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lens
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how to remember the characteristics of i)concave and convex mirrors ii) concave and convex lens? I've been memorizing it but it just seems didn't work.. T_________T i've been drawing wrong image position everytime in exam.. it caused me to lose many marks ...i felt so sad... i just can't understand.. can someone teach me with it (lens)? |
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Re: lens
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by SuicidopoliS
on 2007-05-25 15:06:58
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Well... For starters, mirrors and lenses can be considered opposites: while the first reflects light, the second one lets it pass through. Quite obvious, but the fun thing is that once you know the characteristics of one, you know those of the other one too, cause it works inversely. Then, remember the word "cave" in "concave". Whenever you look at a cavity in a lens or mirror, it's a concave surface. Otherwise, it's plane or convex, and the difference between those two is hardly confusing. Next, think of an antenna! The satellite type, with a dish and a... pointy thing in the middle. We'll assume that all light falling onto the dish is coming in from an axis parallel to the central axis, perpendicular to the middle of the antenna/lens/mirror. Imagine the pointy thing is the focus point of the dish ( which in fact is also the case... ), and the arms that connect it with the dish are light rays. So here you go... Imagine the dish is a lens, and you're a light ray falling onto the lens from the opposite side of the focus point, thus, onto the convex side of the lens. Then you'll get inclined towards the focus point! If on the other hand, you fall onto the lens coming from the other side, so from the concave side, you'll do the inverse. Instead of getting focused towards a single point, you'll be all spread out. If instead of a lens, you consider the antenna dish to be a mirror, it's the same logic... Suppose you're a light ray falling onto the "inside" of the dish, thus the concave side, you'll get reflected towards the focus point. If on the other hand you fall onto the other side, you'll get reflected somewhere into outer space, and never see your fellow light ray buddies again! And of course, you should keep in mind that an image reflected in a concave lens / transmitted through a convex lens will be inverted. That's easy too see if, once again, you imagine you're a light ray. So, you're a light ray! And parallel to you, there's a buddy of yours flying all the way with you. Suppose you're both hitting a concave mirror, so you'll both get reflected towards the focus point. One you've passed the focus point, you'll have swapped places! If you see what i mean, cause i'm afraid this isn't as clear written down as it is in my head... Otherwise, just look at a spoon... If you look into the concave side, you'll see your reflection is inverted, when looking at the other side, you'll see it isn't. I hope this helped... I know i'm not always very clear when i "explain" things... .oO° Life's THE CURE, the rest are details! °Oo. |
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Re: lens
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by
on 2007-05-26 08:02:54 |
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Mirrors Convex mirrors are bent outwards right? (imagine a fat person belly) so they reflect over a large area (like a fat guy eating ALOT of stuff :P no offense) Concave mirrors... on the other hand, they have a lower reflecting range. (like a thin person dejecting all food :D) Lenses: the following info is for general terms As for convex lenses, they usually form inverted and real images (it really depends on where the image is put) Convex lenses causes the lines to come together at one point, which will for a small image. (remember convex like a funnel. when u pour water into a funnel, the water is converged to one point or u might wanna imagine a toilet bowl flush for that matter X.X) Concave/ diverging lens, as its name suggests, cause the light rays to 'spread' out. like a water sprinkler or something. :D  A friend of today is an enemy of tomorrow. I am an empty shell. I don't feel loved at all |
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Re: lens
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by none123456
on 2007-05-27 16:55:44
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General tips for drawing lens-diagrams: 1. Draw 2 lines from farthest point of image/real object AWAY from the center line. 2a.Convex Lenses According to my text: (I attempted an explanation myself, but got way too confusing) 1.Ray that is parallel to principal axis is refracted so it passes through focus on same side of object 2. Ray that passes through focus on opposite side is refracted parallel. 3.A ray that passes through the optical centre goes straight through. 2b. Concave Lenses: The focuses reverse, so: 1.Ray that is parallel to principal axis is refracted so it passes through focus on opposite side of object 2.Ray that passes through focus on same side is refracted parallel. 3.A ray that passes through the optical center goes straight through. 3. Mirrors have one focal point, where side of curve points toward. ) points left, so focal is left of ). Both go through the focal point-one after hitting the mirror, one before hitting the mirror. Still one diagonal then straight, another straight then diagonal. More straightforward than lens, right? ^^ 4. ALWAYS extrapolate the reflected/refracted lines BOTH WAYS. this is to make sure you have always get image if possible, whether real/imaginary. 5. If the above DOES NOT WORK because the object is AT the focal point, assume no image. I'm not 100% sure for concave lens (no matter how i think, there is no image, but I feel there should be one.) But for the rest it works. 6. For real and imaginary : 6a.Lens: same side - imaginary: opposite side - real. 6b.Mirror: same side - real: opposite side - imaginary. |