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 \(1\ \mathrm{eV} = 1.6\times10^{-19}\ \mathrm{J}\).)
The speed with which photons travel in vacuum is
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \( c \)
NCERT (2026-27):
Particle Nature of Light: the Photon – “We can summarise the photon picture of electromagnetic radiation as follows: … (ii) Each photon has energy E (=hν) and momentum p (= h ν/c), and speed c, the speed of light.”