2. 1© Manhattan Press (H.K.) Ltd. 23.6 Continuous spectra Spectra Sun’s spectrum and Fraunhofer lines

3. 2 © Manhattan Press (H.K.) Ltd. 23.6 Continuous spectra (SB p. 170) Spectra 1. Line spectrum – produced by exciting gas atoms at low pressure - consists of discrete lines of definite wavelengths (f of radiation is discrete) 2. Band spectrum – produced by molecular gases or chemical compounds - consists of few groups or band of lines - lines are close together at one end of band

4. 3 © Manhattan Press (H.K.) Ltd. 23.6 Continuous spectra (SB p. 170) Spectra 3. Continuous spectrum – produced by hot solids, liquids and gases at high pressure - consists of all wavelengths of radiation

5. 4 © Manhattan Press (H.K.) Ltd. 23.6 Continuous spectra (SB p. 170) Sun’s spectrum and Fraunhofer lines Fraunhofer lines – dark lines exists in sunlight - example of line absorption spectrum The lines indicate: - some elements vapour (H, He, Na) exists in Sun’s chromosphere - When continuous spectrum is emitted from the central hot region, it passes through the elements vapour - The elements absorb their characteristic wavelengths and dark lines are produced

6. 5 © Manhattan Press (H.K.) Ltd. 1. The energy emitted or absorbed by the atoms is in the form of indivisible packets, which is called quanta. 2. Energy in each quantum (E) = hf where h is Planck constant and f is the frequency of the radiation. 3. The light quanta (or electromagnetic radiation) are known as photons. Energy of photon (E) = hf 23.1 Quantum theory 23.6 Continuous spectra (SB p. 172)

7. 6 © Manhattan Press (H.K.) Ltd. 4. The photoelectric effect is the ejection of electrons from a metal surface when electromagnetic radiation of sufficiently high frequency falls on it. 5. The threshold frequency (f o ) is defined as the minimum frequency of the incident radiation required causing the emission of photoelectrons from the metal surface. 6. Work function of a metal (Φ) = hf o 23.2 Photoelectric effect 23.6 Continuous spectra (SB p. 172)

8. 7 © Manhattan Press (H.K.) Ltd. 7. Photoelectrons are emitted instantaneously. 8. The photoelectric current is directly proportional to the intensity of radiation. 9. By Einstein’s photoelectric equation: Maximum kinetic energy (KE max ) =½ mv 2 max = hf – Φ = hf – hf o 23.2 Photoelectric effect 23.6 Continuous spectra (SB p. 172)

9. 8 © Manhattan Press (H.K.) Ltd. 10. The photoelectron is stopped from reaching the anode when the p.d. between the cathode and anode is equal to –V s, which is called the stopping potential. KE max = eV s = hf – hf o 11. Photoemissive cell and photoconductive cell (light-dependent resistor) are two types of photocells. 23.2 Photoelectric effect 23.6 Continuous spectra (SB p. 172)

10. 9 © Manhattan Press (H.K.) Ltd. 12. Niels Bohr postulated that the hydrogen atom has a proton as its nucleus and one electron in certain allowed discrete orbits. These allowed orbits are known as stable energy levels. 13. The electron in a hydrogen atom normally occupies the lowest energy level called the ground state. The electron is in the most stable state. 23.3 Energy level 23.6 Continuous spectra (SB p. 172)

11. 10 © Manhattan Press (H.K.) Ltd. 14. An atom having an electron in the higher energy level is said to be in the excited state. 15. The energy absorbed by the atom to excite from the ground state to one of the excited states is known as the excitation energy. 16. The energy absorbed by the electron of an atom from the ground state to escape completely from the atom is known as the ionization energy. 23.3 Energy level 23.6 Continuous spectra (SB p. 172)

