# The Naked Scientists Forum

### Author Topic: Why is my reflection on my screen duplicated?  (Read 3151 times)

#### Susan Benfold

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##### Why is my reflection on my screen duplicated?
« on: 30/10/2008 09:08:20 »
Susan Benfold asked the Naked Scientists:

Hi Guys

Love the show.

I have a question that has been bugging me for a while. I understand that you cannot focus on a distant object and a close object at the same time. BUT when I watch my video ipod, I can see myself in the screen. I can either focus on the movie in the screen and my reflection is double or I can focus on my reflection and the ipod is double.  Why is this?  As the reflection and the movie are from the same distance.  What am I missing here?

Thanks
Susan Benfold

What do you think?

#### daveshorts

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##### Why is my reflection on my screen duplicated?
« Reply #1 on: 30/10/2008 12:00:58 »
This is because you have two eyes in slightly different positions. This means that you can judge depth by looking at the difference in direction that your two eyes have to point in order to see just one object. Your eyes move so that you only see one version of the object you are focussing on. but you will see two of everything else. if you focus on you finger close to your face, you will see two of everything else.

the image on your Ipod is slightly deeper than the surface which is creating the reflection of you, so one or other will appear slightly double if you focus on the other.

#### Soul Surfer

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##### Why is my reflection on my screen duplicated?
« Reply #2 on: 30/10/2008 12:41:19 »
The full truth is that the images are not from the same distance.  It is true that the video image and the reflection from the screen are close together but your image on the flat screen is an equal distace behind the screen as you are in front of it so assuming that you are looking at the screen from a typical reading distance of 20 - 30 cm  the image of you is in effect 40 - 60cm away  while the video image is in the ipod.

You can easily check this out by using a pen or finger and a small mirror. Hold the mirror up at your normal reading distance and get an image of the tip of your nose in the mirror.  If you shut one eye and then the other you will easily see that the image of your nose moves on the surface of mirror.  now hold up your finger behind the mirror just by the image of your nose with both eyes open you will see the ipod effect. Now move your finger slowly away from the mirror to arms length and switch between your eyes alternately you will see that eventually you will be able to get the image of your nose in the mirror and your fingertip to move together.  That is the true position of the image from the mirror.

« Last Edit: 30/10/2008 12:45:44 by Soul Surfer »

#### daveshorts

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##### Why is my reflection on my screen duplicated?
« Reply #3 on: 30/10/2008 13:10:05 »
Good point sorry I was being stupid... of course the reflection appears twice as far away as the mirror... duh

#### chris

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##### Why is my reflection on my screen duplicated?
« Reply #4 on: 31/10/2008 23:27:01 »
Maybe you could explain that Dave or Soulsurfer, because not everyone may see why the mirror image is effectively twice as far away...

Chris

#### yor_on

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##### Why is my reflection on my screen duplicated?
« Reply #5 on: 07/11/2008 23:28:12 »
Very nice explanation soulsurfer:)

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##### Why is my reflection on my screen duplicated?
« Reply #5 on: 07/11/2008 23:28:12 »