As level physics

Hey everyone,I need help in June 2013 Unit 2 Q4.

Light from a lamp passes through two polarising filters, P1 and P2, before reaching a
detector. The filters initially have their planes of polarisation parallel.
The intensity of light at the detector will be greatest if
A P1 is rotated by 45° and P2 is rotated by 315° in the same direction.
B P1 is rotated by 90° and P2 is rotated by 270° in the same direction.
C P1 is rotated by 45° and P2 is rotated by 270° in the same direction.
D P1 is rotated by 90° and P2 is rotated by 315° in the same direction.

Why the answer is B?

Thanks in advance for all the help. :smile:

(edited 8 years ago)

Reply 1

iMacJack

What do you know about polarising filters? When are they most effective?

Reply 2

shafia13

Original post by iMacJack

What do you know about polarising filters? When are they most effective?

when both the filters are parallel...but i would like to have your explanation.

Reply 3

iMacJack

Original post by shafia13

when both the filters are parallel...but i would like to have your explanation.

Incorrect. Polaroid filters are at their peak of effectiveness when they are perpendicular to each other. Think about those angles said in answer box B, and why the light may be able to get through both of those polaroid filters with ease?

(edited 8 years ago)

Reply 4

shafia13

Original post by iMacJack

Incorrect. Polaroid filters are at their peak of effectiveness when they are perpendicular to each other. Think about those angles, and why the light may be able to get through both of those polaroid filters with ease?

That`s not true.When Polaroid filters are perpendicular to each other,light cannot pass through them.It`s clearly written in Hodder on Ch-9,sec- Polarised l ight.

Reply 5

iMacJack

Original post by shafia13

That`s not true.When Polaroid filters are perpendicular to each other,light cannot pass through them.It`s clearly written in Hodder on Ch-9,sec- Polarised l ight.

That is true. Hence they are most effective when light cannot pass through them, so they are most effective when perpendicular.

Reply 6

shafia13

Original post by iMacJack

That is true. Hence they are most effective when light cannot pass through them, so they are most effective when perpendicular.

You should read the question once again.It asks,`The intensity of light at the detector will be greatest if...`

Reply 7

iMacJack

Original post by shafia13

You should read the question once again.It asks,`The intensity of light at the detector will be greatest if...`

I am very aware of the question. I asked you what are the conditions needed for the polaroid filters to be at their peak effectiveness, to which you gave me an incorrect answer. I corrected you. How about you draw out the polaroid filters at these said angles, and tell me why all the light will get through?

Reply 8

crashMATHS

Original post by shafia13

You should read the question once again.It asks,`The intensity of light at the detector will be greatest if...`

He has already answered your question.

You asked for an explanation and he is asking you questions to lead you to the correct explanation (i.e. when is a polaroid most effective in not letting light pass). This will help you realise why it is least effective when the filter is parallel to the wavelength of the light.

If it doesn't help, then just consider water moving through a ripple tank. The boundaries are parallel to the water and allow water to move through the tank. Place boundaries at right angles, you will block the water. Light also follows wave properties and can be explained in the same way.

(edited 8 years ago)

Reply 9

shafia13

Original post by iMacJack

Got it!Thanks.

Reply 10

shafia13

Original post by kingaaran

Thanks!

Reply 11

iMacJack

Original post by shafia13

Got it!Thanks.

That's no problem. :smile:

Reply 12

MARYAM1234567

Original post by crashmaths

he has already answered your question.

You asked for an explanation and he is asking you questions to lead you to the correct explanation (i.e. When is a polaroid most effective in not letting light pass). This will help you realise why it is least effective when the filter is parallel to the wavelength of the light.

If it doesn't help, then just consider water moving through a ripple tank. The boundaries are parallel to the water and allow water to move through the tank. Place boundaries at right angles, you will block the water. Light also follows wave properties and can be explained in the same way.