Optics - For what distance must we focus our eyes to see this image

[Kathi201]

An object is placed 1m in front of a plane mirror. An observer stands 3m behind the object. For what distance must the observer focus his eyes in order to see the image of the object?

 

[Mentallic]

5 metres? Was that all there is to the question?

 

[Kathi201]

Yeah that is all the question was. How did you figure that out?

 

[tiny-tim]

An object is placed 1m in front of a plane mirror. An observer stands 3m behind the object. For what distance must the observer focus his eyes in order to see the image of the object?

Hi Kathi!

Hint: for a plane mirror, where is the image in relation to the object?

(Hi Mentallic: please don't give out answers! )

 

[Kathi201]

Do you mean that it is in an angle? I am not quite sure of the answer you are looking for

 

[tiny-tim]

Do you mean that it is in an angle? I am not quite sure of the answer you are looking for

Mirrors are like lenses … every object has an image.

For a plane mirror, where is the image of an object?

 

[Kathi201]

The image is located in front of the mirror. Isn't the image distance equal to the object distance. Thus it would be 3m

 

[Kathi201]

Ok so now that I am thinking about, is the image behind the mirror or in front. I am starting to think it is behind?

 

[tiny-tim]

Ok so now that I am thinking about, is the image behind the mirror or in front. I am starting to think it is behind?

Yup, the image is definitely behind the mirror!

(It's a virtual image: the rays of light "from" it don't actually get anywhere near it, do they? )

Sooo … how far away from the observer is the image of that object she's standing behind?​

 

[Kathi201]

The object is 1m away and the person is 3m away so it would be 3m +1m + the 1m behind the mirror so it would be 5m.

 

[tiny-tim]

That's it!

Any questions? ​