A student studies a beam of light that comes from a lamp and a polarising filter together…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A student studies a beam of light that comes from a lamp and a polarising filter together. The beam is a mixture of two parts. The unpolarised part has an intensity of , and the plane polarised part has an intensity of . She puts an analyser in the beam and turns it slowly through half a circle, and a light meter behind the analyser records the intensity. The light meter has been checked, and it reads the intensity in watt for each square metre. She writes down the readings at many angles, and she wants to know the highest and the lowest values, and how strongly the beam is polarised. She measures the angle of the analyser from the direction of vibration of the polarised part. Find the greatest and the least intensity that she records, and the degree of polarisation of the beam. Find also the intensity at and the angle at which the intensity is .
Need a reminder? The key ideaShow
Unpolarised light that goes through a Polaroid keeps half of its intensity and becomes plane polarised. Plane polarised light of intensity that meets an analyser at an angle gives (Malus' law). Partly polarised light has and .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The unpolarised part. It gives at every angle.
Step 2. The greatest and the least. At the polarised part passes whole: . At it is stopped: .
Step 3. The degree of polarisation. .
Step 4. At . .
Step 5. The angle for . , so . Then .
Answer: ; ; ; ; .
Check. The degree is also the share of the total intensity () that is plane polarised: .
More onOptics
Discussion (0)
Log in to join the discussion.
Loading…