A beam of photons
"A beam of photons" is a free, interactive Class 12 Physics simulation for NEET and JEE, built on the NCERT section "The photon" in Dual Nature of Radiation and Matter. A laser beam drawn as photons: photons per second, energy and momentum of each, and the push on an absorbing or reflecting target. NEET has asked about this idea 4 times in 10 years of papers. JEE Main asked 40 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
From the lesson The photon in Dual Nature of Radiation and Matter. Change the values and watch what happens.
What it shows
NCERT §11.7
A laser beam drawn as photons: photons per second, energy and momentum of each, and the push on an absorbing or reflecting target.
Controls
3 to try
- Sliderpower 0.1–10 mW
- Sliderwavelength 400–700 nm
- Toggleabsorbing / reflecting target
Try this
Make a 450 nm beam deliver as many photons per second as a 2.0 mW, 650 nm beam (≈2.9 mW)
Common mistake
Thinking brighter light means more energetic photons.
In NEET
10 years of papers
Asked 4 times in 10 years of NEET.
See the past questions: Photoelectric Effect, Dual Nature of Radiation and Matter (chapter)
In JEE Main
42 shifts analysed
40 questions from Dual Nature of Radiation and Matter across the 42 JEE Main shifts Lumi has analysed (2017–2026).
The idea behind it
NCERT §11.7
- The photoelectric effect shows that light, when it interacts with matter, acts as if it were made of packets of energy hν.
- Einstein showed that the quantum also carries momentum hν/c. A definite energy together with a definite momentum is a strong sign of a particle, and the particle was later named the photon. Compton's experiment on X-rays scattered by electrons, in 1924, confirmed this particle-like behaviour.
- Einstein received the Nobel Prize in Physics in 1921 for his work in theoretical physics and on the photoelectric effect; Millikan received it in 1923 for his work on the elementary charge and on the photoelectric effect.
- The photon picture: (i) in its interaction with matter, radiation behaves as if made of photons; (ii) each photon has energy E = hν, momentum p = hν/c and speed c.
- (iii) All photons of a given frequency ν, or wavelength λ, have the same energy E = hν = hc/λ and the same momentum p = hν/c = h/λ, whatever the intensity. Brighter light only means more photons crossing a given area per second.
- (iv) Photons carry no charge, so electric and magnetic fields do not deflect them. (v) In a collision between a photon and a particle, total energy and total momentum are conserved, but the number of photons need not be: a photon may be absorbed or a new one created.
- Example 11.1: a laser emits 2.0 × 10⁻³ W of light at 6.0 × 10¹⁴ Hz. Each photon has E = hν = 6.63 × 10⁻³⁴ × 6.0 × 10¹⁴ = 3.98 × 10⁻¹⁹ J, and the number emitted per second is N = P/E = 2.0 × 10⁻³/3.98 × 10⁻¹⁹ = 5.0 × 10¹⁵.
- Exercise 11.1: a 30 kV electron can give at most 30 keV to an X-ray photon, so νmax = eV/h = (30 × 10³ × 1.6 × 10⁻¹⁹)/(6.63 × 10⁻³⁴) = 7.24 × 10¹⁸ Hz, and λmin = c/νmax = 0.0414 nm.
Questions
5 answers
- What does the "A beam of photons" simulation show?
- A laser beam drawn as photons: photons per second, energy and momentum of each, and the push on an absorbing or reflecting target.
- What mistake does the "A beam of photons" simulation fix?
- A common mistake it clears up: thinking brighter light means more energetic photons.
- Has NEET asked about this idea?
- Yes. NEET has asked about this idea 4 times in 10 years of papers.
- Does JEE Main ask questions from Dual Nature of Radiation and Matter?
- Yes. JEE Main asked 40 questions from this chapter across the 42 shifts Lumi has analysed (2017–2026).
- Is this simulation free?
- Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "The photon" in Dual Nature of Radiation and Matter (Class 12 Physics).
Embed this simulation
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