View latest educational results 2026 of class 9th, 10th / Matric / SSC, Intermediate / HSSC / FA / FSc / Inter, 5th / Primary, 8th / Middle, DAE, DBA, D.COM, BA / BBA / BCom / BSc / B.Ed, MA / MSc / MBA / MIT / MCS, MBBA / M.Ed, LLB, Engineering, M.Phil, PhD from all BISE / boards and Universities by roll number & by name and check old result 2025 online.

Home
eLearn
12th Class
12th Class Punjab
Physics
Physics Short Question
Dawn Of Modern Physics Short Questions Preparation

Chapter 19 Dawn Of Modern Physics Physics 12th Class Punjab Short Questions

Question No. 1 - Give two postulate of special theory of relativity.

Answer No. 1 - Postulate of special theory of relativity are states as:
1. The laws of physics are the same in all inertial frames.
2. The speed of light in free space has the same value for
all observes, regardless of their state of motion.

Question No. 2 - Photoelectric effect give the evidence of the particle nature of light, Explain it how?

Answer No. 2 - If light were simple a wave like phenomena, then increasing the intensity and thereby increasing the total energy failing on the surface would be expected to eventually provide enough energy to release electron no matter what the frequency. Furthermore in the classical picture one would expect the maximum energy of the emitted electrons to depend on the intensity of light-but it does not. So this is evidence that light behaves as if it were a particle.

Question No. 3 - Can a pair production take place in vacuum?Explain

Answer No. 3 - No, pair production can not take place in vacuum.
In order to conserve the momentum and energy, their presence of heavy nucleus is essential. The vacuum has no particle or heavy nucleus. Therefore pair production can not take place in vacuum.

Question No. 4 - What are the measurement on which two observers in relative motion will always agree upon?

Answer No. 4 - Two observe in relative motion will always agree upon:
1. Force
2. Acceleration

Question No. 5 - What are black body radiations and how can you get a black body?

Answer No. 5 - An objects that absorbs all radiations falling on it, at all wavelength is called a black body.
When a body is heated, it emits radiation its emission is called black body.
Black body is solid that has hollow cavity within it and a small hole through which radiations can enter or escape. The inside is blackened with suit to make it as good an absorber and as a bad reflector as possible.

Question No. 6 - Write at least two justification for light to behave as wave and as a particle.

Answer No. 6 - Interference and diffraction confirms wave nature of light while photoelectric effect and compton effect confirms particle nature of light.

Question No. 7 - What advantage of an electron microscope has over an optical microscope?

Answer No. 7 - Resolving power of an electron microscope is 1000 times greater than that of optical microscope.
Magnification of an electron microscope is also about 1000 times greater than that of optical microscope.
3-D image of remarkable quality can be obtained by electron.

Question No. 8 - Differentiate between inertial and non-inertial frame of reference.

Answer No. 8 - A coordinate system in which the law of inertia is valid is called inertial frame of reference. It is non-accelerated frame of reference.

Question No. 9 - Why can red light be used in a photographic dark room when developing films, but a blue or white light can not?

Answer No. 9 - As we known red light has longest wavelength in visible spectrum, therefore it has less energy than that of blue or white light. So red light is least scattered on account of its large wavelength. Hence photographic films and the materials concerned are less affected in the presence of red light than high energy blue of white light.

Question No. 10 - Distinguished between photoelectric effect and Compton effect.

Answer No. 10 - The emission of electron from a metal surface when exposed to suitable frequency light is called photoelectric effect and when x rays are scattered by loosely bound electrons from graphite target the phenomena of change in wavelength is known as Compton effect.

Question No. 11 - Will higher frequency of light ejects greater number of electrons than low frequency of light?

Answer No. 11 - No it will not, The number of ejected electrons depend upon the intensity of light. They are independently of frequency.

Question No. 12 - When light shine on surface is momentum transferred to metal surface?

Answer No. 12 - Yes when light shine on surface, momentum is transferred to metal surface and also the energy. So metal is heated up.

Question No. 13 - If you are moving in a spaceship at very high speed relative to the earth. Would you notice difference a In your pulse rate b In the pulse rate of people on the earth?

Answer No. 13 - 1. The pulse rate of the person inside the spaceship moving
with large velocity will decrease.
2. The pulse rate of the people on the earth with respect
to the person inside the spaceship with large velocity
will increase.

Question No. 14 - Define Compton effect.

Answer No. 14 - The phenomena of increase in wavelength of x rays photon scattered by loosely bound electrons from a graphite target is called Compton Effect.

Question No. 15 - Is it possible to create a single electron from energy? Explain

Answer No. 15 - No it is not possible to create a single electron from energy. Creation of single electron will be against the law of conservation of charge and the law of conservation of momentum. In pair production an electron positron pair is produces.

