Dual Nature of Radiation and Matter: Class 12 Physics Notes, Definition, Working Principle, Formula & Real-Life Applications

Wave–particle duality means tiny things—like light particles (photons) and electrons—can behave like both waves (spreading out, creating ripples) and particles (tiny localized bits) depending on how we look at them. In the famous double-slit experiment, individual electrons pass through two slits and build up an interference pattern on a screen that only makes sense if each electron spreads out like a wave and goes through both slits at once. Yet the moment we try to watch which slit the electron takes, it acts like a particle, and the wave-like pattern disappears. Light does the same: at very low intensity it arrives as single dots on a detector (particles), but as more dots accumulate, they reveal a wave-like interference pattern. Early on, scientists argued over whether light was a wave or a particle; by the 1920s, it became clear that both views are needed—this idea is called wave–particle duality.

Photoelectric emission happens when light shines on a metal surface and instantly knocks out electrons (called photoelectrons). Experiments show that there’s a minimum threshold frequency: if the light’s color (frequency) is too low, no electrons escape, no matter how bright the light is. When the light’s frequency exceeds that threshold, electrons fly out right away, and their maximum speed depends only on that frequency, not on brightness. In the lab, scientists use a small opposing voltage (the stopping potential) to figure out how much energy each electron had, confirming that energy comes in whole chunks from single photons.

Do note that this chapter is important for students of CBSE Board exam, NEET and JEE exams.

The Fermi Paradox poses the question, "Why haven't we seen any signs of aliens—no signals, no visits, no artifacts—if there are billions of stars with planets that could support life?" Although we are aware that several planets are located in "habitable zones" where liquid water may exist, we have not found any concrete evidence despite using radio telescopes and sending probes to search. Some potential responses are:

• Most planets never have a spacefaring civilization because of a Great Filter that renders advanced life incredibly rare.
• According to a zoo theory, aliens are aware of humans but decide not to intervene or come forward.
• Before switching to undetectable technology or self-destruction, civilizations may only broadcast for a brief period of time.
• It's possible that our detection techniques are too antiquated to identify their travel routes or signals.

The Casimir effect is a tiny force that pulls two uncharged,flat metal plates together when they’re extremely close (just a few nanometers apart) in a perfect vacuum. Although “nothing” seems to sit between them, quantum physics says empty space is filled with random fluctuations (virtual photons). Between the plates, only certain standing waves of those fluctuations fit; outside, all wavelengths fit. Because there are fewer allowed fluctuations between the plates, that region has slightly lower “vacuum energy.” The higher energy outside pushes the plates inward—this is called the Casimir force.

In an attempt to explain the photoelectric effect prior to Einstein's photon theory, scientists saw light as a wave and assumed that electrons would progressively gain energy from light waves until they had enough to jump out.  However, that viewpoint was unsuccessful because:

• No Energy Build-Up: Wave theory anticipated that, given enough time, even very intense red light (low frequency) should eventually eject electrons, but this was not possible no matter how long you waited.
• Instant Ejection: When exposed to UV light, electrons ejected from the metal surface instantly; wave theory anticipated a delay while the electrons absorbed energy gradually.

Light removing electrons from materials is known as the photoelectric effect, and it can take several forms:

When light strikes a metal, electrons are released into a vacuum, a phenomenon known as external photoemission. We can detect very faint light by amplifying those electrons with devices like photomultiplier tubes, and solar cells use this process to turn sunlight into electricity.

• Internal Photoemission: Some photodetectors and optoelectronic devices function by pushing electrons across a boundary (such as from a metal layer into a semiconductor) using light.
• Multiphoton Photoemission: Scientists can investigate ultrafast electron dynamics in materials by using super-intense lasers to absorb many lower-energy photons simultaneously and then obtain enough energy to escape.