26.4 Polarization and the Reflection and Refraction of Light

Summary

This script delves into the concept of light polarization through Brewster's law, which explains how light becomes polarized when reflected at a specific angle off surfaces. It discusses the practical application of this principle in polaroid glasses, which block horizontally polarized light, enhancing vision on rainy days by reducing glare. The instructor uses Snell's law to derive Brewster's angle, which is crucial for achieving total polarization, and provides an example using the indices of refraction for air and water.

Takeaways

• 🌟 Reflection and scattering of light off particles or surfaces can lead to polarization.
• 🔍 Brewster's law is the principle behind polarized light, which is used in polaroid glasses.
• 📐 When the reflected and refracted rays are at a 90-degree angle to each other, the reflected light is polarized horizontally.
• 🕶️ Polaroid glasses with vertically polarized lenses can block horizontally polarized light, reducing glare on reflective surfaces.
• 🌧️ Wearing polarized sunglasses on a rainy day can make the ground appear strange due to the reduction of reflected light.
• 📚 The concept of Brewster's angle is derived from Snell's law and involves the ratio of indices of refraction.
• 📐 Brewster's angle (θB) is the angle of incidence at which the reflected light is completely polarized.
• 🧭 To calculate Brewster's angle, you need the ratio of the indices of refraction of the two media involved (e.g., air and water).
• 📐 The tangent of Brewster's angle is equal to the ratio of the indices of refraction of the second medium to the first.
• 📊 The angle can be found using the inverse tangent function of the refraction ratio.
• 👓 The effect of polarized glasses is noticeable in everyday situations, such as reducing glare from wet surfaces.

Q & A

What is the main topic discussed in the script?

The main topic discussed in the script is Brewster's law, which explains the polarization of light when reflected off surfaces at a specific angle.

How does Brewster's law relate to polaroid glasses?

Brewster's law is the basis for the polarization in polaroid glasses. These glasses use the law to block horizontally polarized light, reducing glare from surfaces like water or glass.

What is the condition for the reflected and refracted rays to cause polarization according to Brewster's law?

According to Brewster's law, the reflected and refracted rays must be 90 degrees apart for the reflected light to be polarized.

What happens when the reflected light is polarized?

When the reflected light is polarized, it vibrates in a single horizontal plane, blocking out other orientations of light and reducing glare.

Why do surfaces appear different when viewed through polarized sunglasses on a rainy day?

On a rainy day, the ground and other wet surfaces reflect a lot of light, which is horizontally polarized. Polarized sunglasses block this light, making the surfaces appear different or less reflective.

What is the significance of the angle between the reflected and refracted rays being 90 degrees?

When the angle between the reflected and refracted rays is 90 degrees, it is known as the Brewster angle, and at this angle, the reflected light is completely polarized.

How can one calculate Brewster's angle?

Brewster's angle can be calculated using the ratio of the indices of refraction of the two media involved. The tangent of Brewster's angle (θ_B) is equal to the ratio of the refractive indices (n2/n1).

What is the effect of wearing vertically polarized sunglasses?

Wearing vertically polarized sunglasses will block all the horizontally polarized light, reducing glare from surfaces and making the environment appear less reflective.

Why might the ground look 'weird' or 'not real' when wearing polarized sunglasses?

The ground may look 'weird' or 'not real' because the polarized sunglasses are filtering out the horizontally polarized light that usually contributes to the surface's reflective appearance.

Is it possible to always be exactly at the Brewster angle in real-life situations?

No, it is not possible to always be exactly at the Brewster angle in real-life situations. The angle of incidence will typically be slightly larger or smaller than the Brewster angle.

What is the relationship between Snell's law and Brewster's law?

Snell's law describes the refraction of light, while Brewster's law is derived from it and specifically addresses the polarization of light at a certain angle of incidence, known as the Brewster angle.

Outlines

00:00

🌟 Introduction to Brewster's Law and Polarization

The instructor begins by revisiting the concept of light polarization through reflection and scattering off particles or surfaces, previously discussed in chapter 24. The focus then shifts to Brewster's law, which is foundational to understanding how polaroid glasses function, causing light to become polarized vertically or horizontally. The law is illustrated with an incident unpolarized light ray and its interaction with a surface, resulting in a reflected and refracted ray that are perpendicular to each other. At this specific angle, the reflected light becomes horizontally polarized. The instructor explains that wearing vertically polarized sunglasses can block this reflected light, which can make the environment look strange, especially on rainy days, due to the reduced glare. The concept is further explored by connecting it to Snell's law, which helps derive Brewster's angle, where the reflected light is fully polarized. An example using the indices of refraction for air and water is given to demonstrate how to calculate Brewster's angle.

Keywords

💡Reflection

Reflection refers to the process by which light bounces off a surface. In the video, reflection is discussed in the context of light interacting with surfaces, which is crucial for understanding how polarized light is produced. The script mentions that when light hits a surface, it can reflect and scatter, leading to polarization.

💡Polarization

Polarization is the phenomenon where light waves oscillate in a specific direction. The video explains that light can become polarized when it reflects off surfaces or particles, which is the basis for the functionality of polaroid glasses. Polarization is key to the video's theme as it explains how certain types of sunglasses reduce glare.

💡Brewster's Law

Brewster's Law is a principle in optics that describes the relationship between the angle of incidence and the polarization of light. The video uses Brewster's Law to explain how light becomes polarized at a specific angle when it reflects off a surface. This law is fundamental to the video's message, as it underpins the technology used in polaroid glasses.

💡Unpolarized Light

Unpolarized light is light whose waves vibrate in multiple directions. The script introduces the concept by describing an incident light ray with electric field directions perpendicular to the direction of propagation, illustrating the state of light before it interacts with a surface and becomes polarized.

💡Incident Ray

The incident ray is the initial light ray that strikes a surface. In the video, the incident ray is depicted in red, showing the direction of the light before it is reflected or refracted. Understanding the incident ray is important for grasping how light interacts with surfaces to produce polarization.

💡Reflected Ray

The reflected ray is the portion of the incident light that bounces back from the surface. The video explains that when the reflected and refracted rays are at a 90-degree angle to each other, the reflected light becomes polarized, which is a central point in the discussion of Brewster's Law.

💡Refracted Ray

The refracted ray is the light that passes through a surface and changes direction due to a change in medium. The video uses the refracted ray to demonstrate the relationship between the angles of incidence and refraction, which is essential for understanding Brewster's angle and polarization.

💡90 Degrees Apart

In the context of the video, '90 degrees apart' refers to the angular relationship between the reflected and refracted rays. When these rays are perpendicular to each other, the reflected light is polarized horizontally, which is a key observation in demonstrating Brewster's Law.

💡Horizontally Polarized Light

Horizontally polarized light is light where the electric field oscillates in a horizontal plane. The video explains that at Brewster's angle, the reflected light becomes horizontally polarized, which is significant for the function of polaroid glasses in blocking glare.

💡Snell's Law

Snell's Law, also known as the law of refraction, relates the angles of incidence and refraction to the indices of refraction of the two media. The video mentions Snell's Law as the basis for deriving Brewster's Law, showing how the polarization of light can be calculated using the indices of refraction.

💡Indices of Refraction

Indices of refraction are ratios that describe how light slows down in different media. In the video, the ratio of the indices of refraction is used to calculate Brewster's angle, demonstrating how the properties of the materials involved affect the polarization of light.

Highlights

Reflection and scattering of light off particles or surfaces can polarize the light.

Brewster's law is the basis behind polaroid glasses and their polarization.

When reflected and refracted rays are 90 degrees apart, the reflected light becomes polarized horizontally.

Vertically polarized sunglasses block horizontally polarized reflected light.

Wearing polarized sunglasses on a rainy day makes the ground look weird due to reduced reflected light.

Brewster's angle is when the reflected and refracted rays are 90 degrees apart, resulting in polarized light.

Snell's law can be used to derive Brewster's law and calculate the angle of incidence for polarization.

Brewster's angle (θB) can be calculated using the ratio of the indices of refraction.

For air and water, the tangent of Brewster's angle is the ratio of their refractive indices (1.33/1).

The inverse tangent of the refractive index ratio gives Brewster's angle for polarized light.

Polarized light has practical applications in sunglasses to reduce glare and improve visual comfort.

Understanding Brewster's law helps explain the visual effects observed with polarized glasses.

Brewster's law demonstrates how the angle of incidence affects light polarization.

The derivation of Brewster's law from Snell's law provides a deeper understanding of light reflection and refraction.

Polarization is a key concept in optics, with implications for vision and technology.

Brewster's law is an example of how mathematical principles can explain natural phenomena.

The relationship between light polarization and the angle of incidence has practical implications for optical devices.

Transcripts

INSTRUCTOR: We've talked already about the fact

that reflection and scattering of light off

of particles or off of surfaces can polarize the light.

We talked about that in chapter 24, I believe it was.

Here, we're going to put some meat on those bones,

and talk about Brewster's law.

And this is the basis behind polaroid glasses

and why they're polarized up and down.

Brewster's law says that if the--

so here's an incident light ray that's unpolarized.

So this is the ray, shown here in red,

and these red arrows show the directions

of the electric field, which are all

perpendicular to the direction of propagation

like we talked about in chapter 24.

So that's the incident ray.

This is the reflected ray up here, OK?

And then this is the refracted ray.

So there we go here.

If the reflected ray and the refracted ray

are 90 degrees apart, 90 degrees from each other,

so this angle is 90, then the reflected light

will be polarized horizontally.

So the only part of the light that actually makes it out

of that reflection is polarized horizontally.

So if you've got a light source over there.

It hits a surface, it reflects up into your eye,

and if the refracted ray and the reflected ray

are 90 degrees apart, then the reflected light

will be horizontally polarized.

So if you have vertically polarized sunglasses,

they'll block all of that reflected light.

And I don't know if you've ever noticed,

but when you have sunglasses that are polarized on

and you're on a rainy day, the ground looks weird.

It just doesn't look real.

It doesn't look normal.

And it's because of this effect where the polarized glasses cut

out most of the light that comes off that surface.

Now you'll never be exactly at the Brewster angle.

You'd be a little bit bigger, a little bit smaller,

but where this comes from.

If you start off with Snell's law, you can easily derive,

and the derivation is done in the book,

I won't do it for you here, that Snell's law shows

that reflected light is polarized horizontally

at an angle of incidence given by this so-called Brewster's

angle.

It's an angle of incidence.

So I've denoted here theta B, Brewster's angle,

in this diagram, and all you need

is the ratio of the two indices of refraction

in order to calculate it.

And so at that angle, if you were to put in, for example--

let's just do a quick example, if n1 is air and n2 is water,

then we're going to plug those numbers in.

And tangent of theta B is 1.33 over one,

and then we can find that angle by taking the inverse tangent.

I'm not sure what it will be.

Whatever that angle turns out to would be Brewster's angle,

and you get totally polarized light.