CBSE Class 12 Physics VBQs Dual Nature Of Radiation And Matter

Dual Nature Of Radiation And Matter

Question 1. A function was organized in the village hall with 500 sitting arrangement. When people started entering in groups the counting became difficult. The village head asked Rahul and his friends to take the responsibility at the gate. The boys took the responsibility and people started entering one by one . The disciplined entering of the people helped in easy counting with the help of a scientific device given by the village head to boys.

(a) What values are shown by the boys?

(b) Name the scientific device which is based on application of photoelectric effect

(c) What is the principle of such scientific device?

Answer:

(a) The values shown by boys are :

(1) High degree of general awareness

(ii) Sense of responsibility

(b) Photocell

(c) A person approaching a door way may interrupt a light beam which is incident on a photocell. The abrupt change in photocurrent records every interruptions of the light beam caused by a person passing across the beam. In this way it helps count the person entering the hall provided they enter the hall one by one.

Question 2. Rohit one day thought if radiation has a dual (wave particle) nature, might not the particle of nature ( the electrons, protons etc,) exhibit wave like character. So he consulted his physics teacher for its answer, who explained the facts properly.

(a) What are the values displayed by Rohit?

(b) According to your point of view how teacher explained such thing properly?

(c) Why is de – Broglie wave associated with the moving football not visible?

Answer:

(a) The values displayed by Rohit are :

(i) Curiosity

(ii) Creativity

(b) The moving particles of matter display wave like properties under suitable conditions. Because nature is symmetrical and that the two basic physical entities matter and energy must have symmetrical character. If radiation shows dual aspects so should matter.

(c) The de-Broglie wave length associated with moving football is of extremely short wavelength. Thus, it cannot be detected λα 1/m.

Question 3. Meena thought that there are materials which absorb photons of shorter wavelength and emit photons of longer wavelength. But can there be stable substances which absorb photons of larger wavelength and emit light of shorter wavelength? She got confused and could not find its answer. Then she requested her friend Seema to explain her. The first case, energy given out is less than the energy supplied. But in the second case the material has to supply the energy as the emitted photons has more energy which cannot happen for stable substances.

(a) What values do you notice in Meena?

(b) Consider a metal exposed to light of wavelength 600 nm. The maximum energy of the electrons doubles when light of wave length 400 nm is used. Find the work function in eV

Answer:

(a) The values noticed in Meena are

(i) High degree of general awareness

(ii) Concern for her friend

(iii) Helping and caring nature.

(b) Maximum energy = hv - Ф0

{1230/600  - Ф0 }= 1/2 {1230/400 - Ф0}

= 1230/300 - 2Ф0 = 1230/ 400 - Ф0 = {1230/ 1200= 1.02 eV

Question 4. Surya had knowledge that energy and momentum of an electron are related to frequency and wavelength of the associated matter (de- Broglie) wave by the relations E = hv, p= h/λBut when he heard from his friend Srikanth that value of λ is physically significant but the value of frequency has no physical significance. Srikanth explained it clearly.He said ,according to de Broglie a particle behaves as a wave but it is now established that a particlecannot be equivalent to a single wave,but it is equivalent to a group of waves or a wave packet.As p =h/λ , so in the discussion of matter waves a wave packet is significant and hence only wavelength λ, is significant. As a single wave is insignificant, so phase velocity (velocity of a single wave, V = v λ ) is insignificant and thus, frequency v is also insignificant.

(a) What are the values shown by Srikanth?

(b) Calculate the (i) momentum and (ii) de Broglie wavelength of the electrons accelerated through a potential difference of 56 V

Answer:

(a) The values shown by Srikant are ;

(I) High degree of general awareness

(ii) High order of thinking skills

(iii) Concern for his friend

(iv) Helping and caring nature

(b) Mass of electron, m = 9.1 x 10 ^-31kg

(a) Momentum, p = √ 2mEk / = √2 x 9.1 x 10-31 x 1.6 x 10 -19 x56

= 4.04 x 10 -24 kg ms-1

(b) de Broglie wavelength

λ = h/p = 6.63 x 10 ^-34 /  4.04 x 10 ^-24 = .64 x 10 ^-10 m = 0.164 nm

Question 5. It is seen that gases are insulators at ordinary pressures and start conducting at very low pressures. Sunny was interested to know the reason behind it. So he requested his friend Amit to explain the concept behind it properly . Amit explained that at low pressure ions are far away, so they have possibility to reach their respective electrodes and constitute the current, but at ordinary pressure ions collide frequently and recombine with the opposite charged ions.

