Human eye class 10: CBSE previous question paper problems (2021 syllabus)

Dispersion of light

• Solution

  A8. Definition: The splitting of light into its seven constituent colours due to refraction is called dispersion.
  [$1$‍ mark]

  Cause: When entering a medium, like glass, light of different colours bend at different angles with respect to the incident beam. Since white light is a composite of seven different colours, it splits into a spectrum of seven colours when entering a prism, causing dispersion.
  [$1$‍ mark]

  Diagram:

  
  [$1$‍ mark]
• Similar problems (8)

  Q8.1. What is dispersion of white light? What is the cause of such dispersion? Draw a diagram to show the dispersion of white light through a glass prism.
  [$3$‍ marks, AI $2009$‍]

  Q8.2.
  

  In the figure given, a narrow beam of white light is shown to pass through a triangular glass prism. After passing through the prism it produces a spectrum $XY$‍ on a screen.
  ($\text{i}$‍) State the colours seen at $X$‍ and $Y$‍.
  ($\text{ii}$‍) Why do different colours of white light bend through different angles with respect to the incident beam of light?
  [$2$‍ marks, Delhi $2010$‍]

  Q8.3. What is meant by the dispersion of white light? Draw a diagram to show the dispersion of white light by a glass prism.
  [$3$‍ marks, Delhi $2011$‍]

  Q8.4. Draw a labeled ray diagram to illustrate the dispersion of a narrow beam of white light when it passes through a glass prism.
  [$3$‍ marks, AI $2012$‍]

  Q8.5.
  

  If $AO$‍ were a ray of white light,
  ($\text{i}$‍) describe what you will observe on the screen $BC$‍ placed near the prism.
  ($\text{ii}$‍) Write the name of this phenomenon.
  ($\text{iii}$‍) State the cause of this phenomenon.
  ($\text{iv}$‍) What does it prove about the constituents of white light?
  [$4$‍ marks, Delhi $2013$‍ C]

  Q8.6. Define the term 'dispersion of white light'. Name the colour of light which bends ($\text{i}$‍) the most, ($\text{ii}$‍) the least while passing through a glass prism. Draw a ray diagram to justify your answer.
  [$3$‍ marks, Foreign $2014$‍]

  Q8.7. What is dispersion of white light? State its cause.
  [$1$‍ mark, Foreign $2016$‍]

  Q8.8. Why do the component colours of incident white light split into a spectrum while passing through a glass prism? Explain.
  [$1$‍ mark, Delhi $2017$‍]

• Related exercise: Dispersion of light - definition and characteristics
• Related video: Dispersion

• Solution

  A9. The refractive index of glass is different for different colours of light. In a glass prism, a light ray that refracts more while entering the prism also goes through a larger angle of deviation.
  [$1.5$‍ marks]

• Related exercise: Dispersion of light - definition and characteristics
• Related video: Dispersion

Dispersion of light in prism

• Solution

  A10. After a rain-shower, water droplets suspended in the atmosphere act like tiny prisms for the sunlight. They refract and disperse the incident sunlight, then reflect it internally, and finally refract it again when it comes out of the raindrop. We see the colours in a rainbow because of the dispersion of sunlight inside a raindrop.
  [$1$‍ mark]

  
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  Two essential conditions for a rainbow to form:
  ($\text{i}$‍) the sun should at the opposite side of the viewing direction,
  ($\text{ii}$‍) suspended water droplets must be present in the air.
  [$1$‍ mark]
• Similar problems (4)

  Q10.1. Explain the formation of a rainbow in the sky with the help of a diagram.
  [$3$‍ marks, Foreign $2011$‍]

  Q10.2. Draw a diagram to show the formation of a rainbow and mark the points where ($\text{i}$‍) dispersion and ($\text{ii}$‍) internal reflection occurs.
  [$3$‍ marks, Foreign $2012$‍]

  Q10.3. Draw a labeled diagram to explain the formation of a rainbow in the sky.
  [$3$‍ marks, Foreign $2015$‍]

  Q10.4. Draw a labeled diagram to explain the formation of a rainbow.
  [$3$‍ marks, Delhi $2017$‍]

• Related exercise: Rainbows

• Solution

  A11. Newton passed white light from the sun through a glass prism, obtaining its spectrum of seven colours. He still needed proof that there weren't any other possible colours.

  For this, he placed a second identical prism in an inverted position with respect to the first prism, and observed white light emerging from the other side of the second prism. This showed that sunlight was made of seven colours.
  [$1$‍ mark]

  
  [$1$‍ mark]
• Similar problems (2)

  Q11.1. When we place a glass prism in the path of a narrow beam of white light, a spectrum is obtained. What happens when a second identical prism is placed in an inverted position with respect to the first prism? Draw a labeled diagram to illustrate it.
  [$3$‍ marks, Delhi $2012$‍]

  Q11.2. Describe an activity to show that the colours of white light split by a glass prism can be recombined to get white light by another identical glass prism. Also draw ray diagrams to show the recombination of the spectrum of white light.
  [$3$‍ marks, AI $2016$‍]