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**[SOLVED] Short optics question**

**Shiny lawn spheres placed on pedestals are convex mirrors. One such sphere has a diameter of 42 cm. A 13 cm robin sits in a tree 2.1 m from the sphere.**

(a) Where is the image of the robin? in cm

b) How long is the robin's image? in cm

(a) Where is the image of the robin? in cm

b) How long is the robin's image? in cm

**1/f=1/do+1/di**

hi/ho=-di/do

hi/ho=-di/do

**Ok, I know that radius of a concave lens is 2f but when i use that principle i do not get the right answer...what i did is:**

42/2=21=radius

21/2=10.5=focal length

1/10.5=1/210+1/x

x=11.0526 which was my answer for a which webassign says is wrong

once i get this answer part b is very easy as all i will have to do is x/13=-answer to part a/210

plz help me figure out what i did wrong im assuming that radius of a convex lens which is what is used in problem is not 2f but i have no clue what it is then and i tried googling it but could not find anything...thanks

42/2=21=radius

21/2=10.5=focal length

1/10.5=1/210+1/x

x=11.0526 which was my answer for a which webassign says is wrong

once i get this answer part b is very easy as all i will have to do is x/13=-answer to part a/210

plz help me figure out what i did wrong im assuming that radius of a convex lens which is what is used in problem is not 2f but i have no clue what it is then and i tried googling it but could not find anything...thanks