Optical phenomena: Grade 12 Past Paper Questions

Light of different frequencies is incident on a metal plate. The sketch graph below shows the relationship between the maximum kinetic energy, Ek(max), of the photoelectrons and the energy of the incident photons. 10.1 Define the term work function of a metal. 10.2 Write down the numerical value of the gradient of the above graph. 10.3 Calculate the: 10.3.1 Maximum speed of the photoelectrons when photons with energy Y, as shown on the graph, strike the metal plate. 10.3.2 Value of Y. 10.4 Photons with energy 4,02 × 10⁻¹⁹ J strike the metal plate and photoelectrons are emitted. The number of photons with energy 4,02 × 10⁻¹⁹ J striking the metal plate per second is now increased. How will the maximum kinetic energy of the photoelectrons be affected? Choose from INCREASES, DECREASES or REMAINS THE SAME. Give a reason for the answer.

Light is incident on the cathode of a photoelectric cell connected to a battery and a sensitive ammeter, as shown below. 10.1 What conclusive evidence about the nature of light is provided by the photoelectric effect? The cathode has a work function of 3,42 × 10⁻¹⁹ J. 10.2 Define the term work function. Light of frequency 5,96 × 10¹⁴ Hz is shone onto the cathode. 10.3 Calculate the maximum kinetic energy of an electron ejected from the cathode. 10.4 The ammeter registers a constant current of 0,012 A. Calculate the minimum number of photons of light that strike the cathode in a 10 s period. 10.5 The intensity of the incident light is now INCREASED. How will this change affect the reading on the ammeter? Choose from INCREASES, DECREASES or REMAINS THE SAME. Explain the answer.

Which ONE of the following statements CORRECTLY describes the photoelectric effect? A. An electron absorbs the energy of a photon and emits light. B. An electron emits a photon when it collides with another electron. C. An electron absorbs the energy of a photon and is ejected from the surface of a metal. D. A photon is emitted when an electron moves from a lower energy level to a higher energy level.

Which ONE of the following combinations is CORRECT for a line absorption spectrum in terms of the ENERGY TRANSITIONS OF THE ATOMS and the APPEARANCE OF THE NARROW LINES IN THE SPECTRUM? A. Higher to lower energy state, dark lines. B. Lower to higher energy state, coloured lines. C. Lower to higher energy state, dark lines. D. Higher to lower energy state, coloured lines.

10.1 Define the term photoelectric effect. 10.2 Light of wavelength 4,7 × 10⁻⁷ m is shone onto the surface of a piece of caesium metal. If the threshold frequency of caesium is 4,37 × 10¹⁴ Hz, calculate the maximum speed of an electron ejected from the surface of the metal. 10.3 A simple electroscope consists of a zinc disc, a metal stem and a thin length of gold foil. When the electroscope is neutral, the foil hangs vertically, as shown in DIAGRAM 1 below. When the electroscope is negatively charged, the foil is repelled from the stem, as shown in DIAGRAM 2 below. When ultraviolet light is shone on the disc of the negatively charged zinc electroscope, the foil collapses towards the stem (hangs vertically). 10.3.1 How does the frequency of the ultraviolet light compare to the threshold frequency of zinc? Write only HIGHER THAN, LOWER THAN or EQUAL TO. 10.3.2 Explain why the foil of the electroscope collapses. Green light is now shone on another negatively charged zinc electroscope. The foil does not collapse. 10.3.3 Will the foil collapse if the intensity of the green light is increased? Write either YES or NO. Give a reason for the answer.

In a photoelectric investigation, light of different frequencies was radiated on each of two metals, A and B. The graph of maximum kinetic energy of the ejected electrons from metal A and the frequency of the incident photons is shown below. Point X on the graph represents an unknown maximum kinetic energy. 10.1 Write down the numerical value of the gradient of the graph. 10.2 Define the term work function. 10.3 Calculate the: 10.3.1 Work function of metal A. 10.3.2 Value of X shown on the graph. 10.4 How will EACH of the following be affected if light of frequency 12,54 × 10¹⁴ Hz, but of higher intensity, is used? Choose from INCREASES, DECREASES or NO EFFECT. 10.4.1 The value of X. 10.4.2 The number of photoelectrons emitted per unit time. Metal B has a larger work function than metal A. 10.5 Redraw the graph above in your ANSWER BOOK. (Do NOT include values on the axes.) Label this graph as A. On the SAME set of axes, sketch the graph for metal B. Label this graph as B.

