Physics · Dual Nature Of Radiation And Matter · NEET
A photon has zero rest mass, so it can never be at rest. But it still carries momentum p = h*v/c = h/lambda because it has energy and moves at speed c. In NEET, if a question asks for 'rest mass of photon', the answer is zero. If it asks for momentum, use p = E/c.
p = mv is for particles with rest mass moving slower than light. A photon has no rest mass, so we use the relation from relativity E = p*c, which gives p = E/c = h*v/c = h/lambda. So a photon has momentum even though its rest mass is zero.
No. The energy of one photon depends only on frequency: E = h*v. Increasing intensity only increases the NUMBER of photons per second, not the energy of each photon. This is why brighter light of the same colour cannot give photoelectrons more kinetic energy.
A photon is electrically neutral - it has zero charge. So a photon is not deflected by electric or magnetic fields. NEET 2024 tested exactly this: the statement 'a photon possesses positive charge' is FALSE.
Yes, in vacuum every photon moves at c = 3 x 10^8 m/s, whatever its frequency or intensity. A red photon and a blue photon travel at the same speed; the blue one just has more energy because it has higher frequency.
They are two pictures of the same light. The wave picture (frequency, wavelength) explains interference and diffraction. The photon picture (packet of energy E = hv, momentum p = hv/c) explains the photoelectric effect and Compton effect. Which picture we use depends on the experiment - this is the dual nature of light.
If c is the velocity of light in free space, the correct statements about a photon among the following are: A. The energy of a photon is E = hv. B. The velocity of a photon is c. C. The momentum of a photon, p = hv/c. D. In a photon-electron collision, both total energy and total momentum are conserved. E. A photon possesses positive charge.
The number of photons per second on an average emitted by the source of monochromatic light of wavelength 600 nm, when it delivers the power of 3.3 x 10^-3 W will be: (h = 6.6 x 10^-34 J.s)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A photon is one tiny packet of light energy. Light is not a smooth continuous flow; it comes in these packets. Each packet carries energy E = h*v, where h is Planck's constant and v is the frequency of the light.
Energy E = h*v = h*c/lambda; momentum p = h*v/c = h/lambda; speed c in vacuum; zero electric charge; zero rest mass. All photons of the same frequency carry the same energy and momentum, whatever the intensity.
Albert Einstein introduced light quanta in 1905 to explain the photoelectric effect. These quanta were later named 'photons'. Einstein received the 1921 Nobel Prize in Physics for this work.
Use E = h*c/lambda. For example, with h = 6.6 x 10^-34 J.s, c = 3 x 10^8 m/s and lambda = 600 nm, E = 3.3 x 10^-19 J, which is about 2.06 eV (divide joules by 1.6 x 10^-19).
The photon still moves at c in vacuum, but light appears to travel slower in a medium because of absorption and re-emission by atoms. For NEET, take the photon's speed as c = 3 x 10^8 m/s in free space.
