1 Contents: PhotonsPhotons2 Photoelectric effectPhotoelectric effect 4 Bohr AtomBohr Atom11 Line Emission SpectraLine Emission Spectra14 Types of Spectra18Spectra Stimulated EmissionStimulated Emission and Lasers19 Photons, Spectra and Lasers
2 Red : f= 4.76 x 10 14 Hz = 630 x 10 -9 m Blue : f= 7.90 x 10 14 Hz = 380 x 10 -9 m Light travels in packets of energy called photons. Photons have different frequencies and wavelengths Blue light has photons with a higher frequency and shorter wavelength than red light Photons
3 Photons of light carry energy. The energy is proportional to the frequency E = h f frequency of photon / Hzenergy of photon / J Planck’s constant 6.63 x 10 -34 J s Blue photons have a higher frequency and greater energy than red photons.
4 The Photoelectric Effect Electromagnetic radiation can remove an electron from a zinc plate if: the radiation is ultraviolet the plate is clean the plate is charged negatively Zinc plate electroscope ultraviolet if the ultraviolet is more intense, the zinc discharges faster
5 electron Zinc atom photons of red light do not release electrons photons of ultraviolet can release electrons The photoelectric effect with zinc only works with ultraviolet
6 Photoelectric Current ultraviolet radiation thin quartz window vacuum 2 kV + mA anode negative zinc cathode The photons of ultraviolet pass through the window onto the zinc cathode
7 Photoelectric Current ultraviolet 2 kV + mA electrons The photocurrent only flows if the frequency (and hence energy) of the photons is high enough to knock the electrons from the zinc cathode
8 ultraviolet 2 kV + mA electrons photocurrent f 0 frequency of u.v. The THRESHOLD FREQUENCY, f 0, is the minimum frequency needed to release an electron from the surface of the zinc cathode. E 0 = hf 0 The WORK FUNCTION, E 0, is the minimum energy needed to release an electron from the surface of the zinc cathode.
9 Kinetic Energy of the Photoelectron zinc E k = hf - hf 0 electron photon If the photon has more energy than the work function, the extra energy becomes the kinetic energy of the electron. Kinetic energy of electron = photon’s energy - work function
10 Photocurrent Intensity of ultraviolet If the intensity is increased, the photocurrent is increased. Greater intensity means more photons per second, more ejected electrons per second and a greater photocurrent. Doubling the intensity will double the photocurrent. Ultraviolet 2 kV + mA electrons
11 The Bohr Atom nucleus electron The electrons are in fixed orbits round the nucleus The positively charged nucleus is at the centre of the atom
12 nucleus electron Electrons can drop to lower energy orbits.... emitting the excess energy as photons of light
13 E = hf The biggest jump produces photons with the biggest energy …. …. and the highest frequency The smallest jump produces photons with the smallest energy …. …. and the lowest frequency
14 biggest energy jump smallest energy jump brightest line is the most popular jump - more transitions occur Line Emission Spectrum lower frequency longer wavelength higher frequency shorter wavelength
15 Energy Levels E0E0 E0E0 E1E1 E1E1 E2E2 E2E2 E3E3 E3E3 ground state excited states 123 45 6 6 possible lines on the emission spectrum
16 Using numbers! The energy levels for a hydrogen atom : - 21.76 x 10 -19 J E0E0 - 5.43 x 10 -19 J E1E1 - 2.47 x 10 -19 J E2E2 - 1.36 x 10 -19 J E3E3 - 0.84 x 10 -19 J E4E4 The change in energy is: - 2.47 x 10 -19 - (- 5.43 x 10 -19 ) = 2.96 x 10 -19 J An electron drops from E2 E2 to E1E1
17 E = 2.96 x 10 -19 J A photon of light is emitted, its frequency can be found : E = hf 2.96 x 10 -19 = 6.63 x10 -34 x f f = 2.96 x 10 -19 / 6.63 x10 -34 = 4.46 x 10 14 Hz - 21.76 x 10 -19 J E0E0 - 5.43 x 10 -19 J E1E1 - 2.47 x 10 -19 J E2E2 - 1.36 x 10 -19 J E3E3 - 0.84 x 10 -19 J E4E4
18 300 400 500 600 nm Spectra Continuous spectrum Absorption spectrum Line emission spectrum filament bulb discharge tube light from sun
19 E3E3 E4E4 Stimulated Emission of Photons phase direction wavelength The stimulated photon has the same : A photon with the same energy as the difference between the two energy levels causes an electron to fall to the lower level hence stimulates the emission of another photon.
20 Helium-Neon Laser Helium-Neon gas is held in a tube. There are mirrors at the ends of the tube - one lets 1% of the light pass through. The mirrors reflect the photons back into the gas and stimulate more transitions which amplifies the beam. A high frequency generator “pumps” the electrons back up to excited states. The electrons in the gas are stimulated to emit photons of red light. mirror 99% mirror Helium/neon gas generator
21 mirror 99% mirror LASER LIGHT light amplification by the stimulated emission of radiation Helium-Neon Laser Helium/neon gas Laser light is : Monochromatic In phase Intense Parallel