Learn
Games

11.3.2 - Hallwachs’ and Lenard’s observations

Interactive Audio Lesson

Listen to a student-teacher conversation explaining the topic in a relatable way.

Introduction to Photoelectric Effect

Unlock Audio Lesson

Signup and Enroll to the course for listening the Audio Lesson

Teacher
Teacher

Today, we’re diving into photoelectric emission, the process where light causes the release of electrons from a material. Can anyone name the physicists who were key in this discovery?

Student 1
Student 1

Was it Hallwachs and Lenard?

Teacher
Teacher

Correct! They explored how ultraviolet radiation impacts metal plates. Lenard noticed that current only flowed when the ultraviolet light was present. Why do you think that is?

Student 2
Student 2

Because the light must be exciting the electrons to escape the metal?

Teacher
Teacher

Exactly! This connected light directly to the electron emission process.

Hallwachs' Contributions

Unlock Audio Lesson

Signup and Enroll to the course for listening the Audio Lesson

Teacher
Teacher

Hallwachs made observations regarding zinc plates. He connected a negatively charged zinc plate to an electroscope and found that the charge decreased with ultraviolet exposure. Can someone explain why this indicates the emission of electrons?

Student 3
Student 3

When the plate loses its negative charge, it must mean that electrons are leaving the plate due to the ultraviolet light.

Teacher
Teacher

Exactly! The emitted electrons were termed 'photoelectrons.' Let’s remember that, the term 'photoelectron' signifies electrons released due to light.

Lenard's Findings

Unlock Audio Lesson

Signup and Enroll to the course for listening the Audio Lesson

Teacher
Teacher

Lenard’s findings were crucial. He observed that current only flows when ultraviolet light hits the plate, disappearing without it. What does that tell us about the relationship between light and electrons?

Student 4
Student 4

It shows that some minimum energy, provided by light, is required to release electrons.

Teacher
Teacher

Correct again! This minimum light intensity links to the threshold frequency, a key concept for us to understand.

Threshold Frequency

Unlock Audio Lesson

Signup and Enroll to the course for listening the Audio Lesson

Teacher
Teacher

Now, let’s think about threshold frequency. Lenard and Hallwachs found that different metals react differently. Can anyone give an example of metals and their light frequencies?

Student 1
Student 1

Zinc only reacts to ultraviolet light, while alkali metals can react to visible light as well.

Teacher
Teacher

Good observation! Metals have their unique thresholds for electron emission depending on their properties.

Introduction & Overview

Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.

Quick Overview

Hallwachs and Lenard conducted experiments that detailed the photoelectric effect, illustrating how ultraviolet radiation causes electrons to be emitted from metals.

Standard

During the late 19th to early 20th centuries, Hallwachs and Lenard explored the photoelectric effect. Their findings demonstrated that ultraviolet light could cause electron emission from metals, emphasizing the relationship between light frequency and electron behavior. These observations were pivotal in validating the quantum nature of light.

