Waves Questions

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March 2013

7 A learner investigates the difference in patterns obtained on a screen when monochromatic red light passes through a single slit and through a double slit. The diagram below shows two patterns obtained during the investigation.

Pattern A

Pattern B

7.1 Which pattern, A or B, is a diffraction pattern? (1)
7.1 B

7.2 Write down the name of the phenomenon that explains the formation of the red lines (unshaded area) in pattern A. (2)
7.2 Constructive interference

7.3 The monochromatic red light used to obtain pattern B has a frequency of 4,54 x 1014Hz. The broadness of the central band, x, is measured as 20cm when the distance between the screen and the slit is 1,5 m.
Calculate the:
7.3.1 Wavelength of the red light (3)

7.3.2 Width of the slit used (6)

7.4 How will the broadness of the central band, x, change if the monochromatic red light is replaced with monochromatic blue light? Write down only INCREASES, DECREASES or REMAINS THE SAME. (1)
7.4 Decreases

Blue light has a higher frequency and hence a lower wavelength. The lower wavelength produces lesser diffraction.

March 2012

7 Learners investigate the change in the broadness of the central bright band formed when monochromatic light of different wavelengths passes through a single slit. They set up the apparatus, as shown in diagram below, and measure the broadness of the central bright band in the pattern observed on the screen. The width of the slit is 5,6 x 10-7 m.

7.1 Write down an investigative question. (2)
7.1 How will the width of the central band change/differ when the wavelength (of the light) changes / is increased / is decreased?

7.2 Which TWO variables are kept constant? (2)
7.2
• Slit width
• Distance between slit and screen

7.3 In one of their experiments, the distance from the midpoint of the central bright band to the first dark band is measured to be 0,033 m.
Calculate the wavelength of the light used in this experiment. (5)
7.3

7.4 How will the broadness of the central bright band of red light compare with that of blue light? Write down only GREATER THAN, SMALLER THAN or EQUAL TO. Give a reason for the answer. (2)
7.4 Greater than
Red light has a longer wavelength (and is diffracted more.)

OR

Diffraction α λ

Nov 2012

7 Learners use monochromatic blue light to investigate the difference between an interference pattern and a diffraction pattern.

7.1 Apart from the blue light and a screen, write down the name of ONE item that the learners will need to obtain an interference pattern. (1)
7.1 Double slit

7.2 Briefly describe the interference pattern that will be observed on the screen. (2)
7.2 Alternate dark and bright / blue bands.
Bright / blue bands of equal broadness (width).

7.3 In one of their experiments they place the screen at a distance of 1,4 m from a single slit and observe a pattern on the screen.
The width of the central bright band is measured as 22 cm.

Calculate the:
7.3.1 Angle θ at which the first minimum will be observed on the screen (3)
7.3.1

7.3.2 The width of the slit used if the wavelength of the blue light is 470 nm (5)
7.3.2

7.4 The width of the central band INCREASES when the blue light is replaced with monochromatic red light. Explain this observation. (2)
7.4 Red light has a higher wavelength than blue light.
Diffraction is proportional to wavelength. Hence with red light, the central width increases due to the increased diffraction.

March 2011

7 Learners perform an experiment with monochromatic light. They pass the light through a single slit. The distance between the screen and the slit is kept constant.

The diagram below represents the pattern observed during the experiment.

The slit has a width of 0,02 mm and the SECOND dark band is formed on the screen at an angle of 30 from the centre of the slit.

7.1 Define the term diffraction. (2)
7.1 The ability of a wave to bend / spread out (in wave fronts) as they pass through a small aperture / around a sharp edge.

7.2 Calculate the wavelength of this light. (4)
7.2

m = 2 since the angle is for the second dark band

7.3 The light used is either green or red. Given that yellow light has a wavelength of 577 nm, which colour is used? Give a reason for your answer. (2)
7.3 Green
The wavelength calculated is 523 nm and this is a shorter wavelength than that of yellow light which is given as 577nm.
This shorter wavelength implies a higher frequency than that of yellow, hence the light must be green.

highest frequency to lowest frequency:
• violet
• indigo
• blue
• green
• yellow
• orange
• red

7.4 Using the same light as in QUESTION 7.2, write down TWO experimental changes that can be made to decrease the distance x in the diagram above. (2)
7.4 Increase the slit width.
Decrease the distance between the screen and the slit.

