UNIVERSITY OF NAMIBIA
CODeL –Distance Learning
Assignment 1: Modern Physics for Educators (PHE 3751)
SEMESTER 1, 2024
Tutor: Paulus S Weyulu
Due Date: TBA
Instructions: Answer all Question clearly to avoid loosing marks, all work, including
sketches drawn
NB: SHOW ALL YOUR STEPS CLEARLY TO EARN MARKS.
Use the cover page provided on Moodle
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QUESTION 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Total: 8
explain how
(a) A continuous spectrum is formed.
[4]
(b) A line spectrum is formed.
[4]
QUESTION 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Total: 6
Shipi FM radio station in Ondangwa broadcasts at unknown frequency with a radiated power
of 39.0kW and it emits 5.15 × 1028 𝑝ℎ𝑜𝑡𝑜𝑛𝑠/𝑠. determine the unknown broadcasting
frequency of Shipi FM.
[6]
QUESTION 3………………………………………………………………………………Total: 8
A Laser pointer with a power output of 7.0mW emits light, and each photon has 1.55 ×
10−26 𝑘𝑔. 𝑚⁄𝑠 magnitude of momentum.
(a) What is the wavelength of the emitted light?
[3]
(b) How many photons does the laser pointer emit each second.
[5]
QUESTION 4………………………………………………………………………………Total: 10
X-rays of unknown wavelength is used in a Compton-scattering experiment.
(a) at a scattering angle of 55°, the wavelength of incident radiation is 75% of the
wavelength of scattered radiation. Determine the wavelength of the scattered x-rays [5]
(b) If the x-ray used is 0.132nm, at what angle is the wavelength of scattered radiation is
120% of the wavelength of incident x-ray
[5]
QUESTION 5 ………………………………………………………………………………Total: 15
A laser produces light of wavelength 633 nm in pulses that propagate in the +x-direction and
that are 7.00 × 10−6 𝑚 in length. For an average photon in this pulse. Find
(a) the momentum p
[5]
(b) the minimum momentum uncertainty in 𝑘𝑔. 𝑚⁄𝑠 and as a percentage of momentum p
(c) the energy
[3]
2
(d) the minimum energy uncertainty in J and as a percentage of energy
[2]
QUESTION 6 …………………………………………………………..…………Total: 11
In an electron-diffraction experiment using an unknown accelerating voltage, an intensity
maximum occurs for 𝜃 = 50° (see fig 6b) . the electron has a wavelength of 0.21 nm and a
negligible kinetic energy before being accelerated.
(a) find the accelerating voltage used in an electron-diffraction experiment
[6]
(b) calculate the atomic spacing in the target indicated by x-ray diffraction
[5]
QUESTION 7 ……………………………………………………………….Total: 13
The star Betelgeuse has surface temperature 3590 K and be regarded as a blackbody.
(a) Find the wavelength at which Betelgeuse emits most strongly. Is this visible,
ultraviolet, or infrared?
[8]
(b) Find the amount of power radiated per unit area of the surface of Betelgeuse.
[5]
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QUESTION 8 ………………………………………………………………..Total: 8
(a) The temperature of a blackbody is changed so that the intensity 𝐼 of radiation from
the blackbody increases by a factor of 18. By what factor does the peak wavelength
𝜆𝑚 changes?
[8]
Question 9 …………………………………………………………………………..Total: 20
(a) Write the Planck distribution law in terms of the frequency 𝑓 , rather than the wavelength
𝛌 , to obtain. 𝐼(𝑓)
[5]
(b) A show that
∞
2𝜋 5 𝑘 4 4
∫ 𝐼(𝜆)𝑑𝜆 =
𝑇
15𝑐 2 ℎ3
0
Where is the Planck distribution law formula . hint: change the integration variable from 𝜆 to
𝑓 . you ‘ll need to use the following tabulated integral:
[15]
∞
𝑥3
1 2𝜋 4
∫ ∝𝑥
𝑑𝜆 =
( )
−1
240 ∝
0 𝑒
Question 10 ………………………………………………………………………. Total: 8
In an electron-diffraction experiment, the spacing between atoms in the target is 0.172 nm
and the electrons have negligible kinetic energy before being accelerated by an accelerating
voltage of 69.0 V. Find
(a) The de Broglie wavelength of each electron
[5]
(b) The smallest scattering angle for which there is a maximum in the diffraction pattern.
[3]
Total: 100
Available marks: 107
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