Physics · Class 12 Science
Ch 11Dual Nature of Radiation and Matter — Class 12 Physics, concept-first.
By the end of the nineteenth century, physics seemed, to many working physicists of the day, very nearly complete. Newtonian mechanics explained the motion of everything from falling apples to orbiting planets; Maxwell's electromagnetic theory, capped by the discovery of electromagnetic waves, had unified electricity,…
Key concepts
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Photoelectric Effect
Imagine you're throwing tennis balls at a wall covered in loose pebbles. If you throw hard enough, a pebble might get knocked off.
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In previous exams
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Chapter contents
The NCERT structure, section by section. Open a section to see its questions, then read the concept-first solution.
Introduction
By the end of the nineteenth century, physics seemed, to many working physicists of the day, very nearly complete.
Photoelectric Effect: Hertz's Observations
What Hertz was actually doing. In 1887, Heinrich Hertz was not investigating the photoelectric effect at all -- he was carrying out the historic experiment that first produced and detected electromagn…
Hallwachs' and Lenard's Observations
Hallwachs' experiment. Wilhelm Hallwachs, in 1888, devised a much simpler and more direct way to study Hertz's puzzling observation.
Experimental Study of the Photoelectric Effect
The apparatus. The standard arrangement used to study the photoelectric effect quantitatively is described in this section's first figure: an evacuated tube containing an emitter plate C, on which the…
Threshold Frequency, Work Function and the Laws of Photoelectric Emission
Work function. The work function of a metal, denoted , is defined as the minimum energy that must be supplied to free the most loosely bound electron from the surface of that metal and remove it compl…
Failure of the Wave Theory of Light
What the classical wave theory predicts. In the classical (Maxwell) picture, a light wave carries its energy CONTINUOUSLY, spread out over its entire wavefront, with the energy delivered per second to…
Einstein's Photoelectric Equation
Einstein's postulate. In 1905, Albert Einstein proposed that light of frequency is not delivered continuously at all, but arrives, and is absorbed, in discrete packets (later named photons), each pack…
Particle Nature of Light: The Photon
The photon as a particle of radiation. Einstein's equation (Section 11.7) works only if light genuinely behaves, in this kind of interaction, as a stream of localized, particle-like energy packets rat…
Matter Waves: de Broglie's Hypothesis
Turning the argument around. Sections 11.7-11.8 established that radiation, ordinarily understood as a wave, shows a genuine particle aspect (the photon) in the photoelectric effect, with the particle…
de Broglie Wavelength: Formula, Verification and Simple Applications
The formula for an accelerated electron. An electron of charge magnitude and mass , starting from rest and accelerated through a potential difference , gains kinetic energy , which is related to its m…
Summary
Photoelectric effect: the ejection of electrons from a metal surface when light of a sufficiently high frequency falls on it; first noticed by Hertz (1887), investigated qualitatively by Hallwachs (18…
Sample & Board Papers
Sample papers and previous-year board questions for this subject.
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- Q1Write down Einstein's photoelectric equation and mention the symbols used. The photoelectric threshold wavelength for a certain metal is 400…Preview
- Q2For a monochromatic light incident on a metal surface, the maximum velocity of the emitted photoelectrons is v. Then the stopping potential…Preview
- Q3a) Under what potential difference should an electron be accelerated to obtain de Broglie wavelength of 0.6 Å? (h = 6.62×10^-34 J-s, m_e = 9…Preview
- Q4Calculate the ratio of the accelerating potential required to accelerate a proton and an α-particle to have the same de Broglie wavelength a…Preview
- Q5Draw a graph between the frequency (ν) and the maximum kinetic energy of the electrons emitted from the surface of a photosensitive material…Preview
- Q6If photons of frequency ν are incident on the surfaces of metals A & B of threshold frequencies ν/2 and ν/3 respectively, the ratio of the m…Preview
- Q7The slope of the graph between frequency of incident light and stopping potential for a given metal surface will be (a) h (b) eh (c) h/e (d)…Preview
- Q8(a) Why is the frequency and not the intensity of light source that determines whether emission of photoelectrons will occur or not? Explain…Preview
- Q9The magnitude of saturation of photoelectric current depends upon (a) frequency (b) stopping potential (c) work function (d) intensity of li…Preview
