# Concave mirror problem!

premed_love

## Homework Statement

A light bulb is placed 10.7 cm in front of a concave mirror. When the concave mirror is replaced by a plane mirror in the same location, the bulb's image moves 4.00 cm closer to the mirror. Calculate the focal length of the concave mirror.

I am given: do: 10.7
and 4.00 is Di

## Homework Equations

Mirror equation: 1/di+1/do=1/f

## The Attempt at a Solution

I know I have to use the mirror equation so I did. However, I also added 10.7 and 4.00 to get a new di.

1/10.7+1/14.7=1/f
Eventually, I get 6.19 which is wrong! Anyone would like to help me see what is wrong?[/B]

Homework Helper
Gold Member

## Homework Statement

A light bulb is placed 10.7 cm in front of a concave mirror. When the concave mirror is replaced by a plane mirror in the same location, the bulb's image moves 4.00 cm closer to the mirror. Calculate the focal length of the concave mirror.

I am given: do: 10.7
and 4.00 is Di

## Homework Equations

Mirror equation: 1/di+1/do=1/f

## The Attempt at a Solution

I know I have to use the mirror equation so I did. However, I also added 10.7 and 4.00 to get a new di.

1/10.7+1/14.7=1/f
Eventually, I get 6.19 which is wrong! Anyone would like to help me see what is wrong?[/B]
Which side of the mirror is the new image? (It's not made clear.)
What's the sign convention for object and image distances when dealing with mirrors?
(What would your equation produce for the flat mirror?)

premed_love
Which side of the mirror is the new image? (It's not made clear.)
What's the sign convention for object and image distances when dealing with mirrors?
(What would your equation produce for the flat mirror?)

The only information I am given is the one I have provided. I am told it is a concave mirror- that's it.

Homework Helper
Gold Member
The only information I am given is the one I have provided. I am told it is a concave mirror- that's it.
In order to write the equation you used, you have to make an assumption about whether the object and image are on the same side of the mirror or on opposite sides. Which did you assume? Is your equation correct for that assumption?

premed_love
In order to write the equation you used, you have to make an assumption about whether the object and image are on the same side of the mirror or on opposite sides. Which did you assume? Is your equation correct for that assumption?

I was thinking the image should be real and on the same side which is why my Di would not be negative.1/10.7+1/14.7=1/f which is why I get
eventually, I get 6.19. Are you trying to imply that my di is negative and my equation should be 1/10.7-1/14.7=1/f

Homework Helper
Gold Member
I was thinking the image should be real and on the same side which is why my Di would not be negative.1/10.7+1/14.7=1/f which is why I get
eventually, I get 6.19. Are you trying to imply that my di is negative and my equation should be 1/10.7-1/14.7=1/f
I'm saying it's a strong possibility. Note that the question says
When the concave mirror is replaced by a plane mirror in the same location, the bulb's image moves 4.00 cm closer to the mirror
It doesn't say "is 4cm closer" or "moves to 4cm closer and on the opposite side". That suggests to me it moves by 4cm, not by 10.7+14.7=25.4 cm.

premed_love
So you would think it be a simple mirror equation then: 1/10.7+1/4.0=1/f so 2.94

premed_love
but 2.94 cm is off. I am so confused!

Homework Helper
Gold Member
So you would think it be a simple mirror equation then: 1/10.7+1/4.0=1/f so 2.94
No, we're discussing what sign is appropriate to use in your original equation. The distances are correct, but are the image and object the same sign or opposite signs?

premed_love
No, we're discussing what sign is appropriate to use in your original equation. The distances are correct, but are the image and object the same sign or opposite signs?
I think they will be the same size

premed_love
I think they will be the same sign.

Homework Helper
Gold Member
I think they will be the same sign.
Why?

premed_love
Why?
Because it's a concave mirror. The image is also real. Earlier I was thinking they would be different signs as 4.00 cm is moving closer. But then the concave part made me think the sign would be the same.