# Optics

#### ThreeJ

##### Junior Member
10+ Year Member
hello all you potential M.D's I was wondring if one of you could clear this up for me. In the exam krackers book there is a question on optics involving the thin lens equation (1/f=1/di + 1/do). well the question gives the focal distance of 1cm and the object distance of 4cm and ask to solve for the images distance. The way I worked it out is I brought the 1/do (1/4cm) to the side the 1/f (1/1cm) was and then got a common denominator and subtracted them. This is what I did ion brackets below>

[1/1cm=1/4cm + 1/di] [1/1cm-1/4cm= 1/di] [4/4cm-1/4cm= 1/di]

[3/4cm= 1/di]. this is the answer I got yet the answer at the back of the book says it is 4/3 can anyone explain this as the book does not clearly explain it for me.

thank you

#### OCPreMed

##### Member
10+ Year Member
7+ Year Member
ThreeJ said:
hello all you potential M.D's I was wondring if one of you could clear this up for me. In the exam krackers book there is a question on optics involving the thin lens equation (1/f=1/di + 1/do). well the question gives the focal distance of 1cm and the object distance of 4cm and ask to solve for the images distance. The way I worked it out is I brought the 1/do (1/4cm) to the side the 1/f (1/1cm) was and then got a common denominator and subtracted them. This is what I did ion brackets below>

[1/1cm=1/4cm + 1/di] [1/1cm-1/4cm= 1/di] [4/4cm-1/4cm= 1/di]

[3/4cm= 1/di]. this is the answer I got yet the answer at the back of the book says it is 4/3 can anyone explain this as the book does not clearly explain it for me.

thank you
are you saying the answer says 4/3cm= 1/di or 4/3cm= di???

OP

#### ThreeJ

##### Junior Member
10+ Year Member
OCPreMed said:
are you saying the answer says 4/3cm= 1/di or 4/3cm= di???

Yeah it ask for di and I know my mistake now. Because I have 1/di and they ask for di I take the inverse of 1/di which converts 3/4 to 4/3 or 1.33. I just was not thinking hard enough