Read the following passage and answer the questions.
A student studies a photocell. It is an evacuated glass tube with a quartz window. Inside the tube are an emitter plate C and a collector plate A. Plate C is coated with a metal that gives out electrons even in red light. A battery and a commutator let her make plate A positive or negative with respect to C. A microammeter reads the photocurrent.
In the first run, she shines a red laser pointer on plate C. The light has a wavelength of 620 nm, and the pointer gives out a power of 2.4 mW. She records the photocurrent as she changes the potential of A from positive to negative values. In the second run, she uses a red laser of the same wavelength but twice the power. In the third run, she uses a violet laser of wavelength 410 nm. She adjusts it so that the same number of photons reach plate C each second as in the first run.
(Take \(hc = 1240\ \mathrm{eV\,nm}\) and \(1\ \mathrm{eV} = 1.6\times10^{-19}\ \mathrm{J}\).)
Consider the following statements about the photons in the laser beams used by the student:
(A) The photons are not deflected by an electric field or a magnetic field.
(B) When a photon collides with an electron, the number of photons stays the same.
(C) Each photon of the red laser has a momentum \(h/\lambda\), where \(\lambda = 620\ \mathrm{nm}\).
(D) Light shows its photon nature while it travels through vacuum, but its wave nature when it interacts with matter.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A) and (C) only
In the photon picture, light is a stream of particles called photons. Each photon has energy \( h\nu \) and momentum \( h\nu/c \).
(A) The photons are not deflected by an electric field or a magnetic field. — True. Photons are electrically neutral, so these fields exert no force on them.
(B) When a photon collides with an electron, the number of photons stays the same. — False. The photon may be absorbed by the electron, or a new photon may be created. So the number of photons may change in a collision.
(C) Each photon of the red laser has a momentum \( h/\lambda \), where \( \lambda=620\ \mathrm{nm} \). — True. The momentum of a photon is \( p=h\nu/c \). Since \( \nu=c/\lambda \), this is \( p=h/\lambda \), with \( \lambda \) the wavelength of the red light.
(D) Light shows its photon nature while it travels through vacuum, but its wave nature when it interacts with matter. — False. Light behaves as a stream of photons when it interacts with matter. The photoelectric effect in this photocell is one such case.
NCERT (2026-27):
Particle Nature of Light: the Photon – “(iv) Photons are electrically neutral and are not deflected by electric and magnetic fields.”
Particle Nature of Light: the Photon – “(v) In a photon-particle collision (such as photon-electron collision), the total energy and total momentum are conserved. However, the number of photons may not be conserved in a collision. …”
Particle Nature of Light: the Photon – “(iii) All photons of light of a particular frequency ν, or wavelength λ, have the same energy E (=hν = hc/λ) and momentum p (= hν/c= h/λ), whatever the intensity of radiation may be. …”
Particle Nature of Light: the Photon – “(i) In interaction of radiation with matter, radiation behaves as if it is made up of particles called photons.”