7.5 Describe the pattern that will be observed if the single slit is now replaced with a double slit. (2)
7.5 A central band of alternate bright and dark bands of equal width.

Nov 2011

7 A learner investigates the change in broadness of the central bright band in a diffraction pattern when light passes through single slits of different widths. She uses monochromatic violet light of wavelength 4 x 10-7m. The apparatus is set up as shown in the diagram below.

7.1 Define the term monochromatic light. (2)
7.1 Light of a single wavelength OR single frequency.

7.2 Write down the investigative question for this investigation. (2)
7.2
• How will the broadness / width of the central band change / differ when slit width changes / is increased / is decreased?
• What is the relationship between the broadness of the central bright band and slit width?

7.3 Write down TWO variables that are kept constant during this investigation. (2)
7.3
• Wavelength (of light) / Frequency (of light) / Colour of light / Light source
• Distance between slit and screen.

7.4 The learner now uses a narrower slit.
How will the broadness of the central bright band change? Write down only INCREASES, DECREASES or REMAINS THE SAME.
Give an explanation. (2)
7.4 Increases
Diffraction is inversely proportional to slit width.

7.5 Calculate the angle θ at which the second minimum is formed if a slit of
width 2,2 x 10-6 m is used. (5)
7.5

March 2010

9 Light of a single frequency pass through a single slit. The first minimum is observed at point P on a screen, as shown in the diagram below. Point O is the midpoint of the central bright band. The distance OP is 2,5 cm and the slit width is 3,2 x 10-5m.

9.1 What can be deduced about the nature of light from this observation? (1)
9.1 Wave nature

OR

Light has wave properties.

9.2 Explain how the minimum is formed at point P. (2)
9.2
• Wavefronts from the slit arrive at point P out of phase and interfere destructively. 

 OR 

• A crest meets a trough at P and destructive interference takes place.

9.3 If the wavelength of the incident light is 600 nm, calculate the distance Q between the screen and the slit. (5)
9.3

9.4 The original slit is now replaced by a second slit of different width, while the distance Q and the wavelength of the incident light remain the same.
Distance OP changes to 4 cm.
9.4.1 How does the slit width of the second slit compare to that of the first slit? Only write down GREATER THAN, SMALLER THAN or EQUAL TO. (1)
9.4.1 Smaller than

9.4.2 Explain your answer to QUESTION 9.4.1 without performing a calculation. (2)
9.4.2 If OP increases:
This means that sinθ increases OR degree of diffraction increases.

Since

slit width a decreases

Nov 2010

7 Monochromatic red light passes through a double slit, as shown in the diagram below. Circular wave fronts, advancing towards the screen, are shown between the slits and the screen as dotted lines and solid lines. The solid lines represent crests and the dotted lines troughs.

Interference of the circular wave fronts results in an interference pattern observed on the screen. P, Q and R represent the centres of different bands in the interference pattern.

7.1 Define the term interference. (2)
7.1 When two waves pass through the same region of space at the same time, resulting in the superposition of waves.

7.2 What type of interference takes place at point A? Give a reason for the answer. (2)
7.2 Constructive (interference)
The waves crossing each other are in phase.
Two troughs meet.
The path difference is an integer number of λ.

At point A, the diagram shows two dotted lines (troughs) intersecting.
Trough + Trough = deeper Trough
Constructive Interference

At any point where a solid line (crest) meets a solid line (crest), this would also be Constructive Interference.
Like + Like = Constructive

At any point where a solid line (crest) meets a dotted line (trough), this would be Destructive Interference

Like + Unlike = Destructive

7.3 Is band P a dark band or a red band? Refer to the type of interference involved to explain how you arrived at the answer. (3)
7.3 Dark band
It lies on the line combining all the points where crests and troughs overlap resulting in destructive interference.

OR

It lies on the (nodal) line where destructive interference occurs.

8 The relationship between the degree of diffraction of light and slit width is investigated. Monochromatic light of wavelength 410 nm is passed through a single slit at a fixed distance from a screen. The angles at which the first minimum (α) and the second minimum (β) occur are measured.

The experiment is repeated using the same light source but a slit of different width.

The results obtained from the two experiments are represented in the table below

8.1 Define the term diffraction. (2)
8.1 The ability of a wave to bend / spread out (in wave fronts) as they pass through a (small) aperture / opening or around a (sharp) edge/ points /corners / barrier.

