Dawn of modern physics
EXERCISE
MULTIPLE CHOICE QUESTIONS
Choose the best possible answer:
1. Triplets A and B board spaceships and head away from Earth in opposite di rections, each traveling the same distance at the same speed before returning to Earth. Triplet C remains on Earth. Which expression describes the triplets" relative ages once they're reunited on Earth?
A. A<B<C
B. A=B>C
C.A=B<C
D.A=8=C
2. If a spaceship travels from Earth to a planet in the Alpha Centauri star system,
which observer measures the proper time for the trip?
A. an observer on Earth
B. an observer on the planet in the Alpha Centauri star system
C. an observer on the spaceship
D. an observer deep in space far from the planet and Earth
3. When you compress a spring between your fingers, it's mass
A. increase
B. decrease
C. stay the same
D. turns negative
4. By doubling the absolute temperature of black-body, the total energy radi
ated will change by a factor of
A. 0.5 B. 2
C. 4 D. 16
5. According to Einstein, increasing the brightness of a beam of light without changing its color will increase:
A. the number of photons.
B. the energy of each photon.
C. the speed of the photons.
D. the frequency of the photons.
6. Sources of red, blue, and yellow light each emit light with a power of 50 mW. The source which emits more photons is
A. red
B. blue
C. yellow
D. They all emit the same number per second.
7. Of the following energies for photons, which is the least energy that could result in photoelectron production if the work function is 3.00 eV?
A. 1.50 eV
B. 2.90 eV
C. 3.50 eV
D. 6.01 eV
8. In a Compton scattering experiment at which angle the electron carry maxi mum energy? (Assume that initially the target electron is at rest).
A. 0°
B. 90°
C. 135°
D. 180°
9. If X-rays are scattered from protons instead of electrons, the chang e in its
wavelength for a given angle of scattering
A increases B. decreases C. is unchanged
D. is one radian
10. Pair production cannot take place in vacuum because.
A. Mass is not conserved
B. Momentum is not conserved
C. Energy is not conserved
D. Charge is not conserved
11. According to de-Broglie equation which one has the smallest wavelength as sociated with it
A. proton
B. neutron
C. a-particle
D. electron
12. An electron and a proton have the same de-Broglie wavelength. Then the kinetic energy of the electron is
A. Zero
B. Infinity
C. equal to the kinetic energy of the proton
D. greater than the kinetic energy of the proton
13. In Electron microscope, light source is replaced by a beam of very fast moving
A. electron B. neutron
C. proton D. photon
14. According to Heisenberg, as the uncertainty in the measurement of a particle's momentum is reduced by a factor of 2, by what factor is the uncertainty in that same particle's position changed?
A. 1/2 B. 1
C. 2 D. 4
CONCEPTUAL QUESTIONS
1. Imagine a world in which speed of light is only 50 m/s. How would the eve ryday events appear to us?
Ans. There are two possible answers to this question, depending upon the situa tion.
Situation No 1: Let only the speed of light is reduced to 50 m/s and the speed of all other objects are similar to that we observe in this world, then relativistic ef fects would be observed in everyday events very frequently such as length con traction, time dilation and mass dilation.
Situation No 2: If there is a universe in which the speed of light is reduced to 50m/s and all other speed of objects are reduced accordingly, then there would be no relativistic effects observed in the everyday events. Because in that universe, the objects would not be able to attain the speed close to 50 m/s. Therefore, everything would be moving quiet slowly and the objects size would also be reduced accordingly.
Example: Since in this world the fastest speed attained by any vehicle (Parker Solar Probe) so far is 148000 m/s, which is only 0.05% of the speed of light. Therefore, in that world, the fastest speed that would be attainable in comparison to this speed would be 0.025 m/s.
2. Some stars are observed to be reddish, and some are blue. Which stars have the higher surface temperature? Explain.
Ans. The stars that appear to be bluish have higher surface temperatures as com pared to the stars that are observed as reddish. Explanation: Astronomers use the color of stars to estimate their temperature by using Wein's displacement law given by,
AT-2898000nmK
=>T=
2898000nmK
For stars of red color, i.e. λ = 700 nm
Therefore; T 2898000 pp K
700
=>T=4140K ⚫For stars of blue color, i.e. λ=475 nm
2898000 και Κ
Therefore; T 475
=>T-6100K
Therefore, looking at the color of distant stars, we can determine which one will have higher surface temperature.
