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Mirror equation help please

  • Thread starter radiofruit
  • Start date
  • #1
Mirror equation help!! please urgent!!

Homework Statement



The distance from the lens of someone's eye to the retina is 20.0 mm. if the image of a book held 40.0 cm is in front of the eye in sharp focus, what is the effective focal length of the lense.
the back of the book says the answer is 19 mm.

thanks for your help everyone!

Homework Equations


1/f = 1/ do + 1/ di



The Attempt at a Solution

 

Answers and Replies

  • #2
nrqed
Science Advisor
Homework Helper
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Homework Statement



The distance from the lens of someone's eye to the retina is 20.0 mm. if the image of a book held 40.0 cm is in front of the eye in sharp focus, what is the effective focal length of the lense.
the back of the book says the answer is 19 mm.

thanks for your help everyone!

Homework Equations


1/f = 1/ do + 1/ di



The Attempt at a Solution

Just plug in the values.

Don't forget that 40 cm = 400 mm
 
  • #3


Gahh! that's why it didn't work.

I forgot to convert Units.

Ahhh i'm a silly physics 11 student.

Thanks for your help!

Hmm does the distance from the retina to the lense (20 mm ) count as the Do ?
 
  • #4
nrqed
Science Advisor
Homework Helper
Gold Member
3,568
189


Gahh! that's why it didn't work.

I forgot to convert Units.

Ahhh i'm a silly physics 11 student.

Thanks for your help!

Hmm does the distance from the retina to the lense (20 mm ) count as the Do ?
You are welcome:smile:

No, 20 mm is Di, the distance to the image (which is on the retina). 400 mm is the object distance
 
  • #5


Thanks!

So glad I join this message board.

My physics 11 teacher is horrendous, our class average is in the 50's.

I'm managing a 'B', but it's been difficult.
 
  • #6
nrqed
Science Advisor
Homework Helper
Gold Member
3,568
189


Thanks!

So glad I join this message board.

My physics 11 teacher is horrendous, our class average is in the 50's.

I'm managing a 'B', but it's been difficult.
You are welcome. Sorry to hear about your situation! I wish you the best luck!
 

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