Podsumowanie wiadomości z fizyki atomu

Before you start, answer the questions.

1. How do we describe the thermal radiationradiationradiation of bodies?

2. Explain the concepts of continuous, line, emissionemissionemission and absorptionabsorptionabsorption spectra.

3. Explain the concept of photonphotonphoton.

4. Describe the atom models known to you.

5. What is the photoelectric effectphotoelectric effectphotoelectric effect?

1. How do we describe the thermal radiation of bodies?

Bodies at a temperature higher than absolute zero (0 K = -273⁰C) are a source of electromagnetic radiationradiationradiation, called thermal radiation. These bodies can emit and absorb electromagnetic radiation.

The ability of bodies to emit and absorb radiation allows us to explain the existence of colours. The colour of the body depends both on the spectrum of the incident electromagnetic wave and on which wavelengths are absorbed better or worse by a given body. If the blue light is incident on a body which almost completely absorbs it, then the colour detected by the human eye would be black.

To describe thermal radiation, its emissionemissionemission and absorptionabsorptionabsorption by a body, a black bodyblack bodyblack body model was created. A black body is a body that completely absorbs the electromagnetic radiation that is incident on it, regardless of the wavelength and temperature at which this process takes place.

The heated bodies emit thermal energy in the form of electromagnetic waves.

The total energy emitted by a body having a temperature T is described by the formula E = σ · TIndeks górny 4⁴, if the ambient temperature is 0 K (where σ is a constant whose value is equal to 5,67 · 10Indeks górny -8^-8$\frac{J}{s·m^2·K^4}$). This dependence is known as the Stefan‑Boltzmann law. This formula shows that the energy emitted increases rapidly with increasing temperature.

Wien’s displacement law states that objects of different temperature emit spectra that peak at different wavelengths.

where:

b – Wien’s displacement constant, b = 2,89 · 10Indeks górny -3^-3 mK,
T – body temperature expressed in absolute scale (K).

2. Continuous, line, emissionemissionemission and absorptionabsorptionabsorption spectra.

The spectrum is a registered image of electromagnetic radiationradiationradiation. This image consists of different wavelengths (colours). Each wavelength is associated with the corresponding frequency and energy.

Instruments used for imaging and examining of spectra are spectroscopes and spectrometers.

The thermal radiation spectrum of solids and liquids is continuous - in this spectrum all wavelengths are present and there are no gaps between them; an example of a continuous spectrum is a rainbow.

The spectrum, which consists of many separate coloured lines, is called the line spectrumline spectrumline spectrum.

The line spectrum is typical for gases consisting of atoms or molecules. Examples are hydrogen, helium, neon, argon and mercury or sodium vapours. All elements in the gaseous state have a characteristic line spectrum.

RAOFMnOOainPa

Na rysunku przedstawione jest widmo liniowe par uranu. Na czarnym tle widać serię kolorowych linii przechodzących przez praktycznie całe dostępne spektrum światła widzialnego, barwy od ciemnego fioletu, przez niebieską, zieloną i czerwoną.

EmissionemissionEmission spectra are spectra of radiationradiationradiation emitted by bodies excited to glow. Line emission spectrum is generated by heated gases and continuous emission spectrum - by hot solids. Gases, whose molecules have a complex, multi‑atom structure, emit band emission spectra.

The absorptionabsorptionabsorption spectrum is created as a result of absorption of electromagnetic radiation by a body.

If the radiation having a continuous spectrum passes through a cooled gas, then the energy of electromagnetic waves is absorbed exactly at the wavelengths that a given atom can emit.

In the absorption spectrum, dark lines are visible - they are located at the wavelengths that have been absorbed by a given gas.

Fraunhofer was the first to observe such dark lines in the spectrum of sunlight. We call them Fraunhofer lines.

3. What is a photonphotonphoton?

A photonphotonphoton is a portion (quantum) of electromagnetic radiationradiationradiation energy. We can treat it as a particle that has the following characteristics.

- There is no rest mass.
- There is no electric charge.
- It has energy that is expressed by the formula:

where:

h – a universal constant, called the Planck constant, which is equal to 6,63 · 10Indeks górny -34^-34 Js,
ν – the frequency of radiation emitted or absorbed,
c – speed of light,
λ – wavelength of radiation.

4. Atom models.

The atom's model has evolved over time. The subsequent scientific discoveries made it possible to explain the structure of the atom more and more accurately.

Thomson's model

In 1897 Thomson discovered the electron. The electron is a component of all atoms. Atom has a structure that includes electrons. This structure was called the „raisin cake” model.

The Rutherford model

The atom consists of a nucleus and electrons orbiting around it. The atom is electrically neutral (the nucleus has a positive charge, and the electrons are negative). The Coulomb force is responsible for the interaction between the atomic nucleus and its electrons. The size of the nucleus is 100000 times smaller than the size of the atom.

Bohr's model

Bohr created the atom model based on the Rutherford model. He formulated two postulates.

I. The electron can circulate around the nucleus only on selected orbits, called stationary orbits.

II. The change of atomic energy occurs only during the electron transition between stationary orbits - the transition from the higher orbit to a lower one corresponds to the energy emissionemissionemission, and the transition from a lower to higher orbit is caused by the absorptionabsorptionabsorption of energy. Energy is emitted and absorbed by the atom in the form of a portion (quantum) of energy of the value resulting from the formula:

where:

n, k - are the numbers of orbits between which the electron jumps.

With the Bohr model, the spectral line of the hydrogen atom can be explained.

The Bohr atom model allows describing precisely the structure of only a hydrogen atom; it fails when atoms have a more complex atomic nucleus, around which more electrons orbit.

Thanks to the Bohr model, the foundations of a new branch of modern physics - quantum mechanicsquantum mechanicsquantum mechanics - were established.

5. What is the photoelectric effect?photoelectric effectphotoelectric effect?

The external photoelectric effectphotoelectric effectphotoelectric effect (photoemission, photo effect) is the emissionemissionemission of electrons from the metal surface under the influence of radiationradiationradiation incident on this surface. These electrons are called photoelectrons.

The external photoelectric effect occurs only under certain conditions.

• For each metal there is a threshold frequency (wavelength) of radiation, below (and in the case of wavelength - above) which this phenomenon does not occur at all.

• The kinetic energy of emitted electrons does not depend on the intensity of radiation, but only on its wavelength.

• The number of photoelectrons is proportional to the intensity of incident radiation.

The external photoelectric effect is evidence that the electromagnetic wave can be treated as a stream of particles - photons.

Photo effect is a quantum phenomenon; it became the basis of the quantum theory of light. Therefore, classical physics could not explain the photoelectric effect. In 1905 Albert Einstein explained the photoelectric effect by assuming that light is a stream of particles or photons, and one photonphotonphoton incident on metal can transfer energy to only one electron in metal.

The principle of energy conservationenergy conservationenergy conservation in the interaction of photon - electron has been written in the equationequationequation, called the Einstein‑Millikan equation:

This equationequationequation says that the energy of the incident photon in the photoelectric effectphotoelectric effectphotoelectric effect is equal to the sum of the work function and kinetic energy of the electron. The work function is the minimum energy needed for the electron to leave the metal; its relationship to the threshold frequency (wavelength) has the form:

Remember

• Physics of atom, or atomic physics, is a branch of physics dealing with the atom as an isolated system, consisting of the atomic nucleus and electrons orbiting around it.

Exercises

Glossary

Keywords

black bodyblack bodyblack body

line spectrumline spectrumline spectrum

photoelectric effectphotoelectric effectphotoelectric effect

photonphotonphoton

radiationradiationradiation