Which Lens Equation Should I Use for This Optics Problem?

[Groundd]

My teacher assigned us 2 problems to do that were the exact same. One he wanted us to use the thin lenses equation, and the other he wanted us to use think lens equation. i need help because I don't know which one I am doing wrong and my answer is not lining up as it should, since they are the same problem, it should be the same solution. I think I am messing up on the thin lense beucase he worked out part of it out when he wanted us to use think lense

Homework Statement

Find the effective focal length of the two lens system. The focal length of the first lens is 20 cm and the second lens is -20 cm with the separation between the two lens being 10 cm. Locate the principal planes. Locate the image of an object that is 1 m in front of the first lens. Express the location relative to the location of the second lens.

f₁is 20cm
f₂is -20
and the distance between the two is 10 cm
the object is 1m(100 cm) in front of the first lens

Homework Equations

for thick lense the equation is :

(1/s_o) + (1/s_i) = 1/f

and f of both lens is found by ff = ((1/f₁) + (1/f₂) - (d/f₁f₂))^-1

for the thin lens it is

s_i2 = ((1/f₂) - (1/s_o2))^-1

The Attempt at a Solution

For the thick lens I found
h₁ = -(f₁f₂)d/f₂ = -20
h₂ = -(f₁f₂)d/f₁ = -20

s_o = d - (h₁ = 80
and that ff = 40 from the above equation
so
i had (1/80) + (1/s_i) = 1/40
solved for s_i = 80

However for thin lens I found

S_o1 = 100cm
f₁ = 20 cm
S_i1 = ((1/f₁) - (1/s_o1))^-1 = 25

then s_o2 = d - s_i1 = 10 cm - 25 cm = -15 cm

finally s_i2 = ((1/f₂)-(1/s_o2))^-1 = 60

obviously 60 does not equally 80 and I have no idea what I did wrong. I think that what I got for s_o for the thick lens is wrong because when i switch it to 120 i get 60 . However my professor said that s_o was 80.

 

[dlgoff]

Wouldn't you expect the distances to be different? For thick lenses, the thickness of the lens can't be negligible as in the thin lens case.
Welcome to PF, btw.

 

[Groundd]

Thanks. I found PF awhile ago but I have just been browsing for the most part until now.

I would expect the distances to be different beucase of that reason, however my professor said they should be the same.

so I don't know if I am doing it wrong, or he told us wrong.

 

[Groundd]

shameless bump

 

[BerryBoy]

Hey there, your principle plane (h_2), is incorrect, it should yield a positive value. Your s_i, you should get your s_i = 80cm is correct, but this is the distance from the principle plane. So you must subtract the +20cm to get the distance from the second lens.

Both the thick lens and thin lens formulas give you the same answer. Sanity check here, if the experimental setup remains the same. Do you expect using different calculations (with appropriate approximations), to give a different answer? I hope not.Sam

 

[Groundd]

Thanks. I finally got it and forgot to post on here that I figured out what was wrong

This thread can be locked beucase it was solved

 

[Redbelly98]

Groundd, isn't there an option for you to mark your thread "Solved"?

They don't lock up homework threads just because they are solved, locking threads is for other problems that don't apply here.