EXAM QUESTIONS

245Doc Scientia - Physics Textbook and Workbook - Grade 11

UNIT 2: 2D and 3D wave fronts

Multiple-choice questions

Four possible answers are given for the following questions. Each question has only ONE correct answer.Choose the correct answer and mark the applicable LETTER with a cross (X).

1. Which one of the following statements is true in connection with the diffraction pattern of a single slit? A. Blue light has a narrower central band than when red light is used. B. A big slit width will cause a big central colour band. C. The brightness of the coloured bands is equally bright. D. The dark bands which alternate the bright bands are maximums.2. The property of a water wave that influences the diffraction of the wave is: A. wavelength. B. frequency. C. amplitude. D. slit width.3. Which one of the following combinations will result in the largest diffraction of light through a narrow slit with width w? A. Large λ and small w B. Small λ and small w C. Large λ and large w D. Small λ and large w4. Which one of the following combinations will result in the widest central maximum for the diffraction of light through a narrow slit of width w? A. Large λ and small w B. Small λ and small w C. Large λ and large w D. Small λ and large w5. Which statement about wave fronts is not true? A. It consists of many overlapping, circular crests. B. Each point on the wave front will cause a small wave. C. Where crests overlap, destructive interference takes place. D. The “sum” of all the small waves is the new wave front.6. Which one of the following combinations is correct for the properties of water waves?

A. Longitudinal Undergoes diffraction. Consists of quantified photons.

B. Transversal Undergoes refraction. Transfers energy.

C. Transversal Undergoes reflection. Transfers energy as quantified photons.

D. LongitudinalUndergoes diffraction and refraction.

Transfers energy.

246 Doc Scientia - Physics Textbook and Workbook - Grade 11

WAVES, SOUND AND LIGHT

7. Sandelo hides around the corner of a building from his friend. He can hear footsteps from behind the wall as his friend approaches. This phenomenon can be explained by: A. diffraction of sound waves. B. refraction of sound waves. C. reflection of sound waves. D. super position of sound waves.8. Two water waves travel in opposite directions in the same medium. The amplitudes of the waves are 4 cm and 1,5 cm respectively. Which one of the following combinations is correct for the instant when the waves meet?

Before meeting Type of interference Amplitude during meeting

A. Constructive +2,5 cm

B. Destructive -5,5 cm

C. Constructive +5,5 cm

D. Destructive -2,5 cm

9. Red light undergoes greater diffraction than blue light through the same slit because: A. red light has a greater wavelength than blue light. B. red light travels faster than blue light. C. red light has a wider central band in the diffraction pattern than blue light. D. red light has a greater frequency than blue light.10. A photographer notices that the photos he is taking are not entirely in focus. The sides of the photos appear vague. Which adjustment will correct the problem? A. Increase the size of the lens opening. B. Decrease the size of the lens opening. C. Decrease the amount of light entering the lens opening. D. Use a more sensitive camera that has more image elements or “pixels”.11. The phenomena I to IV are defined by one or more wave properties: I. The formation of echoes II. Waves travelling around the edge of an obstacle. III. Change of direction, speed and wavelength with change of medium IV. Change of colour when light travels through filter. Which of the above phenomena is/are caused by refraction? A. Only II B. II and III C. II and IV D. Only III

247Doc Scientia - Physics Textbook and Workbook - Grade 11

2D AND 3D WAVE FRONTS Exam Questions 2

12. The diagram shows water waves that spread out after passing through a single slit. The wave phenomenon observed after the water waves pass through the slit is: A. reflection. B. diffraction. C. dispersion. D. total internal reflection. (Gr. 11: November, 2015 Eastern Cape)13. The ability of a wave to spread out in wave fronts as the wave passes around a sharp edge, is known as ... A. Snell’s law. B. total internal reflection. C. diffraction. D. Huygens’ principle.

(Gr. 11: November 2016, Gauteng)14. A learner hiding behind an open door of a room can hear people talking outside the room even though he cannot see them. The reason for this is that: A. sound has a shorter wavelength than a light wave. B. sound has a longer wavelength than a light wave. C. light undergoes less diffraction than sound waves. D. light undergoes more refraction than sound waves. (Adapted from Gr 11. November 2015, Gauteng)15. A central bright band is observed when light of wavelength λ passes through a single slit of width a. Light of wavelength 3λ is now used. Which one of the following slit widths would produce a central bright band of the same width? A. 1/6a B. 1/3a C. a D. 3a

Contextual questions

1. Give the definition of diffraction of waves.

2. Draw a diagram that shows how the wave fronts of water waves change due to diffraction when travelling through a narrow as well as a wider single slit.

3 The diagram below shows the diffraction pattern when monochromatic red light shines through a single slit, width w.

3.1 What is represented by the dark areas in the diagram? Explain your answer.3.2 How will the width of the central red area, marked 0, change if the width, w, of the slit decreases? Write increase, decrease or stays the same. Motivate your answer.

4 Answer the following questions based on the sketch graph:

red -2 red -1 red 0 red 1

red

2

blac

k

blac

k

ampl

itud

e (m

)

λ (nm)

320A

248 Doc Scientia - Physics Textbook and Workbook - Grade 11

WAVES, SOUND AND LIGHT

4.1 Determine the wavelength.4.2 How many wavelengths are in the sketch graph?4.3 Determine the frequency of the wave if the graph is for a certain type of electromagnetic radiation.4.4 Redraw the graph and indicate a second curve that has 1,5 × the original wavelength with the same intensity. Label this curve B.4.5 Which one of the waves, A or B, will experience greater diffraction? Motivate your answer.4.6 Which one of the waves, A or B, will experience greater refraction? Motivate your answer.

5 Sound waves with a frequency of 12 500 Hz, are produced by a speaker. The speed of sound through air is 335 m⋅s-1. 5.1 Calculate the wavelength of the sound wave.5.2 Is the sound wave transverse or longitudinal?

A second identical speaker is now connected to the same signal generator. The two speakers are turned to face each other. While walking between the speakers you notice that there are areas where the sound is louder and other areas where the sound is softer.

5.3 Explain which property of waves is responsible for the: 5.3.1 louder sounds; and5.3.2 softer sounds.5.4 Later that evening you notice that the sound can be heard behind the solid walls of the house. Explain how it is possible for sound to travel beyond the obstacle.

6 A signal tower emits radio waves; long wavelengths, and TV waves; short wavelengths.

The farm residents that live behind the mountain, position A, notice that the radio signal is stronger than the signal received by the TV.6.1 Explain why this is the case.6.2 The neighbouring farmer, who lives a lot further from the tower at position B, receives both signals equally strong. Explain why this is the case.6.3 The residents of farm A also notice that the TV signal is stronger at night than during the day. Why is this the case?

7 A vibrating, straight plank in a wave tank is creating parallel wave fronts. Each crest forms 0,04 s after the preceding one. The wave speed is 0,6 m⋅s-1. 7.1 Calculate the wavelength of the water waves.7.2 Draw a simple diagram to illustrate how the wave pattern changes if the wave travels through a slit with a width of w.7.3 Draw another sketch to show how the wave changes if w increases.

8 A student observes the following: When she looks at a monochromatic red light source with her eye lids only slightly open, she sees several red and dark lines.

A B