Dual Nature of Light

Overview

This lecture discussed various phenomena of light, Huygens' theory, the dual nature of light, and GI Taylor's diffraction experiment.

Phenomena of Light

• Refraction (bending of light when changing medium) was studied in class 10.
• Interference, diffraction, polarization, photoelectric effect, and Compton effect were studied in class 12.
• Spectra of atoms and black body radiation are also phenomena of light.

Wave and Particle Nature

• Reflection, interference, diffraction, and polarization were explained by wave nature.
• Photoelectric effect, Compton effect, and black body radiation were explained by particle nature.
• Huygens proposed the wave nature theory and explained the wavefront concept.
• Planck introduced the concept of energy bundles (photons) of light; the energy of a photon is given by (E = h\nu).

Dual Nature of Light

• Light has both wave and particle nature, called dual nature.
• The behavior of light depends on the experiment—sometimes wave, sometimes particle.

Human Eye and Light

• An object is visible only when light reaches the retina.
• Light shows wave nature in the lens; particle nature (photons) is seen in the absorption of light on the retina.

GI Taylor's Diffraction Experiment

• Photons were passed one by one through a slit; after a long time, a diffraction pattern appeared on the photographic plate.
• The diffraction pattern indicates wave nature, even though particles (photons) are passing through.
• Conclusion: Each photon is associated with a wave that describes its probability distribution.
• The exact position of a photon cannot be determined; only the region with higher probability can be indicated.

Contribution of D. Broglie

• D. Broglie suggested that if photons can have waves, then particles like electrons and protons can also have waves.
• Such waves are called matter waves or de Broglie waves.

Key Terms & Definitions

• Refraction — bending of light when passing from one medium to another.
• Interference — pattern formation due to the meeting of two light waves.
• Diffraction — bending and spreading of light when passing edges or slits.
• Polarization — vibration of light waves in one direction.
• Photon — energy bundle of light, a particle.
• Dual Nature — simultaneous wave and particle behavior of something.
• Matter Wave/De Broglie Wave — wave nature associated with particles.

Action Items / Next Steps

• Study the lecture on de Broglie waves and matter waves in the next class.
• Review the photoelectric and diffraction phenomena from the previous class.