University:
Kentucky State University
Course:
PHY 212 | General Physics II
Academic year:

2023
Views:

86

Pages:

9
Author:

Madelyn Bradshaw

**Polarisation of Light and Polarimetry** **Abstract:** Typically, electromagnetic waves have vibrating electric and magnetic fields in all directions perpendicular to the direction of propagation. We can restrict the electric field vectors to one direction using a polariser (optical filter). Using polarisation, we can find the intensity of the light (in part one) as well as the effect optically active materials have on its polarisation (in part two). **Introduction** Light is an electromagnetic wave. As light propagates, its changing electric field has vectors in all directions. [Image of an electromagnetic wave with electric field vectors in all directions] Electromagnetic waves (such as light) have changing electric and magnetic fields (vectors) perpendicular to each other and to the direction of propagation. When such waves pass through a polarisation filter, it gets polarised and thus its electric field (and magnetic field) will vibrate in one direction only. We define the direction of polarisation of the EM wave by the direction of the electric field vibration. Some substances are called "dichroic substances". These substances only transmit light (or EM waves) with the electric field vibrating in one direction. In an ideal polariser, all light with E parallel to the transmission axis is passed and all perpendicular is absorbed. There are 3 types of polarisation. The one discussed so far and we will be using is linear polarisation. 1. Linear 2. Circular 3. Elliptical **Part 1)** [Image of a light source, polarizer, and analyzer] We can use any source of light (doesn't have to be monochromatic) with a convex lens (to make it parallel). The unpolarised light will first pass through the polariser. The new polarised light will pass through the analyser. The intensity of light passing through the analyser depends on θ, which is the angle between the transmission axis of the polariser and the analyser. The intensity follows Malus' law: I = I₀ cos²θ When θ = 0, π: I = I₀ (maximum intensity at parallel axes) When θ = π/2: I = 0 (minimum intensity at perpendicular axes) I₀ is the intensity of the polarised light incident on the analyser, not the source. I source is actually equal to 2I₀. This is because when unpolarised light passes through a polariser, its intensity is lowered to half the original value. But how do we measure intensity? We use a convex lens with a photocell that will transform light into current. [Image of a setup with a polarizer, analyzer, convex lens, and photocell] **Part 1)** [Image of a light source, polarizer, tube of optically active material, and analyzer] When polarised light passes through an optically active material, its polarisation angle changes. We can define the "specific rotation" as the rotation caused by a solution of 1 g/ml concentration through a 10 cm path. This is useful in testing the concentration of substances. We use the relationship: θ = (αlc) / 100 Where: θ = angle of rotation α = specific rotation l = length of tube (in cm) c = concentration For this part, we will use a device called "the polarimeter". We can rearrange the previous equation to find α: α = 100θ / lc **Procedures** 1. Set the parts and assemble them as shown previously. Cover the system. 2. Set the angle for the polarizer (transmission axis) at θ = 0° and the analyser at θ = -90°. 3. Take the reading and rotate the analyser to degrees (minus). Take the reading. 4. Repeat step 3 until you reach +90°. Correct your data by subtracting I at θ = 90° from all your data. 5. Draw the first graph of I - θ. 6. Draw the 2nd graph of I - cos²θ, calculate the slope. The slope = I₀