Which one among the following shows particle nature of light?
Photoelectric effect
Understanding Light: Wave Nature vs. Particle Nature
Light exhibits a fascinating dual nature, meaning it behaves as both a wave and a particle depending on the phenomenon being observed. Certain experiments and effects are best explained by considering light as waves, while others require treating light as particles.
Wave Phenomena of Light
Many common optical phenomena can be fully explained using the wave model of light. These include:
- Refraction: The bending of light as it passes from one medium to another due to a change in its speed. This is a classic wave behavior.
- Interference: The superposition of two or more waves resulting in a new wave pattern. This produces constructive interference (brighter regions) or destructive interference (darker regions), like seen in thin films (e.g., soap bubbles) or Young's double-slit experiment.
- Polarisation: This effect applies to transverse waves, including light waves. Polarisation refers to restricting the vibrations of the wave to a specific direction or plane.
These effects demonstrate properties like wavelength, frequency, amplitude, and phase, which are characteristic of waves.
Particle Phenomenon of Light: The Photoelectric Effect
While wave theory successfully explains many phenomena, it fails to explain certain observations, most notably the photoelectric effect. The photoelectric effect is the emission of electrons from a material when light shines on it.
Here's why the photoelectric effect strongly supports the particle nature of light:
- According to classical wave theory, the energy of light is spread uniformly over the wavefront. Therefore, electrons should be emitted eventually, regardless of the light's frequency, as long as its intensity is high enough.
- However, experiments showed that electrons are emitted only if the incident light has a frequency above a certain minimum threshold frequency (called the work function of the material), regardless of intensity. Below this frequency, no electrons are emitted, even at high intensity.
- Furthermore, the kinetic energy of the emitted electrons depends on the frequency of the light, not its intensity. Higher frequency light above the threshold results in higher kinetic energy electrons.
- Electron emission is practically instantaneous, even at low light intensity, which contradicts the idea that electrons need time to accumulate energy from a wave.
These experimental findings were explained by Albert Einstein in 1905, building upon Max Planck's quantum theory. Einstein proposed that light energy is not continuous but comes in discrete packets, or quanta, which were later called photons. Each photon carries energy proportional to its frequency, given by the equation:
\(E = hf\)
where:
- \(E\) is the energy of the photon.
- \(h\) is Planck's constant.
- \(f\) is the frequency of the light.
In the photoelectric effect, a single photon interacts with a single electron. If the photon's energy (\(hf\)) is greater than the work function (\(\phi\)) (the minimum energy required to remove an electron from the material), the electron is ejected. The excess energy is converted into the electron's kinetic energy (\(K\)):
\(K = hf - \phi\)
This particle (photon) model perfectly explains the threshold frequency and the dependence of electron energy on frequency, which wave theory could not.
Comparing Phenomena
Let's summarise which nature of light explains the given phenomena:
| Phenomenon | Nature of Light Explained By |
|---|---|
| Refraction | Wave nature |
| Interference | Wave nature |
| Photoelectric effect | Particle nature (photons) |
| Polarisation | Wave nature |
Based on this comparison, the photoelectric effect is the phenomenon among the options that demonstrates the particle nature of light.
Revision Table: Light Nature Concepts
| Concept | Description | Related Phenomena |
|---|---|---|
| Wave Nature | Light behaves as electromagnetic waves with properties like wavelength, frequency, and amplitude. | Refraction, Reflection, Diffraction, Interference, Polarisation |
| Particle Nature | Light consists of discrete energy packets called photons, each with energy \(E = hf\). | Photoelectric effect, Compton scattering, Blackbody radiation |
| Wave-Particle Duality | Light exhibits characteristics of both waves and particles. | All phenomena depending on the experimental setup. |
Additional Information: Compton Scattering and Blackbody Radiation
Besides the photoelectric effect, other phenomena also provide strong evidence for the particle nature of light:
- Compton Scattering: The scattering of a photon by a charged particle (like an electron), resulting in a decrease in the photon's energy (increase in wavelength). This effect can only be explained by treating light as particles undergoing collisions, conserving energy and momentum, just like particle collisions.
- Blackbody Radiation: The spectrum of electromagnetic radiation emitted by an ideal thermal radiator (blackbody). Max Planck's explanation of this spectrum required him to assume that energy is emitted and absorbed in discrete quanta, which was a precursor to the photon concept and quantum mechanics.
These examples further solidify the understanding that light, in certain interactions, behaves as if it is made up of particles called photons, carrying definite amounts of energy and momentum.
Important Questions from Dual Nature of Radiation and Matter
The work function for an Aluminium surface is 4.2 eV. Find the threshold wavelength for the photoelectric emission.
A potentiometer wire of length L and a resistance r are connected in series with a battery of emf E0 and a resistance r1. An unknown emf E is balanced at a length l of the potentiometer wire. The emf E will be:
The time taken by light to travel normally through a glass plate of thickness 1 mm would be:
(Take refractive index of glass = 1.5)
Energy of a photon corresponding to a wavelength of 600 nm is 2.08 eV. The energy of a photon of wavelength 400 nm will be:
A particle moves three times as fast as an electron. The ratio of the de Broglie wavelength of the particle to that of the electron is 1.813 × 10-4. The mass of the particle is:
