Lens/Focal Length

[Uniqueusername814]

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Hey Guys I came across this question and I just don't understand why the answer is B.

An image of the sun formed by a convex lens of focal length f appears at what distance from the lens.

A. f in front of the lens
B. f behind the lens
C. 2f in front of the lens
D. 2f behind the lens

 

[DrknoSDN]

The sun is assumed to be so far away that light entering the lens is parallel.

Parallel rays all cross each other at the focal point on the opposite side they enter.
Parallel vs Non-Parallel images.

vs

 

[Uniqueusername814]

Thanks!

So is this the case for everything that is far away?

 

[DrknoSDN]

Yes, if it's so far away they wouldn't bother giving you distance you could assume parallel rays.

 

[DeathandTaxes]

Someone should tell me if I'm doing this right, but maybe here's another way to think about it.

The Lensmaker equation tells us: 1/f = 1/o + 1/i,

Given that o (distance of object from lens) is infinity, then i/infinity is about equal to 0. Thus 1/i must be 1/f

So that narrows it down to A or B.

Maybe someone can help clarify how to go from there.

 

[Uniqueusername814]

Someone should tell me if I'm doing this right, but maybe here's another way to think about it.

The Lensmaker equation tells us: 1/f = 1/o + 1/i,

Given that o (distance of object from lens) is infinity, then i/infinity is about equal to 0. Thus 1/i must be 1/f

So that narrows it down to A or B.

Maybe someone can help clarify how to go from there.

I got it down to A, but then I couldn't figure out why B lol but that was the mentality I used 🙂

Yes, if it's so far away they wouldn't bother giving you distance you could assume parallel rays.

Okay and if this was a mirror it would only reflect? Therefore the image would appear on the same side?

 

[DrknoSDN]

No, if you have a Concave mirror with an object inside the focal length the image will be virtual upright and behind the mirror. This topic was extremely frustrating for me at first. Still is but videos helped.

http://mcat-review.org/light-geometrical-optics.php Has some decent bullet point style notes.


This is a link that I saved in my notes, was helpful to me.

==> Skip to 10:20 if you just want TLDR cliff-notes style table to memorize.

Also Khanacademy. =)

 

[Uniqueusername814]

No, if you have a Concave mirror with an object inside the focal length the image will be virtual upright and behind the mirror. This topic was extremely frustrating for me at first. Still is but videos helped.

http://mcat-review.org/light-geometrical-optics.php Has some decent bullet point style notes.


This is a link that I saved in my notes, was helpful to me.

==> Skip to 10:20 if you just want TLDR cliff-notes style table to memorize.

Also Khanacademy. =)

Thank you! Yeah it's definitely really frustrating seeing as my background is not in physics... and I barely remember my physics class, but I'll definitely check out the video!
Thank you again 🙂