# Image Distance

## Homework Statement

You look at yourself in a shiny Christmas tree ball, which has a diameter of 9.63 cm. If your face is 7.81 cm away from the shiny ball, calculate the image distance.

## Homework Equations

???
Thinking I would use
magnification = di/d0

di = 0.0781m
d0 = 0.0963m

## The Attempt at a Solution

-0.811m

This doesn't sound right to me, is this perhaps because di is supposed to be what I'm trying to find out???

If so, would I then assume that the magnification is 0.0781 and I need to times that by 0.0963 to work out di? The the calculations would be as follows;???

m x do = di

di = 0.0781 x 0.0963
= 7.52 x 10^-3 m (3sf)

I think this would be the correct way to work out this question, I just wanted to double check that my second method is correct???

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Wrichik Basu
Gold Member
I don't think magnification formulae will help you here. You're given the object distance and a ball, which acts like a convex mirror. Have you tried using the mirror formula $$\frac{1}{v} - \frac{1}{u} = \frac{1}{f}$$ with the proper signs?

What answer is supplied with the book?

Drakkith
Staff Emeritus