Question 1

As the temperature of a blackbody increases,what happens to the peak wavelength of the light it radiates?

Accepted Answer

According to Wien's Law, as the temperature of a blackbody increases, the peak wavelength of the light it radiates decreases proportionally. This means that the peak wavelength shifts towards the blue end of the spectrum, making the radiation more energetic. Therefore, choice B is the correct answer.

Question 2

Which of the following actions will increase the energy of a photon? (There could be more than one correct choice. )

Accepted Answer

Increasing the frequency or decreasing the wavelength of a photon will increase its energy, as energy is directly proportional to frequency and inversely proportional to wavelength.

Question 3

A blue laser beam is incident on a metallic surface,causing electrons to be ejected from the metal.If the frequency of the laser beam is increased while the intensity of the beam is held fixed,

Accepted Answer

A

Question 4

Two sources emit beams of light of wavelength 550 nm.The light from source A has an intensity of 10 µW/m²,and the light from source B has an intensity of 20 µW/m².This is all we know about the two beams.Which of the following statements about these beams are correct? (There could be more than one correct choice. )

Accepted Answer

Intensity is proportional to the number of photons emitted per unit time per unit area. 
Let the number of photons emitted per second by A be N_A and that by B be N_B. 
Then, 
10 µW/m² = N_A * hc/λ 
20 µW/m² = N_B * hc/λ 
Dividing the second equation by the first, we get: 
2 = N_B/N_A 
So, beam B carries twice as many photons per second as beam A (Option A is correct).

Energy of a photon is given by E = hc/λ. 
Since the wavelength of both beams is the same, the energy per photon is also the same for both beams. So, Option B and D are incorrect.

Frequency is given by f = c/λ, where c is the speed of light. 
Since the wavelength of both beams is the same, the frequency is also the same for both beams. So, Option C is incorrect.

Therefore, the only correct statement is Option A.

Question 5

Which of the following actions will increase the de Broglie wavelength of a speck of dust? (There could be more than one correct choice. )

Accepted Answer

The de Broglie wavelength is inversely proportional to momentum (mass times velocity). Decreasing mass or speed, or decreasing momentum, will increase the wavelength.

Question 6

When the surface of a metal is exposed to blue light,electrons are emitted.If the intensity of the blue light is increased,which of the following things will also increase?

Accepted Answer

Increasing the intensity of the blue light increases the number of photons hitting the metal surface per second, which in turn increases the number of electrons ejected per second. The maximum kinetic energy of the ejected electrons and the work function of the metal are properties dependent on the frequency of the light, not its intensity. There is no time lag between the absorption of light and the ejection of electrons in the photoelectric effect.

Question 7

Protons are being accelerated in a particle accelerator.When the energy of the protons is doubled,their de Broglie wavelength will

Accepted Answer

The de Broglie wavelength is inversely proportional to the momentum of the particle. When the energy of the proton is doubled, its momentum also doubles (since momentum is proportional to energy in the non-relativistic limit). Therefore, the de Broglie wavelength will decrease by a factor of 2 according to the de Broglie relation λ = h/p.

Question 8

If a proton and an electron have the same speed,which one has the longer de Broglie wavelength?

Accepted Answer

The de Broglie wavelength is given by λ = h/p, where h is the Planck's constant and p is the momentum of the particle. Since the momentum of a particle is given by p = mv, where m is the mass of the particle and v is its speed, we can rewrite the de Broglie wavelength as λ = h/mv. Therefore, for the same speed, the particle with the smaller mass will have the longer de Broglie wavelength. Since the electron has a much smaller mass than the proton, it will have the longer de Broglie wavelength.

Question 9

A beam of light falling on a metal surface is causing electrons to be ejected from the surface.If we now double the frequency of the light,which of the following statements are correct? (There could be more than one correct choice. )

Accepted Answer

Doubling the frequency of light increases the energy of each photon, which increases the kinetic energy of the ejected electrons, but not necessarily doubling it. The number of ejected electrons depends on the intensity, not frequency.

Question 10

If the wavelength of a photon is doubled,what happens to its energy?

Accepted Answer

The energy of a photon is inversely proportional to its wavelength. Doubling the wavelength will halve the energy, so the correct answer is A - the energy is reduced to one-half of its original value.