(a) What values are displayed by Amit ?

(b) An electron is accelerated through a potential difference of 100 V. What is the de Broglie wavelength associated with it? To which part of the electromagnetic spectrum does this value of wavelength correspond?

Answer:

(a) The values displayed by Amit are :

(i) High degree of general awareness

(ii) Concern for his friend

(iii) Helping and caring nature.

(b) de Broglie wavelength

Question 6. Shyam knows that red light has greater intensity and so it is much bright but in case of photoelectric emission it cannot produce the emission of electrons from a clean zinc surface while even weak ultra violet radiation can do so. He could not know specific cause of such thing. Then he went to his friend Kumar for its specific explanation. Kumar explained him that the photoemission of electron does not depend on the intensity while it depends on the frequency and the energy of photon of incident light. The energy of photon of red light is less than the work function of zinc, so red light cannot emit photoelectrons. Similarly, the energy of photon of ultraviolet light is greater than the work function of zinc, so ultraviolet light can emit photoelectrons.

(a) What values are noticed in Kumar?

(b) The work functions of lithium and copper are 2.3 eV and 4 eV respectively. Which of these metals are useful for the photoelectric cell working with visible light? Explain

Answer:

(a) The values noticed in Kumar are :

(i) High degree of general awareness

(ii) Concern for his friend

(iii) Helping and caring nature

Question 7. Smitha wanted to give a gift to her cousin a burglar alarm. But she did not know about its working principle and its significance. She discussed it with her friend Shruthi. Shruthi explained her that in burglar alarm, ultraviolet light is continuously made to fall on a photocell installed at the doorway. A person entering the door interrupts the beam falling on the photocell. The abrupt change in photocurrent is used to start an electric bell ringing.

(a) What are the values shown by Shruthi?

(b) An electron and a photon each have a wavelength of 1.00 nm. Find (i) their momenta (ii) the energy of the photon, and (iii) the kinetic energy of electron

Answer:

(a) The values shown by Shruthi are :

(i) High order general awareness

(ii) Able to convince someone

(iii) Helping and caring nature

(b) Given λ = 1.00 nm = 1.00 x 10 ^-9 m

(i) Momentum of electron - momentum of photon

hc / λ = 6.63 x 10 ^-34 / 1.00 x 10 ^-9 = 6.63 x 10 ^-25 kgm /s

(ii) Energy of photon

Value Based Questions

Question 1. In a multi-storeyed building, once a fire broke out at midnight due to Electrical short circuit. Ravi along with others rushed to the spot, informed the fire service and put off the fire. But by that time a huge amount of damage had already been done. Ravi being Secretary of the building decided to fix fire alarms (using photo cell) in all the floors.

(i) What values were shown by Ravi in this situation?
(ii) A human eye can perceive a minimum light intensity is about 10^-10 Wm^-2 Calculate the number of photons of wavelength 5.6 x 10^-7 m/s that must enter the pupil of area 10^-4 m² for vision?

Answer:

(i) Concern for society, social responsibility, application of knowledge.

(ii) Energy falling on area per sec =10^-10 x 10^-4 = Js^-1 Therefore n = N10^-10 x 10^-4 / hc = 3 x 10⁴.photons/sec

Question 2. Davisson and Germer's actual objective was to study the surface of a piece of nickel by directing a beam of electrons at the surface and observing how many electrons bounced off at various angles. Though the experiment was conducted in a vacuum chamber. Air entered the chamber, producing an oxide film on the nickel surface. When they started the experiment again and the electrons hit the surface, they were scattered by atoms which originated from crystal planes inside the nickel crystal. Davisson and Germer's accidental discovery of the diffraction of electrons was the first direct evidence confirming de Broglie's hypothesis that particles can have wave properties as well.

(i) What can we infer about the values shown by Davisson and Germer?

(ii) Write the expression to find the wavelength of an electron when accelerated through a potential difference of V volts.

Answer:

(i) λ = h/√2mev

Question 3. Ravi, while returning home from office late night, saw a person jumping into a house .Suspecting him to be a thief, he slowly followed him. He saw an alarm fixed at the entry of the house. Immediately to alarm the people at the house, he used his laser torch to activate the alarm, as it works with the visible light. The circuit became complete and siren started working and people at the house got up and caught the thief. They thanked Ravi for his action.