8.2 For this investigation, name the following:
8.2.1 Dependent variable (1)
8.2.1
• Angle of / (Degree of) diffraction
• Position of minima
• α or β

8.2.2 Independent variable (1)
8.2.2 (Slit) width / a

8.3 Which ONE of Slit 1 or Slit 2 is the narrower slit? Explain the answer. (2)
8.3 (Slit) 1
Slit 1 represents the most diffraction.

OR

Diffraction /Angle / sin α / α is inversely proportional to slit width.

OR

OR

Larger angle at which first minimum for slit 1 is obtained.

OR

Smaller angle at which first minimum for slit 2 is obtained.

8.4 Use the data in the table to calculate the width of Slit 2. (4)
8.4

March 2009

9 Huygens's principle is used to explain the wave phenomena, interference and diffraction.

9.1 State Huygens's principle. (2)
9.1 Each point on the wavefront acts as a source of spherical secondary waves or wavelets travelling away from source.

9.2 Use Huygens's principle to explain the diffraction of water waves in a ripple tank as they pass through a narrow opening in a barrier. (3)
9.2 Each point on the initial plane wavefront entering the slit acts as a source of secondary wavelets. The wavelets propagate in all directions beyond the slit causing the wave to spread into regions beyond those in line with the slit.

9.3 A single slit of unknown width is illuminated with red light of wavelength 650 nm. Calculate the width of the slit for which the first dark band will appear at 150. (3)
9.3

Nov 2009

9 A learner uses a white light bulb, two pencils and a red filter to investigate a wave phenomenon.
He places the red filter in front of the light bulb and fastens the two pencils together with tape. He then observes the light bulb through the narrow gap between the two pencils from a distance of 2 m, as shown below.

9.1 Name the wave phenomenon investigated by the learner. (1)
9.1 Diffraction

9.2 The learner notes the following observations in his practical book:

Observation 1:
Red and dark bands of different widths are observed on either side of the central red band.

Observation 2:
When the two pencils are brought closer together, the red lines become broader.

Observation 3:
When the red filter is removed, spectral colours are observed on either side of the central band.
9.2.1 Write down Huygens's principle. (2)
9.2.1 Each point on a wave front acts as a source of (spherical) secondary wave fronts / wavelets (that propagates in the forward direction).

9.2.2 Use Huygens's principle to explain the occurrence of red and dark bands in Observation 1. (2)
9.2.2 Dark bands form where wave fronts / wavelets interfere destructively.
Red / bright bands form where wave fronts / wavelets interfere constructively.

9.2.3 Give a reason for Observation 2. (2)
Diffraction is inversely proportional to the slit width

OR

The degree of diffraction / Angle at which minima occurs increases with decreasing slit width

9.2.4 Explain the formation of the spectral colours in Observation 3. (2)
9.2.4 White light consists of different colours with different wavelengths.
Amount of diffraction differs for different colours / different wavelengths.

Nov 2009 Unused

9 A learner uses a single slit to determine the wavelength of a red laser light.
He sets up the slit and screen as shown below and shines the laser through the single slit of width 7,25 x 10-6 m. The distance between the screen and the slit is 0,4 m.

9.1 Name the type of pattern observed on the screen. (1)
9.1 diffraction (pattern)

9.2 State ONE safety precaution that the learner must take when using the above apparatus. (1)
9.2 Do not look directly into the laser.

OR

Do not shine the laser in the direction of other people

9.3 The learner measures the distance between the midpoint of the central bright band and the first dark band as 3,5 cm.
Calculate the wavelength of the red laser light. (5)

9.4 The learner wants to decrease the distance between the midpoint of the central bright band and the first dark band. What change can the learner make to the above arrangement to achieve this? Assume that the same laser is used. (1)
9.4 Increase the slit width

OR

Move the screen closer to the slit / decrease distance between screen and slit

Prep Paper 2008

9 In a set-up to illustrate Young's double slit experiment, Renzo placed a red filter that allows only monochromatic red light to reach the slits between a light bulb and a double slit.

9.1 Define the term monochromatic. (2)
9.1 Light consisting of a single frequency. (or one wavelength)

9.2 Describe the pattern that is observed on the screen with the naked eye once the red light has passed through the double slits. (2)
9.2 Alternate red and dark bands are observed.

