Brewster’s law

If you have ever been trekking, you must have carried polarised sunglasses with you as well. Thus, when the weather is sunny, you can’t help but notice the crystal clear reflection that appears on a lake. These reflections are polarised. However, when you see it via your sunglasses, you will see these reflections are gone. Well, it happens because your sunglasses are polarised. They permit only polarised light to get through them. Moreover, polarizers are often found in monitors and TV displays as well to prevent glare.

Definition

As you all know, the reflected light is polarised but all of it isn’t. When it is in the plane of a reflection, it creates light polarised at 90 degrees to that plane. That is more likely to reflect. Actually, when you beam your light at any given angle, it produces a big influence on how polarised the reflection is going to be.

As per Brewster’s Law, the greatest polarisation takes place when the angle is 90 degrees in between the reflected light and refracted ray. This rule was named for the great Scottish Physicist, Sir David Brewster. It was proposed in the year 1811, according to historical records. Furthermore, Brewster’s angle may be seen in the polarisation angle as well.

Brewster’s Law

According to Brewster’s Law, we may accomplish the greatest polarisation of light by letting the ray fall on the surface of a transparent material in such a way that the refracted ray becomes perpendicular to the reflected ray. There is a developing relationship between the polarising angle ip and the refractive index in this experiment.

If at a certain angle of incidence, all of the reflected light is polarised, the polarising angle is formed, and this precise value of the angle of incidence is recognised as the polarising angle, the polarising angle is formed. The refractive index μ of the transparent material is critical in determining the polarising angle ip.

We may represent the relationship in the following way:

μ = tan ip

The refractive index of the clear material is denoted by the symbol μ.

The polarising angle of incidence (p is in subscript) ip= Brewster angle.

A transparent media is illuminated by unpolarized light reflected at any polarising angle. The rays that transfer and reflect are parallel to one another when the light is incident on the medium at any polarising angle. 

Sir David Brewster

Physicist Sir David Brewster (11/12/1781 – 10/02/1868) was born in Edinburgh, Scotland, and was most known for his experimental work in optics and polarised light. When light strikes a reflecting surface at a specific angle (referred to as the polarising angle), the reflected light is entirely polarised, whereas the incident light is not. Brewster observed that there is a straightforward mathematical link between the polarising angle and the refractive index of the reflecting material. For materials that are either opaque or only available in tiny quantities, this rule can be used to determine the refractive index of the substance.

Applications

Polarised sunglasses are a good illustration of how Brewster’s law may be applied in the real world. The notion of Brewster’s angle is utilised in the design of these glasses. The polarised glasses help to lessen glare that is reflected directly from the sun as well as from horizontal surfaces such as roadways and bodies of water. Using a polarising filter for the lens, photographers may lessen the reflection from reflective objects by applying the same law to their cameras’ lenses.

While there is no reflection of the p polarisation at the Brewster angle, at even larger angles of incidence, the reflection coefficient of the p polarisation is always smaller than that of the s polarisation, almost up to 90° incidence, where the reflectivity of each climbs towards unity. This is due to the fact that reflected light from horizontal surfaces (such as the road’s surface) is strongly s-polarised when it is received at a distance much greater than one height (so that the incidence angle of specularly reflected light is close to, or usually well beyond, the Brewster angle). A layer of polarising material is used in the construction of polarised sunglasses to filter horizontally polarised light and minimise glare in these types of settings. Most effective on smooth surfaces where specular reflection (light whose angle of incidence is the same as the angle of reflection specified by where it is perceived from) is predominant, but even diffuse reflections from roadways, for example, are greatly decreased.

A polarising filter is frequently used by photographers to reduce reflections from water, allowing them to picture subjects that are below the surface of the water. This filter can be adjusted to reduce reflections from objects other than horizontal surfaces by using a rotating polarising camera attachment. An example of this is shown in the accompanying photograph (right), where the s polarisation (approximately vertical) has been eliminated using a rotational polarising camera attachment.

Entrance windows or prisms with their surfaces at the Brewster angle are often employed in optics and laser physics, to name a few of the fields of application. Because of Brewster’s angle, the polarised laser light passes through the prism without incurring any reflecting losses.

Conclusion