Question 11

Monochromatic light falls on a metal surface and electrons are ejected.If the intensity of the light is increased,what will happen to the ejection rate and maximum energy of the electrons?

Accepted Answer

The ejection rate of electrons increases with the intensity of the light because more photons are hitting the surface, leading to more electrons being ejected. However, the maximum energy of the ejected electrons is determined by the frequency of the light, not its intensity, and remains the same as the intensity is increased.

Question 12

If the wavelength of a photon in vacuum is the same as the de Broglie wavelength of an electron,which one is traveling faster through space?

Accepted Answer

Photons always travel at the speed of light in a vacuum, which is the maximum speed anything can travel through space, making them faster than electrons regardless of their wavelength.

Question 13

Two identical metal bars are heated up until they are both glowing.One of them is "red hot" and the other is "blue hot." Which one is hotter,the one that glows red or the one that glows blue?

Accepted Answer

The one that glows blue is hotter. This is because blue light has a higher frequency and shorter wavelength than red light, meaning it carries more energy. Therefore, the bar that glows blue is emitting more energy in the form of light and is hotter than the one that glows red.

Question 14

If you double the frequency of the light in a laser beam,but keep the number of photons per second in the beam fixed,which of the following statements are correct? (There could be more than one correct choice. )

Accepted Answer

The answer of If you double the frequency of the...

Question 15

If a proton and an electron have the same de Broglie wavelengths,which one is moving faster?

Accepted Answer

The wavelength of a particle is inversely proportional to its momentum. Since the mass of a proton is much larger than that of an electron, the proton would need to have a much lower velocity in order to have the same wavelength as an electron. Therefore, the electron must be moving faster.

Question 16

Two sources emit beams of microwaves.The microwaves from source A have a frequency of 10 GHz,and the ones from source B have a frequency of 20 GHz.This is all we know about the two beams.Which of the following statements about these beams are correct? (There could be more than one correct choice. )

Accepted Answer

The energy of a photon is given by E = hf, where h is Planck's constant and f is the frequency of the photon. 
Therefore, since the frequency of the microwaves from source B is twice that of the microwaves from source A, each photon in beam B has twice the energy of a photon in beam A. All the other statements are not necessarily true, as they depend on other factors such as the number of photons and the direction of the beams.

Question 17

Increasing the brightness of a beam of light without changing its color will increase

Accepted Answer

Increasing the brightness of a beam of light means increasing the intensity of the light, which can be achieved by increasing the number of photons per second in the beam without changing its color (i.e., frequency or wavelength). Therefore, choice A is the correct answer. Increasing the energy of each photon would change the color of the light (higher energy photons correspond to higher frequencies) and increasing the speed of photons is not possible without changing the medium through which they travel.

Question 18

A photon of blue light and a photon of red light are traveling in vacuum.The photon of blue light

Accepted Answer

The wavelength of blue light is smaller than that of red light because blue light has a higher frequency. However, both photons travel at the same speed (since they are both traveling in a vacuum). Therefore, choice A is correct.

Question 19

Light of a given wavelength is used to illuminate the surface of a metal,however,no photoelectrons are emitted.In order to cause electrons to be ejected from the surface of this metal you should

Accepted Answer

The photoelectric effect states that the energy of the incident photon must be greater than the work function of the metal for photoelectrons to be emitted. The work function is the minimum energy required to remove an electron from the metal surface. Therefore, by using light of a shorter wavelength, its energy increases, and there is a higher probability that the photons will have enough energy to overcome the work function and eject electrons from the metal surface.

Question 20

Monochromatic light is incident on a metal surface,and the ejected electrons give rise to a current in the circuit shown in the figure.The maximum kinetic energy of the ejected electrons is determined by applying a reverse ('stopping')potential,sufficient to reduce the current in the ammeter to zero.If the intensity of the incident light is increased,how will the required stopping potential change?

Accepted Answer

The stopping potential depends only on the frequency (or wavelength) of the incident light, and not on its intensity. Increasing the intensity of the light will simply increase the number of electrons ejected per unit time, but will not affect their maximum kinetic energy. Therefore, the stopping potential required to stop the ejected electrons will remain unchanged.

Quiz 30: Quantum Physics