Lecture 20 Light and Quantized Energy Ozgur Unal NIS – CHEMISTRY Lecture 20 Light and Quantized Energy Ozgur Unal

The Atom and Unanswered Questions Rutherford’s atomic model was not successful at explaining the locations of electrons around the nucleus. It also failed to explain the chemical behavior of elements in the periodic table. Example: Lithium, Sodium and Potassium have similar chemical properties although they belong to different periods. In order to understand atoms we need to consider the quantum nature of particles, such as electrons, light etc.

The Wave Nature of Light What do you think light is? What descriptive words can you use for light? Visible light is a type of electromagnetic radiation. Electromagnetic (EM) radiation is a form of energy that shows wavelike behavior as it travels. Microwaves and X rays are examples of EM radiation.

Characteristics of Waves How can you describe waves? The wavelength of a wave is the shortest distance between equivalent points on a continuous wave. The symbol for wavelength is λ. It can be measured in meters, centimeters, nanometers etc.

Characteristics of Waves The frequency of a wave, denoted by ν, is he number of waves that pass a given point per second. Hertz (Hz) is the SI unit of frequency. 1 Hz = 1 wave/second = 1/s = 1 s-1. The amplitude of a wave is the wave’s height from the origin to a crest, or from the origin to a trough. The wavelength and frequency does not affect the amplitude of a wave.

Characteristics of Waves Compare the three waves below. Which has higher amplitude, frequency and wavelength? All waves move, so they have speed. All EM waves has a speed of 3 x 108 m/s in vacuum. This speed is a universal value and denoted by c. The speed can be expressed using frequency and wavelength c = λ * ν

Electromagnetic Spectrum White light passing through a prism separates into a continuous spectrum as shown in the picture. Each color has a unique wavelength and frequency. Each color refracts at a different angle.

Electromagnetic Spectrum The visible spectrum of light shown is only a small portion of the complete electromagnetic spectrum. The EM spectrum includes all forms of EM radiation, with different frequencies and wavelengths. Because all EM waves travel at the same speed, you can use the formula c = λ * ν to calculate the wavelength or frequency of any wave.

Lecture 21 The Particle Nature of Light Ozgur Unal NIS – CHEMISTRY Lecture 21 The Particle Nature of Light Ozgur Unal

The Particle Nature of Light Wave nature of light cannot explain some Physics phenomena. Objects glowing in different colors as they are heated up to high temperatures Photoelectric effect When objects are heated, they emit glowing light. As the temperature (average kinetic energy) increases the color (frequency) of the light changes from red to blue. Wave model of light could not explain this phenomenon.

The Particle Nature of Light Wave model could not explain photoelectric effect either. In the photoelectric effect, electrons, called photoelectrons, are emitted from a metal’s surface when light of a certain frequency, or higher than a certain frequency, shines on the surface. Wave model predicts that given enough time, low energy (frequency) light would accumulate and supply enough energy to eject photoelectrons from a metal. In reality, a metal will not eject photoelectrons below a specific frequency of incident light.

The Particle Nature of Light At the beginning of 1900s, Max Planck was able to explain the first phenomenon. He proposed that matter can gain or lose energy only in small, specific amounts called quanta. A quantum is the minimum amount of energy that can be gained or lost by an atom. This means that, although the temperature seems to rise continuously as you heat an object, in fact it rises in small, discrete amounts. Since this increase is so small, we perceive it as continuous.

The Particle Nature of Light Planck also proposed that the energy of a quantum is related to the frequency of the emitted (or absorbed) radiation by a constant caleld Planck’s constant. Equantum = h ν h is Planck’s constant. h = 6.626 x 10-34 J*s According to Planck’s theory, for a given frequency ν, matter can emit or absorb energy only in whole-number multiples of hν; that is 1hν, 2hν, 3hν and so on. Analogy: This is similar to building a wall. Only whole-numer multiples of bricks can be added to or taken out from a wall.

The Particle Nature of Light To explain the photoelectric effect, Albert Einstein proposed in 1905 that light has a dual nature. Light has wavelike and particlelike properties. Light can be thought of as a beam of particles called photons. A photon is a massless particle that carries a quantum of energy. Einstein extended Planck’s idea of quantized energy to light: Energy of a photon = Ephoton = h ν Here, ν is the frequency of light. The concept of photon explains photoelectric effect. Photoelectric Animation

The Particle Nature of Light Example: Sun not only emits visible light but also other electromagnetic waves including radiowaves, microwaves, UV etc. Calculate the energy of a photon from the UV portion of the Sun’s light if it has a frequency of 9.231 x 1014 Hz. Example: The blue color in some fireworks occurs when copper (I) chloride is heated to approximately 1500 K and emits blue light of wavelength 4.50 x 102 nm. How much energy does one photon of this light carry?

Atomic Emission Spectra How is light produced in neon signs? When electricity passes through a tube filled with Ne atoms, these atoms absorb energy and become excited. These excited atoms emit radiation (light) to become stable. This is how neon signs work. If you pass the light emitted from Ne atoms through a prism, you will see atomic emission spectra of Ne atoms. Atomic emission spectrum of an element is the set of frequencies of EM waves emitted by atoms of the element.

Atomic Emission Spectra - Examples Atomic emission spectrum of certain elements

Atomic Absorption Spectra If atoms emits radiation of specific frequencies, that means they absorb the same frequencies of light. This gives us absorbtion spectrum. Absorption spectrum of H

Atomic Absorption Spectra – The Sun By looking at the absorption spectrum of the Sun, we can find out the elements present in the atmosphere of the Sun.