12. 11 © Manhattan Press (H.K.) Ltd. 17. In the inelastic collision of electron with atom, the kinetic energy of the electron is equal to the excitation energy of atom. The atom gains specific amount of energy, i.e. excitation energy, to be in the excited state. 18. In the elastic collision of electron with atom, the kinetic energy of the electron is smaller than the excitation energy of atom. Its energy cannot be transferred to the atom. 23.3 Energy level 23.6 Continuous spectra (SB p. 172)

13. 12 © Manhattan Press (H.K.) Ltd. 19. The electron in a hydrogen atom can be located at the nth stable energy level with energy 20. Ionization energy of hydrogen atom = E ∞ – E 1 = 13.6 eV 21. Excitation energy of hydrogen atom from ground state (n = 1) to first excited state (n = 2) = E 2 – E 1 = 10.2 eV 23.4 Energy level and spectra of hydrogen 23.6 Continuous spectra (SB p. 172)

14. 13 © Manhattan Press (H.K.) Ltd. 22. Energy of photon emitted (hf) = E’ – E’’ where an electron in E’ drops to a lower energy level E’’. 23. Three series of lines in the hydrogen spectrum are called the Lyman series, Balmer series and Paschen series. (a) Lyman series is due to electrons falling from higher excited energy levels to the energy level n = 1. It is in the region of ultra- violet. 23.4 Energy level and spectra of hydrogen 23.6 Continuous spectra (SB p. 173)

15. 14 © Manhattan Press (H.K.) Ltd. (b) Balmer series is due to electrons falling from higher excited energy levels to the energy level n = 2. It is in the region of visible light. (c) Paschen series is due to the electrons falling from higher excited energy levels to the energy level n = 3. It is in the region of infrared. 24. The absorption spectrum of hydrogen consists of some dark lines, which are due to photons of certain frequencies in the incident white light absorbed by the hydrogen atoms. 23.4 Energy level and spectra of hydrogen 23.6 Continuous spectra (SB p. 173)

16. 15 © Manhattan Press (H.K.) Ltd. 25. Fluorescent materials absorb ultra-violet radiation to be in the excited states. The materials emit photons in the visible light region when returning to the ground state. 26. X-ray spectrum consists of a continuous spectrum and the line spectrum. (a) The continuous spectrum is produced by electrons slowing down upon hitting the target. If the p.d. between the cathode and anode is V, 23.5 Fluorescence, X-rays and laser 23.6 Continuous spectra (SB p. 173)

17. 16 © Manhattan Press (H.K.) Ltd. 26. where f max is the maximum frequency of the X-rays, λ min is its minimum wavelength and c = speed of light. (b) The line spectrum is produced when electrons knock out electrons in the lower energy level from the atoms completely. Then another electron falls into the vacancy, X-ray is emitted. 23.5 Fluorescence, X-rays and laser 23.6 Continuous spectra (SB p. 173)

18. 17 © Manhattan Press (H.K.) Ltd. 27. In laser, population inversion and stimulated emission occur. (a) Population inversion is a situation that the majority of atoms are excited to higher energy states but then they are in a metastable state quickly. It is a state in which the electrons remain longer than usual without spontaneous emission. 23.5 Fluorescence, X-rays and laser 23.6 Continuous spectra (SB p. 173)

19. 18 © Manhattan Press (H.K.) Ltd. 27.(b) In the stimulated emission, a photon are incident on the excited atom, the atom may fall to the lower energy state to emit a photon. The photon emitted can also stimulate the emission of other excited atoms. (c) Finally, there are many photons, which is an intense beam of light (laser beam). 23.5 Fluorescence, X-rays and laser 23.6 Continuous spectra (SB p. 173)

20. 19 © Manhattan Press (H.K.) Ltd. 28. There are three types of spectra of radiation: line spectrum, band spectrum and continuous spectrum. 29. There are many dark lines known as Fraunhofer lines in sunlight, which are due to the absorption of elements vapour exiting in the Sun’s chromosphere. 23.6 Continuous spectra 23.6 Continuous spectra (SB p. 173)

21. 20 © Manhattan Press (H.K.) Ltd.

22. 21 © Manhattan Press (H.K.) Ltd. End