Waves and its Characteristics
It is a periodical disturbance causing the transfer of energy without transfer of matter.
➢ Characteristics of Waves
- Amplitude: Maximum displacement from the mean position it remains constant with distance except for stationary or standing waves.
- Wavelength: It is the distance between two adjacent crest or troughs.
- Frequency (m or n): The no of waves passing through a point in 1 second, unit-sec-1 or Hz.
- Wavenumber: No. of waves present in the unit distance. etc.
- Velocity: linear distance travelled by wave in one second.
Question 1:The frequency of a wave is 6 × 1015 s–1. Its wave number would be
Nature of Light
➢ Maxwell Electromagnetic Wave Theory (Wave Nature of Light)
- An accelerated electrically charged particle produces and transmits an electrical and magnetic field. These are transmitted in the form of waves known as electromagnetic waves or electromagnetic radiations.
- He stated that light also possesses an electrical and magnetic field, and it is also known as electromagnetic radiations or e.m.w.
➢ Characteristics of Electromagnetic Radiations
- In this electromagnetic radiation, electrical and magnetic field oscillates perpendicular to each other, and it also propagates perpendicular to both the fields.
- All these electromagnetic radiations do not require any medium and can travel in a vacuum.
- The velocity of all electromagnetic radiations is 3 × 108 m/s in a vacuum.
- The energy of an electromagnetic wave is directly proportional to the intensity, and it is independent of frequency.
- They are also showing diffraction and interference and therefore. Maxwell concluded light to be wave nature.
- But Maxwell theory couldn't explain the results of the photoelectric effect and black body radiations.
➢ Photon Energies for EM SpectrumEM Spectrum
- A black body is an object that emits a well-defined spectrum of radiation solely based on its temperature. We see from the figure at right that the hotter the black body, the more intense it is, and the shorter the peak wavelength.
- The picture does not say anything about what the object is made of, or how heavy it is, etc. It doesn't matter! The only property that determines the spectrum of a black body is its temperature.
- Brick, iron or a dense gas will emit the same spectrum as long as they are at the same temperature. That spectrum will have a peak that lies at a particular wavelength.Graph of Intensity vs Wavelength
➢ Planck's Quantum Theory [Particle Nature of Light]
- He stated that a body radiates energy in the form of discontinuous energy packets or bundles. Each bundle of energy is known as quantum, and the quantum of light is known as photons. The energy of each quantum is directly proportional to the frequency of radiation.
E = hv
h = 6.62 × 10-34 Js.
➢ Plank's Constant
- Total energy absorbed or emitted by a body will be whole no. integral multiple of the energy of quantum. i.e., Eabs or Eemitted = nhν.
Ex. Calculate the no. of photons emitted by the 60-watt bulb in 10 hrs. When the light of wavelength 6000 Å is emitted by it.
Sol. = 6.5 × 1024 J
Question 2:The number of photons of light of wavelength 7000 Å equivalent to 1 J are
Question 3:Which is not characteristic of Planck’s quantum theory of radiation?
➢ Energies in Electron Volts
- Room temperature thermal energy of a molecule = 0.04 eV
- Visible light photons = 1.5-3.5 eV
- Energy for the dissociation of an NaCl molecule into Na and Cl- ions: = 4.2 eV
- Ionization energy of atomic hydrogen = 13.6 eV
- Approximate energy of an electron striking a color television screen (CRT display) = 20,000 eV
- High energy diagnostic medical x-ray photons.= 200,000 eV (=0.2 MeV)
Typical Energies From Nuclear Decay:
- gamma = 0-3 MeV
- beta = 0-3 MeV
- alpha = 2-10 MeV
- Cosmic ray energies = 1 MeV - 1000 TeV
- 1 MeV = 106 eV, 1 GeV = 109 eV, 1 TeV = 1012 eV.
➢ Explanation of Black Body Radiations Using Planck's Quantum Theory
- When a solid substance like the iron piece is heated, it emits radiations. As heating is continued, more and more energy is being absorbed by the atom, and hence, more energy will be emitted and therefore the energy of e.m.w. increases and frequency of e.m.w. increases and therefore, body first becomes red then yellow and finally white.
- Therefore, it can be concluded that light posses particle nature and energy of electromagnetic radiation depends upon frequency.
➢ Explanation of Photoelectric Effect Using Planck's Quantum Theory
- When a metal sheet is subjected to electromagnetic radiation of suitable frequency then some electrons are ejected from the metal surface, and these electrons are known as photoelectron, and the effect is known as the photoelectric effect.
- If electromagnetic radiation of low frequency is used, then there is no ejection of electron despite the continuous increasing intensity. This observation was contradicting to Maxwell theory according to which energy electromagnetic radiation ∝ I but can be explained using Planck's quantum theory, i.e. E ∝ ν.
➢ Dual Nature of Light
- Since the wave nature of light explains the diffraction interference phenomenon while particle nature explains black body radiation and photoelectric effect light was considered to have dual nature particle nature as well as wave nature.
The arrangement of all the electromagnetic radiation in a definite order (decreasing or increasing of wavelength or frequency is known as electron magnetic spectrum.
- When the light coming from a source is passed through a prism, the radiation of different wavelength deviated through different angles and got separated.
The angle of deviation ∝ n ∝
Splitting of light
- This process is called dispersion, and such a collection of dispersed light giving its wavelength composition is known as spectrum.
- Spectrum is of two types:
1. Emission Spectrum
This spectrum of radiations is emitted by any source or atom or molecule of any substance (Which is excited by heating or electric discharge).
- Continuous Spectrum: When white light from a source is dispersed, a bright spectrum continuously distributed on the dark background is obtained. The colours continuously change from violet to red, and there are no sharp boundaries between various colours. These colours appear to be merged into each other, and therefore spectrum is known as a continuous spectrum.
- Discontinuous Spectrum: When an atom is subjected to electromagnetic radiation, it causes excitation of an electron to higher energy level. When electrons return back to lower energy level, it emits certain radiations corresponding to the difference in energy level. Therefore, spectrum obtained in case of an atom is discontinuous. A specific wavelength is known as Atomic spectrum or line spectrum.Spectrum
2. Absorption Spectrum
- When an atom is subjected to white light it absorbs some specific radiations corresponding to the difference in energy levels, the remaining radiations (transmitted radiations) are devoid of certain specific frequencies which are observed in the form of missing line and this spectrum is known as absorption spectrum.
- It is a photographic negative of the emission spectrum, i.e. those bright lines which are present in the emission spectrum of an atom are missing.
- In the absorption spectrum and are observed in the form of dark lines.