- Q10(a) Write down the Einstein's photoelectric equation. Explain the emission of photoelectrons from a metal surface using this equation. [1+1]…Preview
- Q11If the momentum of an electron is changed by P, then the de Broglie wavelength associated with it changes by 0.5%, then the initial momentum…Preview
- Q12What do you mean by dual nature of matter? Calculate the momentum of a photon associated with a radiation of frequency 6×10¹³ Hz. (1+2) **OR…Preview
- Q13If the de-Broglie wavelength of a gas molecule at 0°C be λ, what will be its wavelength at 819°C?Preview
- Q14Write down Einstein's photoelectric equation and mention the symbols used.Preview
- Q15How can you calculate the value of Planck's constant from the graph of stopping potential and wavelength of incident wave? ![A graph of stop…Preview
- Q16Give examples of the production of (i) electron by photon and (ii) photon by electron.Preview
More questions
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- Example 1What is meant by the photoelectric effect? Define, in this context, the terms threshold frequency and stopping potential.Free
- Example 2State the observation Heinrich Hertz made in 1887 while testing Maxwell's electromagnetic-wave theory that, without his realising it at the…Free
- Example 3State the key experimental findings of Hallwachs and Lenard on the photoelectric effect, and explain briefly why these findings could not be…Preview
- Example 4Write down Einstein's photoelectric equation and explain, term by term, what each quantity in it represents.Preview
- Example 5What is a photon? State any two properties of a photon that show it behaving like a particle rather than a spread-out continuous wave.Preview
- Example 6State de Broglie's hypothesis about the wave nature of matter, and write the formula for the de Broglie wavelength of a particle of mass $m$…Preview
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- Q7Using the idea of the work function of a metal, explain why photoelectric emission from a given surface occurs only when the frequency of th…Free
- Q8Describe, in your own words, the basic experimental arrangement (evacuated tube, emitter and collector plates, variable potential difference…Free
- Q9For light of a fixed frequency above the threshold frequency, explain how the photoelectric current varies as the intensity of the incident…Free
- Q10Sketch, in words, the graph of stopping potential $V_0$ versus frequency $\nu$ of the incident light for a given metal surface, and explain…Preview
- Q11The classical wave theory of light predicts that photoelectric emission should occur at any frequency provided the light is intense enough,…Preview
- Q12Explain why the maximum kinetic energy of the emitted photoelectrons depends only on the frequency of the incident light and the nature of t…Preview
- Q13Two metals $P$ and $Q$ have work functions such that $\phi_P > \phi_Q$. Which metal has the higher threshold frequency? If the frequency of…Preview
- Q14Photoelectric emission is observed to begin, for light above the threshold frequency, without any measurable time delay, even when the incid…Preview
- Q15Explain, using de Broglie's own reasoning, why the wave nature of matter is never observed for everyday moving objects such as a thrown ball…Preview
- Q16Describe briefly how an electron-diffraction experiment of the Davisson-Germer type provided direct experimental confirmation of de Broglie'…Preview
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- Q17The work function of caesium is $2.14\ \text{eV}$. Calculate (a) the threshold frequency and (b) the threshold wavelength for photoelectric…Free
- Q18Light of wavelength $400\ \text{nm}$ is incident on a metal surface of work function $2.0\ \text{eV}$. Calculate the maximum kinetic energy…Free
- Q19The threshold wavelength for photoelectric emission from a certain metal is $500\ \text{nm}$. Calculate the work function of the metal in el…Free
- Q20For a certain metal surface, light of frequency $8\times10^{14}\ \text{Hz}$ gives a stopping potential of $0.6\ \text{V}$. Calculate the wor…Preview
- Q21Calculate the de Broglie wavelength of an electron that has been accelerated from rest through a potential difference of $100\ \text{V}$.Preview
- Q22Calculate the de Broglie wavelength associated with a ball of mass $0.20\ \text{kg}$ moving with a speed of $10\ \text{m/s}$, and comment on…Preview
- Q23An electron and a proton have exactly the same kinetic energy. Show that the de Broglie wavelength of the electron is about $42.8$ times tha…Preview
- Q24Calculate the de Broglie wavelength of an electron whose kinetic energy is $150\ \text{eV}$, and state how this wavelength compares with the…Preview
- Q25A photocell records a photoelectric current of $3.2\ \mu\text{A}$ when illuminated steadily by light above the threshold frequency. Calculat…Preview
- Q26The stopping potential for photoelectrons emitted from a metal surface under a certain illumination is measured to be $1.5\ \text{V}$. Calcu…Preview