3. If the photo electric effect is observed for one metal, can you conclude that the effect will also be observed for another metal under the same conditions?
Ans. No, it is not necessary that photoelectric effect will be observed for different metal under the same conditions.
Explanation: A photon of energy E = hf when falls on the surface of a metal, it transfers this much energy to some electron of the metal and disappears itself. A part of this energy is used to eject the electron from the surface of the metal and the rest as the K.E of the electron. The minimum amount of energy required to eject an electron from the surface of the metal is called work function, denoted by 4. Now different metals have different work functions. This means to eject an electron from their surfaces, different amount of energies are required for different metals. So, if the conditions are same and the same photon is made incident on the surfaces of two metals, we should not expect the same effect in both cases of different metals.
4. Does a light bulb at a temperature of 2500 K produce as white light as the sun at 6000 K?
Ans. No, this is not possible for a light bulb at temperature 2500 K to produce as white light as sun at 6000 K. This is because the brightness of light depends upon the intensity of light and sun light at 600 has greater intensity than the light bulb at 2500 K.
Explanation: From Steffen-Boltzmann law the energy radiated per second per unit area of a hot body is directly proportional to the 4th power of the absolute temperature of the body.
Mathematically: Ex T = E=OT'
As the temperature rises, the intensity of the emitted light also increases. There fore, the sun produces more white light (brightness) at 6000 K as compared to the bulb at 2500 K.
5. A beam of red light and a beam of blue light have exactly the same energy. Which light contains the greater number of photons?
Ans. The light beam with greater wavelength will contains greater number of photons.quency of blue light is greater than the frequency of red light or wavelength of blue light is smaller than the wavelength of red light. Therefore, if the number of
Explanation: Physically, energy is directly proportional to the frequency of radia tion or inversely proportional to the wavelength of the radiation. Now the fre photons of blue and red light is equal, the energy of the blue light would be more than the energy of red light. And if energy of both radiations is the same, then the number of photons of red light must be greater than the number of photons of blue light.
Mathematically: E=nhf
(n represents the number of photons)
E=nh
Ελ
2
⇒n= hc
(For E/hc = constant)
⇒n=(constant).
> παλ
Therefore, for larger wavelengths, number of photons must be greater. Since Red light has larger wavelength than the blue light, therefore, for equal energies the number of photons of red light must be greater than the number of photons of blue light.
6. Why must the rest mass of a photon be zero?
Ans. When a material object of actual mass m, moves with high speed then its apparent mass m according to the results of special theory of relativity is given by equation;
Now we know that a photon always travels with speed of light and if we replace v by c in above equation, we get;
m
m=
(1)
m
m
0
√1-1
Similarly, applying Einstein mass-energy equation, we get: E=mcc² =x ->(2)
So, if the photon has some rest mass, its relativistic mass and energy should be infinite (from equation 1 and 2). But photon does not possess infinite mass or energy, therefore, it is concluded that its rest mass must be equal to zero.
7. Why don't we observe Compton's effect with visible light?
Ans. We cannot observe Compton's effect with visible light because of smaller energy content and frequency. Explanation: We do not observe Compton's effect with visible light because for Compton's effect to occur, high energy and high frequency photon is required for instance X-rays but visible light has not got enough energy to carry out Compton'seffect. In comparison to X-rays, energy of visible light is quite smaller than the minimum energy required for Compton's effect to occur. That is why we cannot observe Compton's effect with visible light.
8. If an electron and a proton have the same de-Broglie wavelength, which particle has greater speed?
Ans.Speed of electron will be greater than the speed of proton. Explanation: Since de-Broglie's wavelength can be written as;
7.= h my
h
⇒ V= As given that wavelength of both electron and proton is same (constant). So, we can write the above equation as;
From last expression, it can be concluded that particle with smaller mass will have higher speed. As mass of electron is lesser than mass of proton, so it will have higher speed than proton. V 1|m
Thus, we conclude that speed of electron is greater than speed of proton that both particles have same de-Broglie's wavelength.
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