In an experiment, a metal is identified using the photoelectric effect. Photons of light with different energies are shone onto the metal surface and the corresponding maximum velocities of the ejected electrons are recorded. The graph below shows the relationship between the square of the maximum velocities of the ejected electrons, v², and the energy of the incident photons. The value of X is unknown. 10.1 Define the term work function of a metal. One of the metals in the table below, listing metals and their work functions (including chromium, at 5 × 10⁻¹⁹ J), was used in this experiment. 10.2 Use a suitable calculation to identify the metal used. 10.3 Calculate X. 10.4 How will the maximum velocities of the ejected electrons change when the intensity of the incident light is increased? Choose from INCREASES, DECREASES or REMAINS THE SAME. Give a reason for the answer.

The simplified diagrams below show how different types of spectra are obtained: white light passed through hot gas and a prism produces spectrum Y; white light passed through cold gas and a prism produces spectrum Z. Which ONE of the following statements is CORRECT? A. Y is an emission spectrum and Z is an absorption spectrum. B. Y is an emission spectrum and Z is a continuous spectrum. C. Y is a continuous spectrum and Z is an absorption spectrum. D. Y is an absorption spectrum and Z is an emission spectrum.

The relationship between frequency (f) and maximum kinetic energy (Ek(max)) of photoelectrons emitted from two cathodes, M and N, of different photoelectric cells is investigated. The graphs below have been obtained from the results. 10.1 Define the term threshold frequency. 10.2 How does the maximum kinetic energy of photoelectrons emitted from cathode N compare to the maximum kinetic energy of those emitted from cathode M when light of a frequency greater than 10,40 × 10¹⁴ Hz is shone on each of the cathodes? Choose from GREATER THAN, SMALLER THAN or EQUAL TO. 10.3 Calculate the value of frequency fx indicated on the graph. 10.4 The experiment is now repeated for cathode M using light of frequency fx, but of higher intensity. How will EACH of the following be affected? Choose from INCREASES, DECREASES or NO EFFECT. 10.4.1 The y-intercept of the graph. 10.4.2 The number of photoelectrons emitted per unit time. 10.4.3 The maximum kinetic energy of the emitted photoelectrons.

1.10 High-energy light is shone onto a metal plate. Electrons are emitted from the metal plate. Which combination will be true when the wavelength of the light is decreased: A. kinetic energy of the emitted electrons decreases, number of emitted electrons decreases. B. kinetic energy of the emitted electrons increases, number of emitted electrons decreases. C. kinetic energy of the emitted electrons increases, number of emitted electrons stays the same. D. kinetic energy of the emitted electrons stays the same, number of emitted electrons stays the same.

The graph below represents the maximum kinetic energy of an electron, Ek(max), ejected from the surface of a metal, as the frequency (f) of light shone on it is increased. 10.1 Define threshold frequency. 10.2 Write down the threshold frequency of the metal. 10.3 Determine the value of X as indicated on the graph. 10.4 The intensity of the incident light is increased without changing the frequency. How will this affect the maximum kinetic energy of the emitted electrons? Choose from INCREASES, DECREASES or REMAINS THE SAME. Give a reason for the answer.

QUESTION 10 10.1 Define the term photoelectric effect. 10.2 Red light, blue light and green light are shone simultaneously on a sheet of potassium, as shown in the diagram below. Each colour of light consists of a single frequency. Two maximum kinetic energies of the ejected electrons are possible, namely 6,96 × 10⁻²⁰ J and 2,65 × 10⁻²⁰ J. Each ejected electron has only one of these maximum kinetic energies. 10.2.1 Which colour of light is responsible for ejecting electrons that have a maximum kinetic energy equal to 2,65 × 10⁻²⁰ J? 10.2.2 Explain the answer to QUESTION 10.2.1. 10.2.3 The electrons with a maximum kinetic energy of 2,65 × 10⁻²⁰ J are ejected by light that has a frequency of 5,85 × 10¹⁴ Hz. Calculate the frequency of the light that ejected electrons with a maximum kinetic energy of 6,96 × 10⁻²⁰ J. 10.2.4 The intensity of the red light is increased, while the intensities of the blue light and green light remain the same. What effect will this change have on the rate at which electrons are ejected? Choose from INCREASES, DECREASES or REMAINS THE SAME. 10.3 Some of the atoms of a hot gas, made up of a single element, are in an excited state. The spectrum formed by the hot gas is observed on a screen in a darkened room. The spectrum consists of specific coloured lines on a black background. 10.3.1 Name the type of spectrum formed. 10.3.2 Explain the presence of the coloured lines in the spectrum.

