Chapter 11

 Photoelectric Effect

The phenomenon of emission of electrons from the surface of a metal due to the effect of light is called photoelectric effect.

The electrons emitted in this way are called light electrons or photo electrons. If the circuit is closed, then the flowing current is called photoelectric current.

Hertz and Lenard's experiments

Scientists Hertz, Leonard and Milken conducted many experiments on photoelectric emission. They took plates of different types of metals and made light of different intensities and different frequencies fall on them and measured the maximum kinetic energy and photoelectric current of the emitted electrons in each condition. Thus, they obtained many relations of the photoelectric effect.

Effect of light intensity –

When light is incident on the surface of a metal, if the frequency of light is appropriate, light electrons start emitting from the surface. When the intensity of the incident light is increased, the value of photoelectric current also increases in almost the same proportion.

Effect of frequency of light –

When a graph is drawn by taking the frequency of the incident light on the x-axis and the maximum kinetic energy of the light electrons on the y-axis, a straight line is obtained. This means that when the frequency of the incident light is high, the maximum kinetic energy of the emitted light electrons is also high.

Ek = A(υ - υ o )

Threshold frequency

The minimum frequency of incident light that can emit light electrons from the surface of a metal is called the threshold frequency. It is represented by u o .

  • If u > u o then light electrons will be emitted.
  • If u < u o   then light electrons will not be emitted.
  • The value of threshold frequency is fixed for a given material and its value is different for different materials.

Threshold Wavelength

The maximum wavelength of incident light that can cause emission of light electrons from the surface of a metal is called the threshold wavelength. It is expressed as λo.

  • [λ o = c/υ o ] where c – speed of light
  • If λ < λ o then light electrons will be emitted.
  • If λ>λ o light e will not be emitted.

qualitative nature theory of light

According to Planck's quantum theory, light travels in the form of small bundles or packets of energy. This bundle of energy is called a photon or quantum.

The energy of each photon is E=hυ, where υ is the frequency of light and h is Planck's universal constant. Its value is 6.62X10 -34 joule-second. The intensity of light depends on the number of these photons. If the wavelength of light is λ and the speed of light in vacuum is c, then the energy of the photon = hc/λ

Work function

The minimum energy that can emit light electrons from the surface of a metal is called the work function. It is expressed as w.

Mathematician – is electron volt (ev) or J (joule).

work function

Stopping Potential

The minimum negative potential given to the plate (anode) relative to the cathode, at which the value of photoelectric current becomes zero, is called the stopping potential or stopping potential. It is expressed as V o .

Experimental laws of photoelectric effect

On the basis of experiments the following rules were formulated –

  • The value of photoelectric current depends on the intensity of the incident light.
  • The maximum kinetic energy of photo electrons does not depend on the intensity of the incident light.
  • The maximum kinetic energy of the emitted light electrons depends on the frequency of the incident light.
  • If the frequency of the incident light is less than the threshold frequency for the metal, then no matter how intense the light is, or how long it is incident, emission of light electrons cannot occur.
  • As soon as light falls on the metal surface, light electrons start getting emitted from the surface, that is, there is no time lag between the incidence of light and the emission of light electrons.

Einstein's photoelectric equation

Einstein explained on the basis of German scientist Max Planck's quantum theory that when a light photon falls on a metal surface, the energy (hv) of the photon is divided into two parts. The first part brings the light electron to the metal surface, which is called work function (w). The second part of the energy provides maximum energy (E k ) to the light electrons.

hυ=W+ Ek

Einstein's photoelectric equation is:- ½ mv² max =h (υ-υ o )

Matter Waves

When a particle (photon) moves, a wave is always associated with that particle, this wave is called matter wave.

The wavelength of waves related to different particles is different.

Expression for de Broglie wavelength

λ=h/p 

de Broglie wavelength associated with the electron

de Broglie wavelength associated with the electron

Comments

Post a Comment

Popular posts from this blog

HOW DO ORGANISM REPRODUCE CLASS 10 NOTES BY THE SIDDIQUI ACADEMY

Image
  Introduction All living organisms multiply or reproduce and produce offspring of a similar kind. Reproduction is an essential process for the existence of a species and the continuation of life. Here, in this chapter, we will learn about how different unicellular and multicellular organisms, such as bacteria, algae, plants, animals, and human beings, reproduce. Learn what the different reproductive structures are and about the modes of reproduction, such as cell division, vegetative reproduction, asexual reproduction, and sexual reproduction. Reproduction Reproduction is the process by which all organisms multiply in number and increase their population. Asexual Reproduction Asexual reproduction is a method of reproduction that involves only one organism. A single organism reproduces two or multiple organisms on its own. This is seen in all unicellular organisms, some multicellular organisms and a few plants. Sexual Reproduction The mode of reproduction involves two individuals; ...
Read more

ASSIGNMENT ON ISSUE OF SHARES

  THE SIDDIQUI ACADEMY: ASSIGNMENT ON ISSUE OF SHARES DATED : 13-10-2024 Shares issued at premium ASSIGNMENT QUESTION 1: X Ltd. was registered with a capital of 23,00,000 in shares of Rs. 10 each. It issued a prospectus inviting applications for 23,000 shares at 40% premium payable as follows:-  On application Rs. 5 (including Rs. 1 premium)  On allotment Rs. 4 (including Rs. 1 premium)  On first call Rs. 3 (including Rs. 1 premium)  On second call Rs. 2 (including Rs. 1 premium) Applications were received for 23,000 shares, all money was duly received. Pass the necessary journal entries. ASSIGNMENT QUESTION 2: A Ltd. issued to the public for subscription of 1000 shares of Rs. 10 each at a discount of 10% payable Rs. 4 on application Rs. 3 on allotment and Rs. 2 on first and final call. The issue was fully subscribed and all the money was duly received. Pass the journal entries for the following
Read more

The role of tribal uprisings in the freedom struggle

  The role of tribal uprisings in the freedom struggle of India cannot be undermined or overlooked. The tribes across the country played a significant role in challenging the British rule and fighting for the independence of their motherland. Their revolt against the colonial powers showcased a sense of unity and determination towards achieving freedom. The tribal uprisings were not only a response to the oppressive policies of the British, but they also represented a struggle for the preservation of their unique cultures, traditions, and way of life. The indigenous communities, with their deep-rooted connection to the land, felt a strong attachment to their territories, which were often subjected to exploitation by the British administration. Their resentment towards the encroachment on their lands and disrupting their traditional way of life fueled their desire to fight for independence. The Santhal rebellion of 1855-56, led by the Santhal tribe in eastern India, is one of the ...
Read more