# Solve Concave Mirror Problems: Find Image Distance & Focal Length

• matt72lsu
In summary, the conversation is about a concave mirror that produces a virtual image three times larger than the original object. The equations used are hi/ho and di/do, and the attempt at a solution involved calculating the image distance, which was found to be incorrect. The person asking for help is unsure of what to do and is also unsure of what to do if the image were real instead of virtual. The other person provides a hint to consider the location of the object relative to the mirror's focal length.

## 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?

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

Hello matt72lsu,

matt72lsu said:

## 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?).

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

matt72lsu said:
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.

## 1. What is a concave mirror?

A concave mirror is a reflective surface that curves inward, creating a hollow shape. It is also known as a converging mirror because it causes parallel rays of light to converge at a focal point.

## 2. How do I find the image distance of an object reflected by a concave mirror?

To find the image distance, you can use the formula 1/f = 1/di + 1/do, where f is the focal length of the mirror, di is the image distance, and do is the object distance. Alternatively, you can use the mirror equation, 1/f = 1/do + 1/di, which is derived from the same principles.

## 3. Can I use the same method to find the focal length of a concave mirror?

Yes, the same formulas can be used to find the focal length of a concave mirror. To find the focal length, you can use the formula f = (do * di) / (do + di), or the mirror equation 1/f = 1/do + 1/di.

## 4. How do I know if the image formed by a concave mirror is real or virtual?

If the image distance is positive, the image formed is real, meaning it can be projected onto a screen. If the image distance is negative, the image formed is virtual, meaning it cannot be projected onto a screen.

## 5. What is the difference between a concave and convex mirror?

A concave mirror curves inward and causes parallel rays of light to converge, while a convex mirror curves outward and causes parallel rays of light to diverge. Concave mirrors are used for focusing light and creating real images, while convex mirrors are used for reflecting light and creating virtual images.

• Introductory Physics Homework Help
Replies
5
Views
386
• Introductory Physics Homework Help
Replies
7
Views
777
• Introductory Physics Homework Help
Replies
13
Views
2K
• Introductory Physics Homework Help
Replies
4
Views
2K
• Introductory Physics Homework Help
Replies
3
Views
4K
• Introductory Physics Homework Help
Replies
1
Views
2K
• Introductory Physics Homework Help
Replies
1
Views
1K
• Introductory Physics Homework Help
Replies
2
Views
1K
• Introductory Physics Homework Help
Replies
1
Views
977
• Introductory Physics Homework Help
Replies
13
Views
2K