What is the size of the image of a tree on a man's retina?

[anotherperson]

Homework Statement

A man looks at a tree of height 20.0m a distance of 300m away. the cornea has a radius of curvature of 7.7mm, eye has refractive index of 1.47 and the length of the eye is 24.0mm.

Calculate the size of the image of the tree on the man's retina.

Homework Equations

n1/p+n2/q=n2-n2/R

The Attempt at a Solution

i tired the following but i don't think it is correct
n1=1.47
n2=1.0
R=7.7
p=20

1.47/20+1.0/q=1.47-1.0/7.7
=0.06

 

[yoshtov]

Hint: How is the height of the image related to the distance to the image? See this thread: https://www.physicsforums.com/showthread.php?t=254188

I think this will make everything much clearer. :)

 

[anotherperson]

but i don't know the focal length...

 

[yoshtov]

You don't need it at all. I strongly suspect that at least 2 pieces of given information are extraneous, maybe more. Try this equation, relating magnification, M, to distances and height to the image and object.

M=-\frac{d_{i}}{d_{o}}=\frac{h_{i}}{h_{o}}

 

[anotherperson]

so if i use these values (changing the units of course) is this correct
di=7.7mm
do=300m
hi=24.0mm
ho=20.0m
and then using the magnification i get i can work out the size of the tree?

 

[yoshtov]

Well, since the image is formed on the back of the eye, di=24.0mm, the length of the eye. Remember that you are trying to solve for the height of the image, hi. This is your one unknown. Your do and ho look fine.