Detailed

Youtube Videos

Hallwachs & Lenard's Observation- Dual Nature of Radiation & Matter |Class 12 Physics| CBSE/JEE/NEET
Hallwachs & Lenard's Observation- Dual Nature of Radiation & Matter |Class 12 Physics| CBSE/JEE/NEET
Photoelectric effect || Dual Nature of matter|| Animated explanation in Hinglish||Physics 12th class
Photoelectric effect || Dual Nature of matter|| Animated explanation in Hinglish||Physics 12th class
Class 12 | Physics | Dual Nature of Radiation and Matter | Part 2 - Hallwach and Lenard Observations
Class 12 | Physics | Dual Nature of Radiation and Matter | Part 2 - Hallwach and Lenard Observations
45.Chapter - 10 | Dual Nature Of Radiation & Matter | Lenard's Experiment | Physics Baba 2.O
45.Chapter - 10 | Dual Nature Of Radiation & Matter | Lenard's Experiment | Physics Baba 2.O
Hertz Experiment - Dual Nature of Radiation & Matter | Class 12 Physics Chapter 11 | CBSE/JEE/NEET
Hertz Experiment - Dual Nature of Radiation & Matter | Class 12 Physics Chapter 11 | CBSE/JEE/NEET
Class 12 chap 11 II Dual Nature Of Radiation and Matter 01 : Photoelectric Effect - Part 1 JEE/NEET
Class 12 chap 11 II Dual Nature Of Radiation and Matter 01 : Photoelectric Effect - Part 1 JEE/NEET
Dual nature of Radiation and Matter Class 12 Physics | NCERT Chapter 11 | CBSE NEET JEE | One Shot
Dual nature of Radiation and Matter Class 12 Physics | NCERT Chapter 11 | CBSE NEET JEE | One Shot
Dual Nature Of Radiation and Matter 02 II PhotoElectric Effect - PART 2 -Stopping Potential JEE/NEET
Dual Nature Of Radiation and Matter 02 II PhotoElectric Effect - PART 2 -Stopping Potential JEE/NEET
Photoelectric Effect : Hertz's and Lenard's Observations, NCERT Physics Class 12
Photoelectric Effect : Hertz's and Lenard's Observations, NCERT Physics Class 12
Dual nature and radiation class 12 physics💮💮
Dual nature and radiation class 12 physics💮💮

Audio Book

Dive deep into the subject with an immersive audiobook experience.

Initial Observations by Lenard

Unlock Audio Book

Signup and Enroll to the course for listening the Audio Book

Lenard (1862-1947) observed that when ultraviolet radiations were allowed to fall on the emitter plate of an evacuated glass tube enclosing two electrodes (metal plates), current flows in the circuit (Fig. 11.1). As soon as the ultraviolet radiations were stopped, the current flow also stopped. These observations indicate that when ultraviolet radiations fall on the emitter plate C, electrons are ejected from it which are attracted towards the positive, collector plate A by the electric field.

Detailed Explanation

Lenard's experiments showed that ultraviolet light interacts with a specific metal plate (the emitter plate). When this light hits the plate, it provides energy to electrons, allowing them to overcome the attractive forces holding them within the metal. As a result, these electrons are ejected and flow towards a positively charged plate, creating an electric current. When the light is turned off, this ejected current stops, indicating a direct relationship between the presence of light and electron emission.

Examples & Analogies

Imagine a rubber band that holds marbles (electrons) tightly together. When you shine a light on the band, it gives energy to the marbles, allowing some to break free and roll towards a bucket (the collector plate). The moment you turn off the light, the marbles cannot escape anymore, and the flow of marbles stops.

Hallwachs’ Further Investigations

Unlock Audio Book

Signup and Enroll to the course for listening the Audio Book

Hallwachs, in 1888, undertook the study further and connected a negatively charged zinc plate to an electroscope. He observed that the zinc plate lost its charge when it was illuminated by ultraviolet light. Further, the uncharged zinc plate became positively charged when it was irradiated by ultraviolet light.

Detailed Explanation

Hallwachs built upon Lenard's findings by conducting experiments with a zinc plate connected to an electroscope, which measures electrical charge. He found that shining ultraviolet light on the zinc plate caused it to release negatively charged particles (electrons), leading to a loss of negative charge on the plate, hence resulting in a net positive charge. This demonstrated that light not only made current flow but also actively affected the charge state of materials.

Examples & Analogies

Think of charging a balloon by rubbing it against your hair. When you rub the balloon (like shining ultraviolet light on the zinc plate), it collects negative charges (electrons), making it negative. If you leave it in the air, it can lose its charge (just as the zinc plate loses electrons) and induce a positive charge nearby.

Threshold Frequency and Material Response

Unlock Audio Book

Signup and Enroll to the course for listening the Audio Book

Hallwachs and Lenard also observed that when ultraviolet light fell on the emitter plate, no electrons were emitted at all when the frequency of the incident light was smaller than a certain minimum value, called the threshold frequency. This minimum frequency depends on the nature of the material of the emitter plate.

