Physics question

FudgeBrace

Hi, could someone please show me how to do this question please.?
Q : where must an object be placed so that an image one third the height of the object is formed in a convex mirror of focal length 20cm?

mulciber

Step 1. Start with the formula -1/f= 1/u - 1/v (Convex mirror formula)

Step 2. Magnification= v/u so therefore 1/3=v/u
Therefore: v=1/3u

Step 3. Sub in 1/3u for v in your equation: -1/f= 1/u -1/(1/3)u or -1/f= 1/u -3/u

Step 4. Sub in 20 for f, solve for u and you should get an answer of 40cm.

I hope this helps.

therambler

magnification=v/u

m=1/3

v/u=1/3

u=3v

REAL IMAGE

1/u + 1/v = 1/f ==> 1/3v + 1/v =1/f

4/3v=1/f

4/3v=1/20

3v=80

3v=u

80=u

VIRTUAL IMAGE

1/u-1/v=1/f

1/3v -1/v = 1/f

2/3v = 1/20

3v=40
u=40

For the required real image the object must be placed 80cm from the lens and for the virtual image, the object must be 40cm from the lens.

FudgeBrace

thanks guys that was great help!!