9.3 Explain the observation made in QUESTION 9.2. (2)
9.3 Red bands as result of constructive and dark bands as result of destructive interference.

9.4 Describe and explain how the observed pattern will differ if the red filter is replaced by a blue one. (4)
9.4 The coloured bands are narrower / A greater number of dark bands, closer together are seen.
The wavelength of blue light is shorter than red, resulting in more points of interference.

9.5 How will the pattern observed be affected if the distance between the two slits is increased? (2)
9.5 More dark and light bands are seen.

Nov 2008

9 A helium-neon laser emits red light that passes through a single slit. A diffraction pattern is observed on a screen some distance away from the slit.

9.1 Define the term diffraction. (2)
9.1 The spreading (or bending) of a wave passing through a small aperture / slit / around a sharp edge / obstacle.

9.2 If the wavelength of red light is 644,4nm and the slit width is 3437nm, calculate the angle at which the third minimum occurs. (3)
9.2

9.3 Briefly describe the diffraction pattern that will be observed on the screen. (2)
9.3 A broad central red / bright / light fringe (bands) followed by alternate dark and red (bright) fringes (bands) on either side

The single slit is replaced with a double slit.

9.4 Name ONE similarity and ONE difference in the pattern observed when the single slit is replaced with a double slit. (2)
9.4
Similarity
Alternate red and dark bands

Difference
The red bands are of equal width / no broad central band is observed / The red bands are of equal intensity (brightness)

9.5 Will this pattern be observed if the laser is replaced with a light bulb? Give a reason for your answer. (2)
9.5 No, it is not a coherent source / not monochromatic
Bands of different colours are observed , colour fringes

9 During a demonstration of a wave phenomenon, monochromatic red light passes through a slit of width 1,8 x 10-4 m and shines on a flat screen a distance of 0,4 m away from the slit. The wavelength of the light is 675 nm.

9.1 Name the phenomenon demonstrated above. (1)
9.1 Diffraction

9.2 Briefly explain how the dark bands in the observed pattern are formed. (2)
9.2 Wavelets originating from different points in the slit reach the screen out of phase and interfere destructively on the screen.

9.3 Calculate the width 2y of the central bright band. (6)
9.3

Width of central bright band

9.4 How will your answer to QUESTION 9.3 change if the width of the slit is
changed to 1,8 x 10-6m? Write only INCREASES, DECREASES or REMAINS THE SAME.
Give a reason for your choice. (3)
9.4 Increases
The slit is now narrower. Diffraction is inversely proportional to the width of the slit. 

OR 

Amount of diffraction is determined by the ratio :

If a decreases,
then increases.

i.e. if a decreases, amount of diffraction increases

9.5 (Not in this years exams)
The red light incident on the slit now passes through a yellow filter and then through a magenta filter before reaching the slit. What colour will now be observed for the central bright band? Explain your answer. (3)
9.5 Red
 The yellow filter transmits red light. - yellow only transmit red and green light. When the red light reaches the magenta filter it will be transmitted - magenta only transmits red and blue light.

Exemplar Paper

9 Red light from two stationary narrow slits, S1 and S2, reaches a large white screen PON, indicated in the diagram below.

A dark band is observed at point P on the screen. The brightest band is observed at point O on the screen. Bands are arranged such that the band at point N on the screen is dark.

9.1 State Huygens' principle in words. (2)
9.1 Every point on a wavefront acts as a source of secondary waves.

9.2 Write down the type of interference that occurs at point O. Write down only DESTRUCTIVE or CONSTRUCTIVE. Briefly explain your answer. (3)
9.2 CONSTRUCTIVE 
- waves are interfering constructively to increase the amplitude of the wave. 

9.3 Describe the change in brightness, if any, of the light bands formed on the screen as you walk closer to the screen from point M to point O. Briefly explain your answer. (3)
9.3 Brightness of red light remains the same.  The distance from each source to line MO is the same. (The difference in path length is zero)
(The light source has not moved.)

The red light is now replaced with a green light.

9.4 How will the new pattern differ from the previous one? (2)
9.4 The bands would be green and dark
The green and dark bands would be narrower. 

Green has a smaller wavelength and would display lesser diffraction.
Light from highest to lowest frequency
(from lowest to highest wavelength)
• violet
• indigo
• blue
• green
• yellow
• orange
• red
Red has the largest wavelength, and displays the widest diffraction.
Violet would not display as much diffraction as red.