# Homework Help: Convex mirror question how long the image is?

1. May 11, 2014

### batcave1985

Shiny lawn spheres placed on pedestals are convex mirrors.
One such sphere has a diameter of 40 cm. A
12 cm long robin sits in a tree 1.5 m from the sphere.
(a) Where is the image of the robin?
(b) How long is the image?

1/v=1/f-1/u to find image distance (the mirror equation)
M=v/u magnification and length (maginification equation)

my answer just not to sure if its right.
a)
f=r/2
=20/2
f=10cm
u=150cm

1/v=-1/1-1/50
v= -9.4cm

b) m=v/u
= -9.4/150
= 0.06cm

not to sure if that was the way to go.

2. May 11, 2014

### ehild

I do not understand the line with ?????

For question b, you gave the magnification, (with wrong sign) instead of the size.

ehild

3. May 11, 2014

### batcave1985

its suppose to be
1/v=1/f-1/u (trying to find image distance)
1/v=1/-10 -1/50
v= -9.4cm

then with part b i think i use the magnification and multiply it with the height of object to get image size

0.06 x 12cm= 0.75cm

4. May 11, 2014

### ehild

Where does 50 come from? The object distance is 150 cm.

that is correct.

5. May 11, 2014

### batcave1985

Yes sorry another typo it is 150 i did the calculation with 150 but typed in 50.