Malus law - Polarisation properties of light

Malus law describing the polarisation properties of light.

1. MALUS’ LAW Presented By, Kaviya S 23PHY21 I MSc Physics

2. Malus law  is crucial if we want to learn or understand the polarisation properties of light. The law helps us to study the light intensity relation of the polariser-analyser. Malus law is named after Étienne-Louis Malus, who, in the year 1808, discovered that natural incident light could be polarised when it was reflected by a glass surface. He used calcite crystal for his experiment. MALUS’ LAW:

3. When a plane polarised light is seen through an analyser, the intensity of transmitted light varies as the analyser is rotated throught an angle perpendicular to the incident direction. When a beam of plane polarised light of intensity is incidenton an analyser, the intensity of light I transmitted from the analyser varies directly as the square of the cosine of the angle θ between the transmission axes of polariser and analyser . This is known as Malus’ law. I = θ  

4. Malus’ law

5. The proof of Malus’ law is as follows. Let us consider that the transmission axes of the polariser and the analyser are inclined by an angle θ is as shown. Let be the intensity and a be the amplitude of the electric vector transmitted by the polariser. The amplitude a of the incident light has two rectangular components, a cosθ and a sinθ which are the parallel and perpendicular components to the axis of transmission of the analyser . Only the component acosθ will be transmitted by the analyser. The intensity of light transmitted from the analyser is proportional to the square of the component of the amplitude transmitted by the analyser.  

6. I  I = k Where k is constant of proportionality . I = k I = θ Where k = , is the maximum intensity of light transmitted through the analyser.  

7. The following are few special cases. Case (i); when θ = ,cos =1, I= When the transmission axis of polariser is parallel to that of the analyser, the intensity of light transmitted from the analyser is equal to the incident light that falls on it from the polariser . Case (ii); when θ = , cos =0, I=0 When the transmission axes of polariser and analyser are perpendicular to each other , the intensity of light transmitted from the analyser is zero.