Dispersion, scattering, etc.

Physics 30 Lesson 10: Dispersion, Scattering, Colour, Polarisation

I. Dispersion

  • Definition of Dispersion: Dispersion occurs when light rays are refracted, with the change in direction depending on the difference in speed between two mediums.

  • Index of Refraction (n):

    • The index of refraction is dependent on the wavelength of light to a small degree.

    • Typically, this wavelength dependency is negligible; however, in the case of glass triangular prisms, it results in the separation of white light into its spectrum of colors.

  • Practical Example of Dispersion:

    • Rainbows are an outcome of dispersion.

    • When sunlight shines and interacts with water droplets in the air (

    • sources include rain, mist from waterfalls, or sprinklers), the different wavelengths of light are refracted differently, leading to the separation of colors and forming a rainbow.

    • The observer's location influences their perception of the rainbow; it moves as the observer moves. This principle is humorously noted regarding the myth of locating a pot of gold at the rainbow's end.

  • Diagram Notation:

    • The schematic involves two media, with the index of refraction noted as follows:

    • Medium 1: $n_1$

    • Medium 2: $n2 > n1$

    • Speeds: $v2 < v1$

    • Wavelengths: $ ext{λ}2 < ext{λ}1$

    • Frequency remains constant.

    • Color spectrum noted: Red, Orange, Yellow, Green, Blue, Violet.

II. Scattering

  • Understanding Visibility: For an object to be visible, light must reflect off particles (e.g., dust, lint) and enter our eyes. This principle is termed scattering.

  • Scattering Process:

    • Light interacts with atmospheric particles or molecules, scattering in various directions. Observable light is a result of scattered light directed into our eyes; unscattered light continues past unnoticed.

    • Diagram Notation:

    • Unscattered Light: travels unaffected.

    • Scattered Light: reflected into the observer's vision.

  • Why is the Sky Blue?:

    • The scattering of light is proportional to the fourth power of frequency ($f^4$).

    • Higher frequencies like blue and violet light scatter more than lower frequencies such as red and orange.

    • The omnipresent scattering of blue light from all directions results in a blue sky.

  • Sunset Explanation:

    • During sunset, due to maximum atmospheric travel, blue light scatters out, allowing other hues to reach the observer, resulting in a reddish hue due to light scattering off larger dust particles.

  • Scattering Validity:

    • The fourth power relation holds if scattering objects (like oxygen and nitrogen molecules) are much smaller than the light's wavelength.

  • Cloud Appearance:

    • Clouds consist of larger water droplets or crystals that scatter all light frequencies uniformly, rendering them white.

III. Colour

  • Composition of White Light: White light comprises all colors of visible light.

  • Definitions:

    • Black: Defined as the absence of light.

  • Subtraction Theory of Colour:

    • Explains how objects appear based on light absorption and reflection.

    • Example: Red objects absorb blue and green, reflecting red.

    • Diagram:

      • Incident light: white

      • Absorption: blue and green

      • Reflection: red.

  • Addition Theory of Colour:

    • When different colors of light are combined, they produce new colors.

    • Demonstrated through a color light box.

    • Cone Cells in Eyes:

      • Three types: blue, green, and red; essential for detecting color combinations.

    • Color Combinations:

      • Blue + Green = Cyan

      • Blue + Red = Magenta

      • Red + Green = Yellow

      • Red + Green + Blue = White.

IV. Polarisation

  • Nature of Light Waves: Light vibrates in all planes as it travels through space.

  • Polarising Filters:

    • A polarising filter only allows light waves that are aligned with the filter plane to pass through.

  • Effect of Polarisation:

    • When unpolarised light passes through a filter oriented for horizontal waves, it is selectively passed; if a second vertical filter is applied, verticular light is absorbed, confirming only light at parallel axes passes.

  • Longitudinal vs Transverse Waves:

    • Longitudinal waves vibrate in the travel direction and cannot be polarised, unlike transverse waves.

  • Polarisation Effect:

    • With parallel filter axes, light transmits; with perpendicular axes, light is blocked.

V. Polarisation Activity

  • Investigations with a Polarisation Kit:

    1. Rotate a polarising disk in light; observe maximum and minimum light penetration. Record observations with descriptions and diagrams.

    2. Use a calcite crystal over text and notice brightness changes, then cover with a polarising filter and rotate. Note visual changes and record.

    3. Observe a watch or calculator display with polarised filters; note appearance changes upon rotation.

    4. Examine glare on horizontal surfaces with polarised filters; document effects and propose usage for glare reduction.

    5. Rotate a mica crystal disc over light, noting changes with polarising discs.

    6. Repeat with a benzoic acid crystal.

VI. Hand-in Assignment

  1. Analyze why sunlight is not dispersed in air despite being made of the spectrum colors.

  2. Determine what signals different refractive indices in diamonds for spectrum colors.

  3. Explore the significance of particle size in light scattering for photography definition.

  4. Discuss comfort differences in clothing color under sunlight.

  5. Determine physical differences between red and orange light.

  6. Explain how spotlights could render blue clothes black or red on stage.

  7. Justify why window glass does not disperse white light.

  8. Predict color appearance changes of green objects under different colored light.

  9. Compare cat and human eye differences in color perception.

  10. Explain why moonlight creates a colorless perception of objects.