Solve Concave Mirror Problems: Find Image Distance & Focal Length

[matt72lsu]

Homework Statement

1)A concave mirror produces a virtual image that is three times as tall as the object.
a) If the object is 21 {\rm cm} in front of the mirror, what is the image distance?
b) What is the focal length of this mirror?

Homework Equations

hi/ho
di/do
1/f= 1/do + 1/di

The Attempt at a Solution

hi/ho = 3 thus do/di = 3 a) di = -do /3 = -7.0cm (was wrong)
thus i could not do the second part. Where did I go wrong? And if it was a real image instead of a virtual image, what what you do different? Thanks

 

[collinsmark]

Hello matt72lsu,

Homework Statement

1)A concave mirror produces a virtual image that is three times as tall as the object.
a) If the object is 21 {\rm cm} in front of the mirror, what is the image distance?
b) What is the focal length of this mirror?

Homework Equations

hi/ho
di/do
1/f= 1/do + 1/di

hi/ho and do/do are not really equations. If you put these in full equation form, ensuring that they apply to virtual images in concave mirrors, it might help.

The Attempt at a Solution

hi/ho = 3 thus do/di = 3 a) di = -do /3 = -7.0cm (was wrong)

I believe you've got your do and di switched around.

thus i could not do the second part. Where did I go wrong? And if it was a real image instead of a virtual image, what what you do different? Thanks

For one thing, knowing whether the image is real or virtual gives you information about where the object is relative to the mirror's focal length (is the object between the focal length and the mirror or is it on the far side of the focal length?).

 

[matt72lsu]

i'm sorry, i just have no clue what to do. the whole 3 times larger part is throwing me off

 

[collinsmark]

i'm sorry, i just have no clue what to do. the whole 3 times larger part is throwing me off

If the image is 3 times as large, than it's 3 times as far away.

Start by going back to your original equations.

Earlier you calculated

hi/ho = 3 thus do/di = 3

But that should be di/do = 3.

 

[matt72lsu]

i got this one already