DIFRAÇÃO e POLARIZAÇÃO - ONDULATÓRIA - AULA 6

Professor Boaro

20 Aug 201917:39

Summary

TLDRIn this lesson on wave phenomena, Professor Marcelo Mora covers diffraction and polarization. He explains how diffraction allows waves to bend around obstacles, using sound and light examples such as a car horn bending around a wall. The discussion also highlights the principle of Huygens-Fresnel and the distinction between longitudinal and transverse waves, showing that only transverse waves, like light, can be polarized. Practical demonstrations, including a laser and a hair example, illustrate the concepts, with exercises reinforcing the understanding of diffraction and polarization effects.

Takeaways

🌊 The video discusses the concept of wave diffraction, explaining how waves bend around obstacles and continue to propagate.
🚗 An example of diffraction is given with a car approaching from the left, where the car's sound is heard before it is seen due to the wave diffracting around a corner.
🌈 The video also covers the phenomenon of light diffraction using a laser and a strand of hair to illustrate how light waves interfere and diffract.
📢 Sound waves can diffract around objects like windows and walls, allowing us to hear sounds even when the source is not in direct line of sight.
🚦 The principle of Huygens is introduced, stating that every point on a wavefront can be considered a source of secondary wavelets that spread out in the forward direction at the same speed as the original wave.
🌉 Fresnel's contributions to the principle of wave propagation are highlighted, particularly his improvements upon Huygens' principle to better explain reflection and refraction.
🚧 The script touches on the historical debate between Newton and others about the nature of light, whether it is a particle or a wave.
📏 The concept of wave polarization is explained, noting that it only occurs in transverse waves, and longitudinal waves like sound cannot be polarized.
🕶️ The use of polarizers to filter out certain orientations of light waves is discussed, with examples of how this is used in sunglasses to reduce glare.
💡 The video concludes with exercises to test understanding, including questions on diffraction, the behavior of light passing through slits, and the use of polarized lenses to reduce glare from reflected light.

Q & A

What is the diffraction phenomenon mentioned in the script?
-Diffraction is a wave property that allows waves to bend around obstacles or pass through small openings. This is observed when waves, such as sound or light, change their direction and spread out after encountering a barrier or a slit.
How does sound diffraction differ from light diffraction?
-Sound waves, which are longer in wavelength, can easily diffract around obstacles like walls or through openings like windows. This allows sound to be heard even if the source is not directly visible. Light waves, being much shorter in wavelength, do not diffract as easily, which is why we cannot see around obstacles in the same way we can hear around them.
What is the Huygens-Fresnel principle, and how is it related to diffraction?
-The Huygens-Fresnel principle states that every point on a wavefront can be considered a source of secondary wavelets that propagate forward. This principle helps explain how waves, such as light or sound, can diffract when passing through slits or around obstacles.
What is the significance of the fender width in diffraction?
-The width of the opening (fender) must be of the same order as the wavelength of the wave for noticeable diffraction to occur. If the opening is too large compared to the wavelength, the wave passes through with minimal diffraction.
What example does the speaker use to illustrate sound diffraction?
-The speaker uses the example of hearing a car approaching from around a corner before actually seeing it. The sound waves emitted by the car horn can diffract around the corner, allowing the listener to hear the car before it is visible.
What is polarization, and which types of waves can be polarized?
-Polarization is a phenomenon where the oscillations of a wave are restricted to a particular direction. It only occurs in transverse waves, like light. Longitudinal waves, such as sound, cannot be polarized.
What practical application of light polarization is mentioned in the script?
-One practical application of light polarization is in Polaroid lenses, which are used to reduce glare by blocking horizontally polarized light reflected off surfaces like roads or water.
Why do sunglasses with polarized lenses help reduce glare?
-Polarized lenses are designed to block specific polarized light waves, such as those reflected horizontally off surfaces like water or roads. This helps reduce glare and makes vision clearer.
What is the Doppler effect, as briefly mentioned in the script?
-The Doppler effect is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. For example, the pitch of a car horn appears higher as the car approaches and lower as it moves away.
How did Isaac Newton's view of light differ from the wave theory?
-Isaac Newton believed that light was composed of small particles, while the wave theory, supported by figures like Huygens and Fresnel, proposed that light is a wave. This debate was prominent until experiments like diffraction and polarization demonstrated light's wave properties.

Outlines

00:00

📚 Introduction to Wave Theory and Diffraction

In this section, Professor Marcelo Mora introduces the topic of wave theory, focusing on diffraction and polarization. He begins by discussing the concept of resonance, using sound waves as an example, and explains how waves bend around obstacles. The example of a car horn being heard before the car is visible due to sound diffraction demonstrates this phenomenon. Marcelo also introduces the concept of light diffraction using a laser and a human hair, showing how waves behave when encountering obstacles of similar dimensions to their wavelength.

05:00

🌊 Huygens' Principle and Wave Propagation

Here, Marcelo explains Huygens' Principle, which states that every point on a wavefront acts as a source of secondary spherical waves. These secondary waves spread out in all directions, maintaining the speed of the original wave. This principle helps explain various wave phenomena, including diffraction, reflection, and refraction. Marcelo highlights how Augustin-Jean Fresnel improved upon Huygens' principle, making it more comprehensive in explaining diffraction.

