Section Bank C/P Question 11

[Zebracarns]

The question asks for the focal length of the lens. All parallel lines going through converging lens should pass through focal point, and light from a single source going through converging lens should exit in parallel rays...but the figure shows neither. I went with focal length =4cm as my choice but apparently you should use lensmaker's equation. If you just put the numbers in with 12 cm as object distance and 4cm as image distance you get 3cm focal length, but I don't think it makes sense given their diagram?

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[aldol16]

1/f = 1/o + 1/i = 1/12 + 1/4 = 1/12 + 3/12 = 4/12. Thus, f = 12/4 = 3 cm.

The drawing they have is not a ray diagram.

 

[sfsn]

I'm wondering about this question. At least for MCAT purposes, we can use 1/f = 1/o + 1/i to solve for one of the variables in this type of problem, even though it's not a ray diagram? Basically, I should just plug and chug with that equation?

And OP mentioned the lensmaker's equation (from https://en.wikipedia.org/wiki/Lens_(optics)):

where

is the focal length of the lens,

is the refractive index of the lens material,

is the radius of curvature (with sign, see below) of the lens surface closest to the light source,

is the radius of curvature of the lens surface farthest from the light source, and

is the thickness of the lens (the distance along the lens axis between the two surface vertices).
but how is that relevant to the figure from the problem? We aren't given any of those variables.

 

[itsalwayssunny96]

1/f = 1/o + 1/i = 1/12 + 1/4 = 1/12 + 3/12 = 4/12. Thus, f = 12/4 = 3 cm.

The drawing they have is not a ray diagram.

I know this post was made a while ago, but in the question description it does say "based on the ray diagram." How would you be able to tell that it wasn't a ray diagram?

 

[aldol16]

I know this post was made a while ago, but in the question description it does say "based on the ray diagram." How would you know be able to tell that it wasn't a ray diagram?

It does not follow the conventions of a conventional ray diagram. See: http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/raydiag.html