Any MCAT Optics Tricks?

[aspiringdoc09]

Do any of you have any MCAT tricks that have proved successful in remembering lenses, mirrors, focal lengths, and whether image is large or small based on the focal length distance? I would like to learn how to recal this information conceptually vs memorizing or calculations (where not required). For instance:

TPRH Question 392 asked: An object is placed at a great distance from a converging lens and gradually moved toward the focal point of the lens. The image will move:

The answer is whited out: D. away from the lens and increase in size.

Are any of you capable of determining the answer intuitively w/o setting up a hypothetical situation using the lens and magnification equations? I'm lacking in this area and would prefer to figure out a system to attack these questions instead. Thanks.

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[MedPR]

Not really intuitively, but the equations are easy to manipulate. Magnification = i/o, so as the image distance gets bigger and the object distance gets smaller, the image gets bigger.

Also, 1/f=1/o+1/i, so as the object distance gets smaller, the image distance necessarily gets larger.

I stuck to the equations for optics because they are so simple to work with and memorize.

 

[aspiringdoc09]

Not really intuitively, but the equations are easy to manipulate. Magnification = i/o, so as the image distance gets bigger and the object distance gets smaller, the image gets bigger.

Also, 1/f=1/o+1/i, so as the object distance gets smaller, the image distance necessarily gets larger.

I stuck to the equations for optics because they are so simple to work with and memorize.

I was wondering if there was a faster way to do it. I need to definitely work on timing for mathematical calculations. Thanks.

 

[MedPR]

I was wondering if there was a faster way to do it. I need to definitely work on timing for mathematical calculations. Thanks.

If you have those 2 equations in your head and have a good understanding of the relationships between i, o, f, and m, then the only faster way to do it is to have every lens situations memorized. Answering that question took about 5 seconds for me, in case you're wondering how fast that method can be.

 

[chiddler]

medpr, what i've done is memorize the trends for each type of lens. easier and faster than equation.

make a table of fixed focal length and change object distance (x) to see how image distance varies. you get a good feel for it that way.

 

[Buttafuoco]

The Khanacademy vids on optics are pretty good. I think ray diagrams are very fast an intuitive when you get the hang of them. I don't know what the EK guys are talking about when they call them a last resort. It takes like 10 seconds to give you a qualitative answer.

http://www.khanacademy.org/#physics

The series is in the last column, starting with Virtual Images.

 

[aspiringdoc09]

Thanks. MedPr, I do know the equations, but I have never been fast with math, so that is what always get me. Chiddler, I understand that works for you and that's great, but for me memorizing a bunch of stuff is a waste of time. I rather understand the concept rather than memorize it. I will check out Khanacademy too. Is wikipremed a good source for optics, Buttafuoco? Thanks everyone.

 

[Buttafuoco]

I'm not really sure TBH. But once you understand how to use a ray diagram with 2 rays like Khan draws, it really helps you both memorize the concepts and quickly get a qualitative answer for a problem. Hell, for the MCAT, it will often be quantitative enough too just by roughly comparing your image distance to object distance.

 

[chiddler]

Thanks. MedPr, I do know the equations, but I have never been fast with math, so that is what always get me. Chiddler, I understand that works for you and that's great, but for me memorizing a bunch of stuff is a waste of time. I rather understand the concept rather than memorize it. I will check out Khanacademy too. Is wikipremed a good source for optics, Buttafuoco? Thanks everyone.

what I suggested is not memorization without concept. but either way, suit yourself.

 

[Class1P]

TBR method really made this crystallize for me.

For converging lenses mirrors: set two imaginary points, one at the focal point (f)and one at the radius of curvature (twice the distance of the focal point) R

If your object is beyond R way out distant, your image appears very closely to the focal point and is tiny.

as you move the object closer to R your image increases in size and moves toward R. When your object is AT R your image will also be at R and the same size.

As you continue to move your object past R and closer to f, your image will continue to get further away and larger. Your image is always upside-down and real. unless you move past the focal point toward the lend/mirror. then everything flips. Your image is very large, upright and virtual and gets smaller the closer you move it toward the lens.

Play with this tool I found.

http://www.mtholyoke.edu/~mpeterso/classes/phys301/geomopti/lenses.html



so in general as you move your object closer to the lens/mirror, the image gets further away from the focal point and grows in size. They are only the same size at the radius of curvature.

 

[DrRichand1]

freelanceteacher.com has some awesome vid's on mirrors and lens. best explaination i've ever seen.

 

[SaintJude]

a good understanding is key but optics is the one area where quick memorization has helped me a lot b/c of time restrictions

- diverging optics (convex mirrors, concave lenses) = negative, virtual and upright images
-converging optics (concave mirrors, convex lenses)= positive, real and inverted images unless the object is within the focal point (then it'll be virtual)

Those two factoids pretty much sum most of it up.

 

[MedPR]

a good understanding is key but optics is the one area where quick memorization has helped me a lot b/c of time restrictions

- diverging optics (convex mirrors, concave lenses) = negative, virtual and upright images
-converging optics (concave mirrors, convex lenses)= positive, real and inverted images unless the object is within the focal point (then it'll be virtual)

Those two factoids pretty much sum most of it up.

Except for where the image will appear and whether or not it is smaller, larger, or the same size!

 

[SaintJude]

True.

 

[medical.balla]

Do any of you have any MCAT tricks that have proved successful in remembering lenses, mirrors, focal lengths, and whether image is large or small based on the focal length distance? I would like to learn how to recal this information conceptually vs memorizing or calculations (where not required). For instance:

TPRH Question 392 asked: An object is placed at a great distance from a converging lens and gradually moved toward the focal point of the lens. The image will move:

The answer is whited out: D. away from the lens and increase in size.

Are any of you capable of determining the answer intuitively w/o setting up a hypothetical situation using the lens and magnification equations? I'm lacking in this area and would prefer to figure out a system to attack these questions instead. Thanks.

So, here's how I think about it: With a converging lens I think myopia. People who are myopic are by definition near-sighted so they need the focal length to be small to be able to see it. As the focal length decreases the image distance increases so the image can be formed properly on the retina of the eye. With the image distance being increased, think of a projector, the farther away a projector projects the display is larger. So, that's why the image will be bigger.

 

[pretty_positron]

I just have this memorized.