## Lens and mirror: the eye example

lotus0618
Posts: 182
Joined: Sun Dec 29, 2019 5:47 pm

### Lens and mirror: the eye example

I understand this is a multi-lens. But how do you know that the value for object and the lens accounts for the TOTAL frequency? shouldn't the distance from her lens (glasses) to her retina 2cm be the frequency of just her lens?
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the human eye problem.png
NS_Tutor_Mathias
Posts: 616
Joined: Sat Mar 30, 2019 8:39 pm

### Re: Lens and mirror: the eye example

Let's be careful here - we are talking about focal length, not a frequency. They may use a similar symbol, but the two concepts are not related.

Second, you know that there is an in-focus image at the back of the retina. The only way for this to be true is that 1/f=1/o+1/i for the eye and glasses as a whole system. That is why we can assume that there is some total focal length for this system of two lenses - it would be counter-productive for one lens or the other to by itself create an in-focus image on the retina (if this person's eye could do that, they wouldn't need glasses in the first place).

In this problem, you are also allowed to assume that the lens of the glasses and lens of the human are on top of each other since the problem doesn't seem to specify any distance between the two. That also means that image distance is the distance from our 2-lens-blob to the retina, and object distance is distance from our 2-lens-blob to the object. So at that point, you are really treating them as two lenses merged almost into one - determining their combined power (1/f, make sure this is in meters^-1) and from that deducing what the power of the human lens must be.
lotus0618
Posts: 182
Joined: Sun Dec 29, 2019 5:47 pm

### Re: Lens and mirror: the eye example

That makes sense. I appreciate your help. And yeah frequency was a typo on my part