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Lenses - Magnification
Kerolos_4875
#1 Posted : Friday, August 07, 2020 5:38:30 PM
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Hi everyone,

Page 132 of the CC states that if magnification is positive it will yield a virtual image. However, the first example of page 133 (Concave mirror, object b/w focus and radius) the magnification is positive but it is somehow real. Same thing goes for the second example on page 135.

Am I misunderstanding the rule?

Thanks!
INSTR_Radhika_42
#2 Posted : Friday, August 07, 2020 7:28:57 PM
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Hi!

These are some good questions to clarify.

I like to begin with the mnemonic: RIP VUN.
- RIP - real images are inverted and positive
- VUN - virtual images are upright and negative

Also remember the Lens/Mirror Equation: m = - (i/o) where i is the image distance and o is the object distance. For mirrors, i is positive when the image appears on the same side as the optic. For lenses, i is positive when the image appears on the opposite side of the optic.

When m is positive, the image is upright, when m is negative the image is inverted. I have a feeling you may be mixing up a positive/negative i with positive/negative m.

On p 133 the first example, the image is magnified because it appears larger than the object (I believe what they mean to say is that the ratio is greater than one - aka it is enlarged!). We can see from the ray diagram that the actual rays converge there, so it is a real image and therefore inverted. The actual 'm' however, is negative because i is positive. The magnitude of |m| is greater than 1 because it is larger. Does this make sense?

See if you can reason this way with the second example! Let me know if you have follow up questions.




Kerolos_4875
#3 Posted : Sunday, August 09, 2020 8:35:43 PM
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Thanks for the response!

I am still confused though because you said when m is positive the image is upright - but the example on page 133 (top one) shows a positive m but a real, inverted image. Is magnification not referring to m?

Sorry it just seems like there are examples where it is RIP and magnified (positive m) and some where it is RIP yet diminished (page 132) and that is confusing me.

Another question (sorry!): are magnitude and sign (+/-) related? Ie. could an object have a negative m and be less than 1 but still be considered RIP? In other words, does the ratio of height between image and object have anything to do with RIP and VUN or is it just the sign that matters when determining RIP vs VUN?
INSTR_Kailey_109
#4 Posted : Wednesday, August 19, 2020 2:53:16 PM
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Hi Kerolos,

m, does refer to magnification but there are 2 aspects of it that you should recognize. The absolute value of m (whether it is greater than |1| or less than |1|) tells you about SIZE of the image. Such that if the absolute value is greater than 1, the image is enlarged. If the absolute value of m is less than 1, the image is diminished. The SIGN of m tells you the image orientation. Positive m means the image is upright and negative m means it is inverted.

I think you may have been merging knowledge of the +/- with the magnitude and I hope this clarifies. As a suggestion, I think it would be helpful if you read through this link: https://www.physicsclass...ics-for-Concave-Mirrors. It classifies the concave mirror situation into 5 situations based on where the object is placed and I think it will help visualize everything on a single page.

Please let me know if you have additional questions!
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