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#1 2013-11-15 23:54:17

niharika_kumar
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From: Numeraland
Registered: 2013-02-12
Posts: 1,059

mirror- help me!

pls help
Q. Two plane mirrors A and B are aligned parallel  to each other as shown in fig.
A light is incident at an angle 30° at a point just inside one end of A .The plane of incidence coincides with the plane of figure.Find the maximum number of times the ray undergoes reflections before it emerges out.


friendship is tan 90°.

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#2 2013-11-15 23:59:54

anonimnystefy
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From: Harlan's World
Registered: 2011-05-23
Posts: 15,937

Re: mirror- help me!

Do you have the picture?


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#3 2013-11-16 00:00:38

bobbym
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From: Bumpkinland
Registered: 2009-04-12
Posts: 97,245

Re: mirror- help me!

I thought you had figured out how to post a picture?


In mathematics, you don't understand things. You just get used to them.

If it ain't broke, fix it until it is.

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#4 2013-11-16 00:34:57

Agnishom
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From: Riemann Sphere
Registered: 2011-01-29
Posts: 21,988
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Re: mirror- help me!

Alternately you can upload the picture in a familiar site and post the image hotlink here


'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'
'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'
'But our love is like the wind. I can't see it but I can feel it.' -A Walk to remember

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#5 2013-11-16 04:28:14

niharika_kumar
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From: Numeraland
Registered: 2013-02-12
Posts: 1,059

Re: mirror- help me!

pls copy paste the following link for the fig.:-

http://www.mathisfunforum.com/viewtopic.php?pid=290049#p290049

post #77


friendship is tan 90°.

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#6 2013-11-16 04:29:25

niharika_kumar
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From: Numeraland
Registered: 2013-02-12
Posts: 1,059

Re: mirror- help me!

i am still not able to find the upload option in this section.


friendship is tan 90°.

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#7 2013-11-16 04:30:59

bobbym
Administrator
From: Bumpkinland
Registered: 2009-04-12
Posts: 97,245

Re: mirror- help me!

It is in post reply on the bottom, I can make a picture of it if you need it.


In mathematics, you don't understand things. You just get used to them.

If it ain't broke, fix it until it is.

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#8 2013-11-16 04:36:14

Agnishom
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From: Riemann Sphere
Registered: 2011-01-29
Posts: 21,988
Website

Re: mirror- help me!

showimage.php?pid=290049&filename=mirror.gif


'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'
'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'
'But our love is like the wind. I can't see it but I can feel it.' -A Walk to remember

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#9 2013-11-16 06:13:36

bobbym
Administrator
From: Bumpkinland
Registered: 2009-04-12
Posts: 97,245

Re: mirror- help me!

Hi;

The formula says 30.


In mathematics, you don't understand things. You just get used to them.

If it ain't broke, fix it until it is.

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#10 2013-11-16 06:26:36

ShivamS
Member
Registered: 2011-02-07
Posts: 3,646

Re: mirror- help me!

What a coincidence! This is the same question I encountered while taking the IIT JEE some years back. The answer is 30.

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#11 2013-11-16 08:44:50

Nehushtan
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Registered: 2013-03-09
Posts: 631
Website

Re: mirror- help me!

I got 35 instead but I don’t know where I went wrong. sad

EDIT: I got it now. I carelessly used sin instead of tan.

Last edited by Nehushtan (2013-11-16 17:39:26)


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#12 2013-11-16 11:44:06

bob bundy
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Registered: 2010-06-20
Posts: 7,191

Re: mirror- help me!

hi niharika_kumar

When the ray hits mirror B for the first time it has travelled in the right/left direction

This can happen until it has travelled 2root 3 in that direction.  Say it happens n times then

But the ray has also hit A at the start so add one, but 'just' misses B at the end so take one off.

Hence the maximum number of reflections is 30.

Bob

View Image: ray.gif

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#13 2013-11-18 16:29:24

niharika_kumar
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From: Numeraland
Registered: 2013-02-12
Posts: 1,059

Re: mirror- help me!

thank you! smile


friendship is tan 90°.

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#14 2013-11-19 09:13:56

ShivamS
Member
Registered: 2011-02-07
Posts: 3,646

Re: mirror- help me!

Niharika, you are preparing for the JEE?

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#15 2013-11-20 04:59:23

niharika_kumar
Member
From: Numeraland
Registered: 2013-02-12
Posts: 1,059

Re: mirror- help me!

JEE is my goal and its preparations are going in parallel,  but now I am in class10 so my focus is more on boards.
Did you give JEE,if yes then how was it?


friendship is tan 90°.

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#16 2013-11-20 05:16:54

Agnishom
Real Member
From: Riemann Sphere
Registered: 2011-01-29
Posts: 21,988
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Re: mirror- help me!

What is JEE? Joint Entrance Examinations?


'And fun? If maths is fun, then getting a tooth extraction is fun. A viral infection is fun. Rabies shots are fun.'
'God exists because Mathematics is consistent, and the devil exists because we cannot prove it'
'But our love is like the wind. I can't see it but I can feel it.' -A Walk to remember

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#17 2013-11-20 16:33:19

niharika_kumar
Member
From: Numeraland
Registered: 2013-02-12
Posts: 1,059

Re: mirror- help me!

yes (as far as I know).


friendship is tan 90°.

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#18 2014-03-09 04:08:26

ShivamS
Member
Registered: 2011-02-07
Posts: 3,646

Re: mirror- help me!

niharika_kumar wrote:

JEE is my goal and its preparations are going in parallel,  but now I am in class10 so my focus is more on boards.
Did you give JEE,if yes then how was it?

Just saw this post right now...
It wasn't extremely hard. I managed to crack the main and the advanced (AIR 52) but decided to go to MIT instead. For your preparation, try plancess.com (it gives tutorials by IIT JEE crackers AIR <100). Use NCERT books for all the subjects, but for physics go for HC Verma.
Good luck.

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#19 2014-03-09 20:06:56

niharika_kumar
Member
From: Numeraland
Registered: 2013-02-12
Posts: 1,059

Re: mirror- help me!

wow, that's really great.

thanks for your suggestion.will try that after my exams are over. smile


friendship is tan 90°.

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