Detailed Explanation

They found that each material has a specific frequency of light below which it cannot emit electrons. This is known as the threshold frequency. If the frequency of the incident light is lower than this threshold, the energy provided is insufficient to free the electrons from their binding within the metal, resulting in no emission, regardless of light intensity. This concept highlights that not all light can cause photoelectric emission; only specific frequencies can.

Examples & Analogies

Consider a seed that requires a specific amount of sunlight to sprout. If it's placed in a dark room (lower frequency), it won't sprout no matter how long it stays there. But with enough sunlight (above a threshold), it will start to grow and break through the soil, mirroring the concept of frequency being the key to releasing electrons.

Sensitivity of Different Metals

Unlock Audio Book

Signup and Enroll to the course for listening the Audio Book

It was found that certain metals like zinc, cadmium, magnesium, etc., responded only to ultraviolet light, having short wavelength, to cause electron emission from the surface. However, some alkali metals such as lithium, sodium, potassium, caesium and rubidium were sensitive even to visible light.

Detailed Explanation

Different materials respond differently to light. While metals like zinc require ultraviolet light to emit electrons, alkali metals can emit electrons when exposed to visible light. This reveals that not only the type of light but also the material's properties play a significant role in the photoelectric effect, influencing their sensitivity to different parts of the light spectrum.

Examples & Analogies

Think of a variety of plants that need different amounts of sunlight. Some plants (like zinc) thrive with bright sunlight (ultraviolet light), while others (like alkali metals) can grow well even in partial shade (visible light). Each one has a unique set of requirements for growth, just as different metals have different threshold light wavelengths.

Photoelectric Effect Identification

Unlock Audio Book

Signup and Enroll to the course for listening the Audio Book

All these photosensitive substances emit electrons when they are illuminated by light. After the discovery of electrons, these electrons were termed as photoelectrons. The phenomenon is called photoelectric effect.

Detailed Explanation

The culmination of Hallwachs and Lenard’s work led to the identification of the photoelectric effect, where certain substances emit electrons (photoelectrons) when illuminated by light. The understanding that light could induce electron emission formed a foundational idea in quantum mechanics and the study of light's particle nature.

Examples & Analogies

It's similar to how a floor is made slippery when a water balloon bursts on it. Just like the balloons cause changes to their environment, the interaction of light with sensitive metals causes a reaction that results in the emission of electrons, changing the state of the metal.

Definitions & Key Concepts

Learn essential terms and foundational ideas that form the basis of the topic.

Key Concepts

  • Photoelectric Effect: A process in which electrons are emitted from a material when light shines upon it.

  • Threshold Frequency: Each material has a certain minimum frequency of light needed to emit electrons.

  • Hallwachs and Lenard: Key scientists who significantly contributed to understanding the photoelectric effect.

Examples & Real-Life Applications

See how the concepts apply in real-world scenarios to understand their practical implications.

Examples

  • When ultraviolet light is shone on a zinc plate, electrons are released, demonstrating the photoelectric effect.

  • Different metals require varying frequencies of light; zinc does not respond to visible light while alkali metals do.

Memory Aids

Use mnemonics, acronyms, or visual cues to help remember key information more easily.

🎵 Rhymes Time

  • In light of UV rays so bright, electrons fly out of sight!

📖 Fascinating Stories

  • Imagine zinc at a party where UV light is the DJ; when the music plays, the electrons get excited and dance away!

🧠 Other Memory Gems

  • Remember 'PEC' for Photoelectric Effect and Current: Photoelectric Effect leads to emitted Current.

🎯 Super Acronyms

HLP

  • Hallwachs and Lenard study Photoelectric effect.

Flash Cards

Review key concepts with flashcards.

Glossary of Terms

Review the Definitions for terms.

  • Term: Photoelectric Effect

    Definition:

    The phenomenon where electrons are emitted from a material when exposed to light.

  • Term: Threshold Frequency

    Definition:

    The minimum frequency of light required to emit electrons from a metal surface.

  • Term: Photoelectron

    Definition:

    Electrons that are emitted from a material due to exposure to light.