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VISUAL PHYSICS ONLINE 7.1 THE NATURE OF LIGHT ELECTROMAGNEITC WAVES P71 003 (A) Heinrich
Hertz used a set-up like the one shown to investigate the production and
detection of electromagnetic radiation. (A1) In
the diagram identify the transmitter, receiver, transmitter, spark gaps, high voltage source. (A2) List
the components of the electromagnetic spectrum (A3) Explain the production and
detection of the radio waves. A glass
sheet was placed between the transmitter and receiver. (A4) Did the radio waves pass through
the glass? Explain. (A5) Were ultraviolet waves blocked when the
glass sheet was in place? (A6) Describe
and explain the change in the spark length when the glass sheet was in place. (A7) How does
this relate to the Photoelectric Effect?
(B) Assess the impact of the discovery of the photoelectric effect of the development of the quantum model of light. (C) How did Einstein’s theory of special relativity and his explanation of the photoelectric effect lead to the re-conceptualisation of the model of light? View solution below only after you have completed the answering the question. |
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Solution (A1)
(A2) In order of increasing energy radio microwave IR visible UV X-rays g-rays (A3) Oscillating charges à oscillating electric fields à oscillating magnetic fields à production of radio wave Radio wave – oscillating electric & magnetic fields à oscillating electric force on free electrons in wires – detection of radio waves (A4) The glass in transparent to radio waves. (A5) UV does not pass through glass. (A6) When glass plate blocks UV, spark shorter because less energy absorbed by electrons – less easily removed à more difficult to create the spark. (A7) Photoelectric effect – release of electrons from a surface by absorption of incident em radiation. (B) Photoelectric effect – release of electrons from a surface by absorption of incident em radiation. The predictions of classical physics could not explain the experimental observations made. Einstein used Planck’s idea of energy quantization to explain the photoelectric effect by applying the law of conservation of energy where light behaves as a stream of particles called photons and the energy of each photon is given by E = hf. This explanation also supported the radical idea introduced by Planck. The impact of his explanation was great and was the starting point for modern physics where the ideas of classical physics were not valid. (C) Light – wave model – interference effect Light – particle model - Photoelectric Effect Einstein – special relativity speed of light is constant independent of motion of source or observe
equivalence of mass and energy
E = m c2 Einstein – light stream of particles called photons energy of photon E = hf photon energy absorbed by electron and given sufficient energy to escape the surface of a metal photons have mass m = E/c2 Photoelectric Effect The photoelectric effect: release of an electron from the surface of a metal exposed to electromagnetic radiation. Classical physics was unable to explain - the threshold frequency, which is the frequency of light at which electrons are emitted from the surface of a metal. It predicted that energy from light would be absorbed over time until an electron had sufficient energy to leave the surface of the metal. Einstein used Planck’s theory and the concept that photons carried energy to explain the photoelectric effect. Einstein assumed that light existed as photons, each with an energy equal to E = h f and that the number of photons determined the light intensity. Photons with the highest energy correspond to the highest frequency, and an electron would not be emitted from a metal surface unless the photons possessed energy equal to or greater than the energy needed to overcome the energy holding the electron on the metal surface. The energy required to release the electron from the surface was called the work function, W. Einstein was able to combine the photon energy with the work function and the KE of the emitted electrons in the equation h f = Kmax + Wmin |
