Physics Help Forum Mirror Question

 Light and Optics Light and Optics Physics Help Forum

 Nov 12th 2010, 11:08 AM #1 Junior Member   Join Date: Oct 2010 Posts: 28 Mirror Question Standing 2.5 m in front of a plane mirror with your camera, you decide to take a picture of yourself without turning the camera around. To what distance should the camera be focused to get a sharp image? Is there an equation for mirror questions like this?
 Nov 12th 2010, 02:27 PM #2 Member   Join Date: Jul 2009 Posts: 72 2.5m to the mirror the image is formed behind the mirror 2.5m so 2.5*2
Nov 13th 2010, 12:46 PM   #3
Junior Member

Join Date: Oct 2010
Posts: 28
 Originally Posted by Haytham 2.5m to the mirror the image is formed behind the mirror 2.5m so 2.5*2
Is it really just that simple? Wow, thanks though I should have just thought it through a little more!

Anyways, here's another question but with velocity this time.

"A dog is sitting in front of a plane mirror. At a command it jumps at the mirror with a speed of 0.50 m/s. How fast does the dog approach its image?"

How would you do that one?

Nov 13th 2010, 03:37 PM   #4
Senior Member

Join Date: Apr 2008
Posts: 815
 Originally Posted by jrodd321 "A dog is sitting in front of a plane mirror. At a command it jumps at the mirror with a speed of 0.50 m/s. How fast does the dog approach its image?" How would you do that one?
If the dog moves 1 m toward the mirror, what do you think the image will do?
Can you take it from there?
__________________
Isaac
If the problem is too hard just let the Universe solve it.

Nov 13th 2010, 09:31 PM   #5
Junior Member

Join Date: Oct 2010
Posts: 28
 Originally Posted by arbolis If the dog moves 1 m toward the mirror, what do you think the image will do? Can you take it from there?
It will get larger? I'm really confused in this question. I understood the first one I asked where you are 2.5 away and the image is formed 2.5 meters behind the mirror so it would be 5. But I don't understand how you would know the dogs speed if it just tells you how he starts out and doesn't give his distance away from the mirror initially.

 Nov 14th 2010, 01:39 AM #6 Physics Team     Join Date: Jun 2010 Location: Mauritius Posts: 609 Take it for one second. If the dog made 0.5 m in one second towards the mirror, how far would the image travel during the same time? From this, you should be able to deduce the speed of the image towards the surface of the mirror, and hence the speed of approach with respect to the dog. __________________ Jerry (Got my results!) It is easier to protect your feet with slippers than to cover the earth with carpet. No one can go back and change a bad beginning; but anyone can start now and create a successful ending. If a problem can be solved, no need to worry about it. If it cannot be solved what is the use of worrying?
Nov 14th 2010, 08:00 AM   #7
Junior Member

Join Date: Oct 2010
Posts: 28
 Originally Posted by Unknown008 Take it for one second. If the dog made 0.5 m in one second towards the mirror, how far would the image travel during the same time? From this, you should be able to deduce the speed of the image towards the surface of the mirror, and hence the speed of approach with respect to the dog.
It would travel the same speed. 0.5 m/s right?

 Nov 14th 2010, 08:06 AM #8 Physics Team     Join Date: Jun 2010 Location: Mauritius Posts: 609 Right. Now, the dog approaches the mirror with a speed of 0.5 m/s and the image approaches the mirror with a speed of 0.5 m/s. How fast does the image approach the dog? __________________ Jerry (Got my results!) It is easier to protect your feet with slippers than to cover the earth with carpet. No one can go back and change a bad beginning; but anyone can start now and create a successful ending. If a problem can be solved, no need to worry about it. If it cannot be solved what is the use of worrying?
Nov 14th 2010, 10:37 AM   #9
Junior Member

Join Date: Oct 2010
Posts: 28
 Originally Posted by Unknown008 Right. Now, the dog approaches the mirror with a speed of 0.5 m/s and the image approaches the mirror with a speed of 0.5 m/s. How fast does the image approach the dog?
It approaches the dog at 1 m/s?

 Nov 14th 2010, 10:41 AM #10 Physics Team     Join Date: Jun 2010 Location: Mauritius Posts: 609 Right! Well done __________________ Jerry (Got my results!) It is easier to protect your feet with slippers than to cover the earth with carpet. No one can go back and change a bad beginning; but anyone can start now and create a successful ending. If a problem can be solved, no need to worry about it. If it cannot be solved what is the use of worrying?

 Tags mirror, question