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

niharika_kumar
Member
From: Numeraland
Registered: 2013-02-12
Posts: 1,062

### 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
Real Member
From: Harlan's World
Registered: 2011-05-23
Posts: 16,037

### Re: mirror- help me!

Do you have the picture?

Here lies the reader who will never open this book. He is forever dead.
Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

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

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### 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.
Always satisfy the Prime Directive of getting the right answer above all else.

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

Agnishom
Real Member
From: Riemann Sphere
Registered: 2011-01-29
Posts: 24,838
Website

### 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'
I'm not crazy, my mother had me tested.

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

niharika_kumar
Member
From: Numeraland
Registered: 2013-02-12
Posts: 1,062

### 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
Member
From: Numeraland
Registered: 2013-02-12
Posts: 1,062

### 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
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### 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.
Always satisfy the Prime Directive of getting the right answer above all else.

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

Agnishom
Real Member
From: Riemann Sphere
Registered: 2011-01-29
Posts: 24,838
Website

### Re: mirror- help me!

'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'
I'm not crazy, my mother had me tested.

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

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### 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.
Always satisfy the Prime Directive of getting the right answer above all else.

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

ShivamS
Member
Registered: 2011-02-07
Posts: 3,648

### 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
Member
Registered: 2013-03-09
Posts: 905
Website

### Re: mirror- help me!

I got 35 instead but I dont know where I went wrong.

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
Administrator
Registered: 2010-06-20
Posts: 8,137

### 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

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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

niharika_kumar
Member
From: Numeraland
Registered: 2013-02-12
Posts: 1,062

### Re: mirror- help me!

thank you!

friendship is tan 90°.

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

ShivamS
Member
Registered: 2011-02-07
Posts: 3,648

### 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: 1,062

### 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: 24,838
Website

### 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'
I'm not crazy, my mother had me tested.

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

niharika_kumar
Member
From: Numeraland
Registered: 2013-02-12
Posts: 1,062

### 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: 3,648

### 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: 1,062

### Re: mirror- help me!

wow, that's really great.

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

friendship is tan 90°.

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