Physics 60 Interference of Light (6 of 8) The Thin Film: Oil

Michel van Biezen
14 May 201305:15
EducationalLearning
32 Likes 10 Comments

TLDRThe video discusses the phenomenon of light reflection in a thin film, using an example of a puddle with a thin oil layer. When light shines on the puddle, different colors appear due to varying wavelengths. The video explains how an observer sees yellow light reflected and determines the oil film's thickness needed for this reflection. It covers concepts such as phase shifts at boundaries, wavelength adjustments in different mediums, and calculations to find the oil film's thickness, concluding that it should be 89 nanometers for yellow light reflection.

Takeaways
  • 馃寛 The phenomenon described is the interference of light reflecting off an oil film on water, creating colorful patterns due to varying thicknesses of the oil layer.
  • 馃敩 The observer sees yellow light being reflected from a particular spot on the oil film, which suggests a specific thickness related to the wavelength of yellow light.
  • 馃専 Sunlight contains a spectrum of colors, but only yellow light is being reflected back to the observer, indicating a resonance condition with the film's thickness.
  • 馃搹 The thickness of the oil film is critical for the constructive interference of yellow light, which requires the path difference to be an integer multiple of the wavelength.
  • 馃攧 A phase shift occurs at the boundary where light reflects from a medium with a lower to a higher index of refraction, affecting the interference pattern.
  • 馃攧 There is a 180-degree phase shift (half-wavelength) at the air-oil boundary due to the higher index of refraction of oil compared to air.
  • 馃攧 No phase shift occurs at the oil-water boundary because the light is moving from oil (higher index) to water (lower index).
  • 馃搻 The extra distance traveled by the light through the oil film must be half a wavelength to achieve constructive interference for yellow light.
  • 馃敘 The formula to calculate the thickness of the oil film is derived from the condition that twice the thickness (2T) equals half the wavelength of yellow light in the oil (位/2n).
  • 馃搻 The wavelength of light in the oil is adjusted by the index of refraction (n), which is why 位/2n is used instead of just 位/2.
  • 馃摑 The calculated thickness for the oil film to reflect yellow light is approximately 89 nanometers, based on the given index of refraction for oil and the wavelength of yellow light.
Q & A

  • What causes the different colors seen when oil is floating on water?

    -The different colors are caused by the interference of light waves reflecting off the thin oil film on the water's surface. This phenomenon is known as thin-film interference.

  • Why is only yellow light being reflected in the given example?

    -Yellow light is being reflected because the thickness of the oil film has been such that it constructively interferes with the yellow wavelength of light, while other wavelengths are either transmitted or destructively interfered with.

  • What is the significance of the extra distance traveled by the light ray that reflects off the boundary between oil and water?

    -The extra distance traveled by the light ray is crucial because it determines the phase difference between the light waves reflecting off the top and bottom surfaces of the oil film, which in turn affects the color seen by the observer.

  • Why is there a phase shift when light is reflected off a boundary where the index of refraction is greater on the other side?

    -A phase shift occurs at a boundary when light is reflected because the light wave changes from a medium with a lower index of refraction to one with a higher index, causing a half-wave shift in the reflected light.

  • Why is there no phase shift when light reflects off the boundary between oil and water?

    -There is no phase shift at the oil-water boundary because the light is moving from a medium with a higher index of refraction (oil) to one with a lower index (water), which does not cause a half-wave shift.

  • What is the initial phase difference between the two beams of light reflecting off the oil film?

    -The initial phase difference is a half-wavelength because of the phase shift at the air-oil boundary and the lack of a phase shift at the oil-water boundary.

  • What condition is needed for the two waves to constructively interfere and produce yellow light?

    -For constructive interference to occur and produce yellow light, the extra distance traveled by the light through the oil must be a half-wavelength, compensating for the initial half-wavelength phase difference.

  • How does the index of refraction affect the wavelength of light in the oil?

    -The wavelength of light changes when it enters a medium with a different index of refraction. In the case of oil, the wavelength is reduced because the index of refraction of oil is greater than that of air.

  • What formula is used to calculate the thickness of the oil film that results in the reflection of yellow light?

    -The formula used is T = \frac{\lambda}{4n}, where T is the thickness of the oil film, \lambda is the wavelength of yellow light in air, and n is the index of refraction of the oil.

  • What is the calculated thickness of the oil film for the observer to see yellow light being reflected?

    -The calculated thickness of the oil film is 89 nanometers, which is sufficient for the observer to see yellow light being reflected off the film.

Outlines
00:00
馃寛 Understanding Light Reflection on Thin Films

This paragraph explains the phenomenon of light reflection on a thin film, such as a layer of oil on water. It describes how sunlight, composed of various colors, can cause different colors to appear when reflected from such a film. The focus is on determining the thickness of the oil layer required for an observer to see yellow light being reflected. The explanation includes the concept of light traveling through the oil, reflecting off the oil-water boundary, and how the thickness of the oil affects the phase difference between light waves, ultimately determining the color observed.

