Question

An object of height 8 cm is placed 28 cm in front of a concave mirror of focal length 20cm. At what distance from the mirror should a screen be placed , so that a sharp image can be obtained? Find the size and nature of the image. Sketch a ray diagram?

Answer 1

I found:
Position`=70cm`
Image bigger and upside down.

Explanation:

Consider first the ray diagram:

You can see that the image will be upside down and quite far from the surface of the mirror.

We now use the mirror equation:
`1/p+1/q=1/f` to find the location `q` of the image:
`1/28+1/q=1/20`
`q=70cm` corresponding to the position of the screen.

The characteristics of the image can be found using the `m` formula:
`m=-q/p=(h')/h`
Where:
`h=8cm` height of object;
`h'=` height of image;
`p=28cm>0`
`q=70cm>0`

We get:
`m=-70/28=-2.5` this tells us that the image will be upside down (`-` sign) and bigger (it will be `h'=2.5*8=20cm` upside down)

Answer 2

with a convex mirror...

Explanation:

...you should get: