## 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.

## Homework Equations

1/f = 1/ do + 1/ di

## The Attempt at a Solution

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nrqed
Homework Helper
Gold Member

## 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.

## 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

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

I forgot to convert Units.

Ahhh i'm a silly physics 11 student.

Hmm does the distance from the retina to the lense (20 mm ) count as the Do ?

nrqed
Homework Helper
Gold Member

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

I forgot to convert Units.

Ahhh i'm a silly physics 11 student.

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

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

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.

nrqed
Homework Helper
Gold Member

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!