05:01
馃敩 Calculating Oil Film Thickness for Yellow Light Reflection

This paragraph continues the explanation by calculating the exact thickness of the oil layer needed for yellow light reflection. It delves into the physics of phase shifts at different boundaries, the importance of considering the index of refraction, and how these factors contribute to the light's wavelength in the oil. The calculation concludes that for yellow light (500 nm), the oil layer must be 89 nanometers thick for the observer to see yellow light reflected, rather than transmitted into the water. This provides a practical example of solving a thin-film interference problem.

Mindmap
Keywords
馃挕Thin film
A thin film in this context refers to a very thin layer of a substance, such as oil, that is spread over a surface, like water. It is a key element in the video's demonstration of light interference and color reflection. The script uses the example of a puddle with an oil layer to illustrate how light interacts with different media, resulting in the observer seeing reflected colors.
馃挕Interference
Interference is a phenomenon in physics where two waves superimpose to form a resultant wave of greater or lower amplitude. In the video, the concept is used to explain how light waves reflecting off the thin oil film can interfere constructively or destructively, leading to the observation of specific colors like yellow.
馃挕Reflection
Reflection is the change in direction of a wavefront at an interface between two different media so that the wavefront returns into the medium from which it originated. The script describes how yellow light is specifically reflected off the oil film, which is central to understanding the optical effects being discussed.
馃挕Phase difference
Phase difference refers to the difference in the phase angle between two waves at a given time. It is crucial in the context of the video because the constructive interference of light waves, resulting in the reflection of yellow light, depends on the phase difference being a multiple of 2蟺 (a full wavelength, half wavelength, etc.). The script explains how the phase difference is influenced by the thickness of the oil film and the refractive indices of the media involved.
馃挕Wavelength
Wavelength is the spatial period of a periodic wave鈥攖he distance over which the wave's shape repeats. The video script discusses the importance of wavelength in the context of light waves and how the thickness of the oil film must be related to the wavelength of the yellow light to achieve constructive interference.
馃挕Constructive interference
Constructive interference occurs when the peaks or troughs of two or more waves align, resulting in a wave with a larger amplitude. In the script, this concept is essential for explaining why the observer sees yellow light being reflected from the oil film, as it is a result of the constructive interference of light waves with a specific phase difference.
馃挕Destructive interference
Destructive interference happens when the peak of one wave aligns with the trough of another, leading to a cancellation of the waves and a reduction in amplitude. Although not explicitly mentioned in the script, the concept is implicitly discussed in the context of the colors not being reflected, which would be due to destructive interference.
馃挕Refraction
Refraction is the change in direction of a wave due to a change in its speed when it passes from one medium to another. The script explains how sunlight, which contains all colors of the rainbow, changes speed and direction as it enters the oil film, affecting the wavelengths and the resulting interference pattern.
馃挕Index of refraction
The index of refraction is a measure of how much a wave (such as light) slows down and changes direction when entering a medium from a vacuum. In the script, the index of refraction of the oil is used to calculate the required thickness of the film for the yellow light to be reflected, indicating its importance in understanding the optical properties of the media involved.
馃挕Phase shift
A phase shift is a change in the phase of a wave. The script describes how a phase shift occurs at the boundary between air and oil due to the difference in refractive indices, which is critical for calculating the phase difference between the light waves reflecting off the oil film.
馃挕Sunlight
Sunlight, in the context of the video, is the source of the light that is being analyzed for its interaction with the thin oil film. It is composed of a spectrum of colors, and the script uses sunlight to illustrate how different wavelengths are affected by the oil film, leading to the observer seeing specific colors like yellow.
Highlights

The phenomenon of different colors being reflected off an oil film on water when light shines on it, creating colorful rings.

The requirement for the oil film to be a specific thickness to reflect yellow light back to the observer.

Sunlight contains all colors of the rainbow, but only yellow light is being reflected in the example.

The concept that the extra distance traveled by light through the oil film is equal to twice the thickness of the oil.

Phase difference for yellow light reflection to be 0, implying constructive interference.

The importance of considering phase shifts at boundaries with different indices of refraction.

A phase shift occurs when light is reflected off a boundary where the index of refraction on the other side is greater.

No phase shift when the index of refraction on the other side is smaller than where the light came from.

The initial phase difference due to the reflection off the first boundary is a half-wavelength.

The condition for constructive interference requires making up the other half-wavelength.

The wavelength of light in the oil is different due to the change in the index of refraction.

The formula for calculating the thickness of the oil film: 2T = (1/2 * 位) / n.

Solving for the thickness T gives T = 位 / (4n), accounting for the wavelength and index of refraction.

The example calculation for yellow light at 500 nanometers with an index of refraction of 1.4 for oil.

The resulting thickness of the oil film required for yellow light reflection is 89 nanometers.

The explanation of how the thickness of the oil film affects the color seen by the observer.

Transcripts

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Optical PhenomenaThin FilmInterferenceRefractionWavelengthColor TheoryPhysicsLight ReflectionPhase ShiftOil on Water