# Homework Help: Compund microscope problem

1. Feb 9, 2009

### thundercats

1. The problem statement, all variables and given/known data
The barrel of a compound microscope is 14.8 cm in length. The specimen will be mounted 1.03 cm from the objective, and the eyepiece has a 4.92 cm focal length. Determine the focal length of the objective lens

2. Relevant equations
(1/Do)+(1/Di)= (1/f)

3. The attempt at a solution
what did was i added 1.03 with 13.8 to get the distance of object from the eye piece. then i plugged the numbers in the equation ( with the focal point of the eye piece) to get the distance of the image from the eyepiece. then i used the image distance i got and subtracted it from the 14.8 to get the image distance from the objective lense to the image and i plugged in to solve for f. but my answer comes down to be .99 cm which is wrong

2. Feb 9, 2009

### tiny-tim

Hi thundercats!

I can't really tell what you did without seeing your equations …

but you don't seem to have used the position of the intermediate image.

3. Feb 9, 2009

### thundercats

i think i see what you are saying but now im more confused. this is how i have started to set it up again. what i dont seem to understand is how to use the 14.8 which is the lenght of the barrel. im not sure if im suppose to use it to find Df (distance of final image)or something
(1/1.03)+(1/Di1)=(1/F1)
(1/Di1)+(1/Df)=(1/4.92)

4. Feb 9, 2009

### tiny-tim

14.8 is the sum of two of the distances.

5. Feb 9, 2009

### thundercats

hmm. by saying its the sum of the two distances im assuming ur saying that its the distance between the two lenses of the compund microscope so i guess i can plug that into my 2nd equation making it look like this
(1/Di1)+(1/14.8-Di1)=(1/4.92)
am i close :D. by the way how do u know to reply back to my posts i was just wondering :D

6. Feb 9, 2009

### thundercats

there is still something wrong with it i cant figure out what i was thinking maybe the sign has to be negetive since the final image is going to be to the left of the second lens meaning
(1/Di1)-(1/14.8-Di)=(1/4.92)

7. Feb 10, 2009

### tiny-tim

Just got up … :zzz:
That's right.

It should work … show us what you got.