8 Most VSAQ’s of Dual Nature of Radiation and Matter Chapter in Inter 2nd Year Physics (TS/AP)

VSAQ-1 : What are “Cathode rays”?

Cathode rays are streams of electrons originating from the negative plate (cathode) and moving towards the positive plate (anode) within a vacuum tube. They are generated when a high voltage is applied to a gas-filled tube at very low pressure. This low-pressure gas becomes ionized and conducts electricity, resulting in the flow of electrons in the form of cathode rays. These rays played a crucial role in the discovery of the electron and have important applications in electronics, especially in cathode ray tubes (CRTs).

VSAQ-2 : What is “work function”?

Work function is a material-specific property defining the minimum energy required to remove an electron from the surface. When electromagnetic radiation (e.g., light) strikes, electrons can be emitted if the incident energy exceeds the material’s work function. Crucial for photoelectric effect and electron emission. Varies among materials, measured in electron volts (eV).

VSAQ-3 : What is “photoelectric effect”?

The photoelectric effect is a phenomenon where electrons are emitted from the surface of a material when it is exposed to electromagnetic radiation, typically in the form of light. For this emission to occur, the incident light must have sufficient energy (frequency) to surpass the material’s work function (the minimum energy required to remove an electron). The intensity of the emitted electrons depends on the intensity of the incident light, while the kinetic energy of the emitted electrons is influenced by the frequency of the light. The photoelectric effect played a pivotal role in establishing the particle-like behavior of light and contributed significantly to the development of quantum mechanics.

VSAQ-4 : Write down Einstein’s photoelectric equation.

Einstein’s photoelectric equation relates the energy of photons to the kinetic energy of emitted electrons in the photoelectric effect. The equation is as follows:

E = hf – Φ


  1. E represents the energy of the emitted electron.
  2. h is Planck’s constant (approximately 6.626 x 10^-34 Joule-seconds).
  3. f is the frequency of the incident light.
  4. Φ is the work function of the material (the minimum energy required to release an electron).

This equation shows that the energy of the emitted electron (E) is equal to the energy of the incident photon (hf) minus the work function (Φ) of the material. If hf is greater than Φ, the electron is emitted with kinetic energy.

VSAQ-5 : Write down de Broglie’s relation and explain the terms therein.

De Broglie’s relation:

  1. Wavelength (λ): Characteristic length of a particle’s associated wave.
  2. Planck’s Constant (h): Fundamental constant relating energy to frequency.
  3. Momentum (p): Measure of a particle’s motion (p = mv).

This relation explains that particles exhibit both particle-like and wave-like behavior, where the wavelength (λ) is inversely proportional to the momentum (p). Particles with higher momentum have shorter wavelengths, resembling particles more, while those with lower momentum have longer wavelengths, displaying more wave-like characteristics. Crucial in understanding quantum mechanics and particle behavior.

VSAQ-6 : State Heisenberg’s uncertainty principle.

Heisenberg’s uncertainty principle states that it’s impossible to simultaneously measure both the position (Δx) and momentum (Δp) of a particle with perfect accuracy. The product of these uncertainties is always greater than or equal to a constant value (h/2π), where h is Planck’s constant. This principle is a cornerstone of quantum mechanics and reflects the inherent limitations in our ability to precisely measure and predict the behavior of subatomic particles.

VSAQ-7 : An electron, an α particle and a proton have the same kinetic energy. Which of these particles has the shortest de Broglie wavelength?

Among the electron, α particle, and proton with the same kinetic energy, the proton has the shortest de Broglie wavelength due to its higher mass and lower velocity compared to the other particles.

VSAQ-8 : What is Photoelectric effect? How did Einstein’s photo-electric equation explain the effects of intensity (of light) and potential on photo-electric current?

The photoelectric effect is the emission of electrons from a material’s surface when exposed to light, typically in the form of photons. It played a crucial role in understanding the behavior of light and electrons.

Einstein’s photo-electric equation explained the effects of intensity (brightness) of light and potential (voltage) on photo-electric current:

  1. Intensity (Brightness) of Light: The intensity of light (I) determines the number of incident photons per unit time. Higher intensity leads to more incident photons, increasing the chances of electron emission and thus the photo-electric current.
  2. Potential (Voltage): The kinetic energy of emitted photoelectrons depends on the difference between the energy of incident photons (determined by their frequency) and the material’s work function (φ). Increasing the potential (voltage) applied to the material increases the kinetic energy of emitted electrons, allowing a broader range of electrons to contribute to the current.