1.10 White light is passed through a cold gas and then through a prism, as shown below. A line spectrum is observed on the screen. Which ONE of the following correctly describes the ENERGY TRANSITION of the atoms of the gas and the TYPE OF LINE SPECTRUM observed on the screen? A. Higher to lower energy level; Emission. B. Lower to higher energy level; Emission. C. Higher to lower energy level; Absorption. D. Lower to higher energy level; Absorption.

1.9 The simplified diagram below represents a DC motor. The diagrams below indicate some changes made to the above motor. Which of the changes to the motor above will change the original direction of rotation of the coil? A. (i) and (ii) only B. (i) and (iii) only C. (ii) and (iii) only D. (iii) only. 1.10 An atom has a ground state energy of x. When the atom moves to a higher energy state y, a line spectrum is observed. Which ONE of the following combinations is CORRECT for the ENERGY CHANGE of the atom and the TYPE OF LINE SPECTRUM observed during the transition? A. Energy change y – x, emission. B. Energy change x – y, emission. C. Energy change x – y, absorption. D. Energy change y – x, absorption.

1.9 In which ONE of the following electrical machines is electrical energy converted to mechanical energy? A. AC generator B. DC generator C. AC dynamo D. DC motor. 1.10 Which ONE of the following combinations correctly links an emission spectrum and an absorption spectrum to the energy transitions of an electron in an atom? A. Emission spectrum from low to high energy levels, absorption spectrum from high to low energy levels. B. Emission spectrum from low to high energy levels, absorption spectrum from low to high energy levels. C. Emission spectrum from high to low energy levels, absorption spectrum from high to low energy levels. D. Emission spectrum from high to low energy levels, absorption spectrum from low to high energy levels.

1.6 The absorption spectra of the same element surrounding both star A and star B are observed from Earth. The spectral lines for star B are more red-shifted than those for star A. How do the frequencies of the observed spectral lines and the speed of star B compare to that of star A? A. Frequencies higher, speed greater. B. Frequencies higher, speed smaller. C. Frequencies lower, speed greater. D. Frequencies lower, speed smaller. 1.7 R and S are two small charged spheres placed a distance apart. P is a point to the right of sphere S. Eᵣ and Eₛ are the electric fields at point P due to the charges on spheres R and S respectively. See the diagram below. Which ONE of the following combinations is CORRECT for the net charge on spheres R and S? A. Sphere R: positive; sphere S: negative. B. Sphere R: negative; sphere S: positive. C. Sphere R: negative; sphere S: negative. D. Sphere R: positive; sphere S: positive.

1.9 The diagram below shows a simplified electric motor. The rotation of the coil is observed from the battery. Which ONE of the following statements is CORRECT while the motor is in operation? The coil and the … A. slip rings rotate anti-clockwise. B. slip rings rotate clockwise. C. commutator rotate clockwise. D. commutator rotate anti-clockwise. 1.10 Which of the following statements is/are TRUE for the photoelectric effect? The photoelectric effect demonstrates that: (i) Light has a wave nature. (ii) Light has a particle nature. (iii) Light energy is quantised. A. (i) only. B. (ii) only. C. (i) and (iii) only. D. (ii) and (iii) only.

1.6 Light emitted from a distant star contains a spectral line X of frequency f. The spectral lines of this star when observed on Earth are red-shifted. Which ONE of the following combinations of the OBSERVED FREQUENCY of spectral line X and the MOTION OF THE STAR is CORRECT? A. Observed frequency greater than f; star moving away from Earth. B. Observed frequency greater than f; star moving towards Earth. C. Observed frequency smaller than f; star moving away from Earth. D. Observed frequency smaller than f; star moving towards Earth. 1.7 A proton and an electron are a distance r apart. The magnitude of the electrostatic force that they exert on each other is F. Which ONE of the following graphs shows the relationship between F and r² as the proton and the electron approach each other? (Four F-versus-r² graphs are given as options.)