(a) What moral value can you see in Ravi?

(b) The photoelectric cut off voltage in a certain experiment is 1.5V. What is the maximum kinetic energy of the photoelectrons emitted?

Answer:

(a) Presence of mind/social responsibility / attitude to help others.

(b) Kmaz = eV₀ = 1.6x 10^-19 x 1.5J = 2.4x10^-19 J

Question 4. A seminar was conducted in an auditorium in Chennai. It was a big auditorium and some seats at the back were vacant. The coordinator had a problem in making the order for lunch, as he was unable to estimate the number of people present in the auditorium. Vikram, associate of the coordinator, saw a digital counter working on the principle of photo electric effect, at every entry and exit gate of the auditorium. The people were allowed to enter through the entry gate and leave the hall by the exit gate. He also noticed that all the counters were in good condition. He immediately noted the display of every counter and gave the coordinator the exact number of people in the auditorium.

(a) What moral values can you see in Vikram?
(b) What is photo electric effect?

Answer:

(a) Proper application of knowledge/ concern for his boss/ presence of mind.

(b) The phenomena of emission of electron when light of certain frequency falls on any photosensitive material.

Value Based Questions

Question 1. In a multistoried building, once a fire broke out at midnight due to electrical short circuit .Ravi along with others rushed to the spot, informed the fire servive and put off the fire. But by that time a huge amount of damage had already been done. Ravi being Secretary of the building decided to fix fire alarms (using photo cell) in all the floors .

Q – (i) What values were shown by Ravi in this situation?

Answer:

^{Concern for society, social responsibility, application of knowledge.}

Question 2. Q-(ii) A human eye can perceive a minimum light intensity is about 10^-10 Wm^-2 .Calculate the number of photons of wavelength 5.6 x 10^-7 m/s that must enter the pupil of area 10^-4 m² for vision ?

Answer:

Energy falling on area per sec =10^-10 x 10^-4 = nhc / λ

Therefore n= λ10^-10 x 10^-4 / hc = 3 x 10⁴.photons/sec

Question 3. Davisson and Germer's actual objective was to study the surface of a piece of nickel by directing a beam of electrons at the surface and observing how many electrons bounced off at various angles. Though the experiment was conducted in a vacuum chamber. air entered the chamber, producing an oxide film on the nickel surface.When they started the experiment again and the electrons hit the surface, they were scattered by atoms which originated from crystal planes inside the nickel crystal.Davisson and Germer's accidental discovery of the diffraction of electrons was the first direct evidence confirming de Broglie's hypothesis that particles can have wave properties as well.

(i) What can we infer about the values shown by Davisson and Germer?

Answer:

Perseverance, not giving up and patience.

Question 4. Ravi, while returning home from office late night, saw a person jumping into a house .Suspecting him to be a thief, he slowly followed him. He saw an alarm fixed at the entry of the house. Immediately to alarm the people at the house, he used his laser torch to activate the alarm, as it works with the visible light. The circuit became complete and siren started working and people at the house got up and caught the thief. They thanked Ravi for his action.

(a) What moral value can you see in Ravi?

(b) The photoelectric cut off voltage in a certain experiment is 1.5V. What is the maximum kinetic energy of the photoelectrons emitted?

Answer:

(a) Presence of mind/ Social responsibility/ attitude to help others.

(b) eV₀

Question 5. A seminar was conducted in an auditorium in Chennai. It was a big auditorium and some seats at the back were vacant. The coordinator had a problem in making the order for lunch, as he was unable to estimate the number of people present in the auditorium. Vikram , associate of the coordinator , saw a digital counter working on the principle of photo electric effect,at every entry and exit gate of the auditorium.The people were allowed to enter through the entry gate and leave the hall by the exit gate. He also noticed that all the counters were in good condition. Heimmediately noted the display of every counter and gave the coordinator the exact number of people in the auditorium.

(a) What moral values can you see in Vikram?

(b) What is photo electric effect ?.

Answer:

(a) Proper application of knowledge/ concern for his boss/ presence of mind

(b) definition

Please refer to the link below for CBSE Class 12 Physics VBQs Dual Nature Of Radiation and Matter Set A