Optics Clarification

[tf2medic]

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just to be clear...i really like EK's method of memorizing the mirrors+lens rules from that yummy cracker guy.

i just want some clarification to make sure i'm reading it right:
1) focal length and image distance will always have the same sign. i think cracker's #1 rule proves this.

2) the +/- flipping for lenses is only for the focal and image.
so the object distance when in front is always POSITIVE for both mirrors and lens. for lens, the + side is flipped and is behind the lens for when dealing with the image and focal point distances, correct? i'm pretty sure the cracker guy's #2 rule proves this.

am i correct?

thanks

 

[tncekm]

1) I'm not positive, but I don't think that applies to convex mirrors and converging lenses because both are capable of producing real and virtual images depending on where the object is with respect to the focal point; But it would seem that for convex mirrors and diverging lenses this would always be the case since they have pseudo focal points and cannot produce real images as single mirror/lens systems.

2) Just remember that for lenses the side with the object is the "virtual side" because a lens cannot produce a "real image" on that side. Light passes through the lens, so there will be no light rays focusing on the side of the object. Remember, unless something can focus light rays back into a point it cannot produce a real image.

3) This is hard to explain without a ray diagram. It just so happens that with concave mirrors and converging lenses that when an object is between the focal point and the mirror/lens, the mirror/lens is no longer capable of forming light rays back into a point, and thus cannot create a real image.

 

[tf2medic]

yep, i got it.

i actually erased #3 right after you saw it i guess b/c i realized what EK was saying.

#1 is always true. it says specifically in EK, just checked. same with #2.
you seem to have it figured out your own way so you're probably fine but i'll share with you the EK's method anyway. EK's method is a lot more simple, though understanding it your way actually makes you use your brain, which is always good haha. on MCAT tho the EK way may save you some time.

their method is:
1) I (eye) am positive real is inverted. (this refers to focal lengths and image distances, not object distance) ("eye" refers to which side of the lens/mirror your eye normally sees from. for mirrors, you obviously see from the front of the mirror so + is in front. for lens, think of a camera which uses a lens. your eye looks from behind the camera lens, so behind lens is positive. this is why you flip the sign for the lenses)
2) Objects in front of lens/mirrors are always positive and objects behind are always negative.
3) As long as object is in front of lens/mirrors: concave mirrors and converging lens are positive, real, inverted EXCEPT if the object is within the focal distance. if the object lies within the focal distance, it is the opposite (negative, virtual, upright.) convex mirrors and diverging lens are always negative, virtual, upright.

make sense? btw that strategy is copyright of Examkrackers no one sue me!! haha

BTW, your #3 IS the exception that EK mentions. niiccee.
another interesting exception, which is pretty intuitive, is if your object is AT the focal point, then there is no image period because all the rays are parallel and never converge to a point to make an image. i know this is true for converging lens because this was in an EK problem. i assume it's true for all lens and mirrors but i haven't really thought about it for the other cases. whatever

 

[tncekm]

Check out this page: http://physics.bu.edu/~duffy/semester2/c28_lenses.html

The pseudo (negative) focal points are the ones that "literally" cannot exist because no light can focus there. Similarly, the virtual (negative) images are the ones that cannot exist because they exist where light rays cannot be focused.

 

[tncekm]

They're strategy makes me flip my current game around, but I think its a good way to think about it. I always just analyze which side is virtual/real based off of where the light rays are going, and can go, with respect to the object.

 

[tf2medic]

yeah that works. i did it that way back in physics when we learned it but i usually forgot the differences of the ray diagrams between kinds of lens and mirrors. BTW, EK says if you are using ray diagrams to solve these problems on the MCAT, then you're wasting time. it definitely works and it's the logical reason why each scenario is + or -, but if you're trying to save time, then use their way since you don't have to think about it. i know it's blindly memorizing, which is counter-intuitive to actually learning the concepts, but hey it's faster. you know the concepts behind it anyways, though, so you aren't losing if you do memorize it.

and yeah, the stuff in your link i remember learning about in physics. makes perfect sense.

good luck

 

[tf2medic]

wait, just curious, where do bathroom mirrrors come into this? they aren't concave or convex shaped....when you look in the mirror it's definitely a virtual image and upright...but your left and right are flipped. hmm?

 

[tncekm]

another interesting exception, which is pretty intuitive, is if your object is AT the focal point, then there is no image period because all the rays are parallel and never converge to a point to make an image. i know this is true for converging lens because this was in an EK problem. i assume it's true for all lens and mirrors but i haven't really thought about it for the other cases. whatever

That makes sense.

If 1/f = 1/o, then 1/i = 0/1, so i = ∞.

M=-i/o, so does that mean that there is an infinitely large image at an infinite distance away (i.e. no image)? I guess that would work out well with parallel light rays, like EK said.

& Good luck to you, too.