] (5 pts) Ans: c h E = and 1eV=1.610-19 J Therefore: eV eV J m m s J E 117 . 10 6 . 1 10 06 . 1 sec 10 3 10 626 . 6 19 5 8 34 = = 1 Problem 4 The work function of iron is =4.31eV. Which of the following photons can extract an electron from an iron cathode and produce a current in a photoelectric experiment? (5pt) [Answer A, B, C, D] A- 256 nm B- 324 nm C- 954 nm D- 584 nm Ans: In order to extract an electron from an iron cathode a photon must have at least an energy E= =4.31eV, hence the maximum photon wavelength to remove a electron from iron is: nm eV J eV m s J 288 10 6 . 1 31 . 4 sec 10 3 10 626 . 6 19 8 34 = = Hence only photon A is energetic enough to extract an electron from iron....
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Homework1 - MSE 110 Problem Set 1, Problem 1 Photons...

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