Question No. 16 - Define pair production and annihilation of matter.

Answer No. 16 - Pair production: The change of very high energy photon into an electron, positron pair is called pair production.
Annihilation of matter: When a positron comes close to an electron, they annihilation and produce two photons in the gamma rays range. It is called annihilation of matter.

Question No. 17 - Does the dilation mean that time really passes more slowly in moving system or that it only seems to pass more slowly. Explain briefly.

Answer No. 17 - According to dilation formula this relation show that a clock moving with respect to an observe appears to move slow then it does when it is at rest with respect to him. So the moving clocks just appears to run slowly to the observer at rest. Hence time dilation is an apparent change and it only seems to pass more slowly but not actually.

Question No. 18 - Is it possible to create a single electron from energy?

Answer No. 18 - No it is not possible to create single electron from energy. Because electron has negative charge and photon has no charge.

Question No. 19 - Will the bright light ejects more from metal surface than dimmer light of the same colour?

Answer No. 19 - Since intensity number of electrons and bright light is more intense than dimmer one. So bright light will ejects more electrons than dimmer light.

Question No. 20 - What are advantages of NAVSTAR navigation system?

Answer No. 20 - NAVSTAR is used to locate the position and find the speed of an object anywhere on Earth up to to an accuracy of 2 cms-1.

Question No. 21 - What are conclusion made from the pair production?

Answer No. 21 - Pair production is the creation of an elementary particle and its antiparticle. Pair production offers refers specifically to a photon creating an electron positron pair near a nucleus but can more generally refer to any particle antiparticle pair creation.
Energy can be converted into mass according to E=mc2

Question No. 22 - Which has the lower energy quanta, radio waves or x rays? Explain.

Answer No. 22 - Energy of quanta is given as E=hf-hc/lamda
Radio waves length has longer wavelength, Therefore radio waves has lower energy quanta.

Question No. 23 - The rest mass of photon is zero. Is its momentum also zero?

Answer No. 23 - No, The rest mass of photon is zero.It travels in the form of small energy packets which are of equal wavelength. When it is in motion, it possesses some mass.So it which it will also have momentum.

Question No. 24 - Define ionization potential and excitation potential.

Answer No. 24 - Ionization potential: The potential necessary to remove an electron from the atom is called ionization potential, It is expressed in volts.

Question No. 25 - State uncertainty principle.

Answer No. 25 - The product of uncertainty in the measurement of momentum and uncertainty in the measurement of position of an electron is approximately equal to Plancks constant.
The product of uncertainty in the measurement of energy and uncertainty in the measurement of time of an electron is approximately equal to Plancks constant.

Question No. 26 - What is wave particle duality?

Answer No. 26 - It says that light has dual nature;it travels in the form of waves but interacts with matter in the form of energy particle called as photon. E.g in interference, reflection and polarization it show the properties of wave nature but in phenomena like pair production and Comptons effect light acts as energy particle.

Question No. 27 - Can an object move with the speed of light?

Answer No. 27 - As an object approaches the speed of light, its masses rises abruptly. If an object tries to travel 186,000 miles per seconds, its mass become infinite and so does the energy required to move it, For this reasons, no normal object can be travel as fast as or faster than the speed of light.

Question No. 28 - What do you understand by work function and stopping potential?

Answer No. 28 - The minimum amount energy required to remove an electron from the surface of a metal is called work function.
And the stopping potential is the potential difference applied to stop the electrons from being ejected from the surface when the light falls on it.

Question No. 29 - As a solid is heated and begins to glow, why does it first of its absolute.

Answer No. 29 - Since the red light has longest wavelength, so it will be emitted first and solid appears red first.

Question No. 30 - Define Stopping potential and Threshold frequency.

Answer No. 30 - Stopping potential: The stopping potential is the potential difference applied to stop the electrons from being ejected from the surface when the light fall on it.
Threshold frequency: The minimum of frequency of incident light at which electrons are emitted from a surface is called threshold frequency.

Question No. 31 - Why dont we observe a Compton Effect with visible light?

Answer No. 31 - The frequency of visible light is less than x rays and the wavelength of visible light is much greater than the Compton of wavelength of electron. So Compton effect can not observed with visible light.

Question No. 32 - Define Compton effect. Write formula of Compton shift for scattering angle.

Answer No. 32 - When X-rays are scattered by loosely bound electrons from a graphite target, the phenomena of change in wavelength is known as Compton Effect.

Question No. 33 - A satellite is orbiting around earth. Is its frame of reference inertial or non-inertial? Justify your answer.

Answer No. 33 - The motion of the satellite is synchronized with the earth so it is in the same frame of reference in which earth lies. Hence the frame of reference will be inertial.

Previous Question

1-33

Next Question