10:01

🎓 Understanding Diffraction and Polarization

This paragraph delves deeper into the characteristics of waves and diffraction, focusing on how wave properties like frequency and wavelength interact with obstacles. Marcelo explains how sound waves can bend around doors or windows due to their larger wavelength compared to light waves, which cannot. He also discusses the historical debate about the nature of light—whether it is a particle or a wave—with Newton supporting the particle theory and others supporting the wave theory. This section transitions into a discussion on the polarization of transverse waves.

15:03

🕶️ Polarization of Light and Practical Applications

Marcelo discusses how polarization applies primarily to transverse waves and not longitudinal waves like sound. He uses an example of a cord vibrating in different directions and explains how a polarizer filters these oscillations. When light passes through a polarizing filter, only waves oscillating in the filter's direction pass through. He also explains how this concept is used in Polaroid lenses, which block light waves from specific orientations, reducing glare for drivers.

Mindmap

Keywords

💡Wave Diffraction

Wave diffraction refers to the bending or spreading of waves as they pass around an obstacle or through a narrow opening. In the video, this concept is illustrated through the example of sound waves bending around a wall, allowing the sound of a car horn to be heard even before the car is visible. This phenomenon is key to understanding how waves interact with obstacles, and it demonstrates that diffraction is more noticeable when the size of the obstacle or opening is comparable to the wavelength of the wave.

💡Resonance

Resonance occurs when an object or system vibrates at its natural frequency due to an external periodic force of the same frequency. In the video, the professor refers to resonance in the context of sound waves oscillating within a tube, showing how waves can resonate under certain conditions. This concept is essential for understanding many physical systems, including musical instruments, where resonance enhances sound.

💡Huygens' Principle

Huygens' Principle states that every point on a wavefront can be considered as a source of secondary spherical wavelets that spread out in the forward direction at the same speed as the wave. In the video, the principle is used to explain how wave diffraction occurs, with each point along a wavefront acting as a source of secondary waves, leading to the bending of the wave as it encounters an obstacle or passes through an opening.

💡Interference

Interference is the phenomenon that occurs when two or more waves overlap, resulting in a new wave pattern. In the video, interference is mentioned when light passes through a hair and creates a diffraction pattern that can be explained by the interaction of the light waves. This principle is crucial for understanding wave behavior in many contexts, including sound and light.

💡Polarization

Polarization is the process by which the oscillations of a wave, especially light, are restricted to a particular direction. The professor explains that polarization occurs only in transverse waves, like light waves, and demonstrates this with an example of a polarizing filter that only allows waves oscillating in a specific direction to pass through. Polarization is an important concept in optics, particularly for applications like sunglasses and camera lenses.

💡Transverse Waves

Transverse waves are waves in which the oscillations are perpendicular to the direction of wave propagation. In the video, light waves are described as transverse, and it is explained that these waves can be polarized. This distinction is important because transverse waves behave differently from longitudinal waves, such as sound, which cannot be polarized.

💡Longitudinal Waves

Longitudinal waves are waves in which the oscillations occur in the same direction as the wave travels. The video contrasts longitudinal waves, like sound waves, with transverse waves, noting that sound cannot be polarized. This difference is significant when studying wave behavior and properties, such as diffraction and interference.

💡Diffraction Grating

A diffraction grating is an optical component with a pattern that splits and diffracts light into several beams. In the video, the professor uses the example of a laser beam passing through a hair to demonstrate diffraction, showing how light can create interference patterns. Diffraction gratings are used in various scientific instruments to analyze light and measure wavelengths.

💡Sound Waves

Sound waves are longitudinal waves that travel through a medium, such as air, and are perceived as sound. In the video, sound waves are used to demonstrate diffraction, showing how they can bend around obstacles, such as a wall, allowing a person to hear something before seeing it. The video also highlights the different behaviors of sound and light waves, particularly in terms of diffraction and polarization.

💡Fresnel's Extension of Huygens' Principle

Fresnel extended Huygens' Principle to explain diffraction more comprehensively, incorporating the idea of wave interference. The professor mentions Fresnel's contribution to understanding wave behavior, particularly diffraction, improving on Huygens' original idea. This extension is crucial for explaining complex wave phenomena, such as how light behaves when it encounters obstacles.

Highlights

Introduction to wave phenomena and focus on diffraction and polarization.

Explanation of diffraction using sound waves, like how sound can bend around obstacles.

Demonstration with a car approaching a corner, showing how sound reaches before the car is visible due to diffraction.

Introduction of the laser experiment with a hair, showing diffraction through small obstacles.

Definition of diffraction: the property of waves to bend around obstacles or pass through small openings.

Illustration of diffraction using a tank of water and obstacles, explaining wave behavior through a slit.

Comparison of sound and light waves, explaining why sound waves diffract more easily than light waves.

Description of the Huygens-Fresnel principle, which explains how each point of a wavefront acts as a source of secondary wavelets.

Detailed explanation of how the Huygens-Fresnel principle helps understand diffraction, reflection, and refraction.

Introduction to polarization, noting that it only occurs with transverse waves, like light, but not with longitudinal waves like sound.

Demonstration of polarization using a rope, where oscillations in specific directions are allowed through a polarizing filter.

Explanation of how polarized light passes through filters and how blocking orthogonal polarizations prevents light from passing.

Application of polarization in everyday objects, such as sunglasses using polarizing lenses to block glare from surfaces.

Connection of diffraction to practical examples, such as hearing conversations through a door while not seeing the people speaking.

Explanation of diffraction in terms of wave properties like wavelength, relating it to practical phenomena like sound passing through windows.