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Lesson 3: Lenses

# Multiple lens systems

Some examples of using the thin lens equation with multiple lenses. Created by David SantoPietro.

## Want to join the conversation?

• can someone explain why (1/12) - (1/36) = (1/18), please? thanks.
• 1/12-1/36=(3-1)/36=2/36=1/18
• At :40sec, why do you decide to use the lens on the left as the "first lens" and not the second one on the right?
• Because a photon coming from the original image to eye will pass the lens on the left "first."
• Object distance has to be negative right it should be - 1/36 as per sign conventions
• No that's for the mirror formula... Since lens is made up of two spherical mirrors , both of its sides are positive..
• I understand that the image created is virtual, but can you explain why?
Is it because the image went to a concave lens and they always have a virtual image
• did you see a diagram of the light rays?

If so, you will see that the rays look as though they 'come from a point'. so the image LOOKS as though it should be somewhere, but really it is not. you can not shine the rays of light onto the wall and see an image on the wall. You only see an image when you eye focusses the light again to make it real (on the back of your eye)

Hope that helps
• So Im confused, In previous videos the other instructor (Sal I think) always drew the real image of a convex lens LARGER than the object itself. Sal used 2 light rays defracting through the lens that both went through the focal points on either side of the lens to accomplish this. He also drew a diagram in his video "object image height and distance relationship", where the image was larger. SO WHY IS THE IMAGE IN THIS VIDEO SMALLER THAN THE OBJECT FOR THE CONVEX LENS?!
• In Sal's video, the image of an object seen through a convex lens was larger when the object was placed a distance between f and 2f from the lens. When the object is placed at a point past 2f (i.e. 2f or greater), the inverted real image is smaller.

In the example used in this video, we see that f = 12cm, and the object is placed at 36cm (3f). So, the inverted real image is smaller.
• After getting -6 cm as answer, shouldn't he take 6 cm from right hand side of lens as he took +15 from left hand side of lens as object.
• No because for an image a negative means it is on the opposite side of the eye
• So to know how high the image is, the magnification isn't all that precise. So why dont you do the M = hi/ho which is equal to -di/do? Like wouldnt that be simpler to know the height of the object?
• I think in this video the magnification factor was used and not the height because we don't know the original height of the object (h0). If we knew that then yes, using that formula we'd get h1= h0 x (-d1/d0), where (-d1/d0) is the magnification factor.
• How did he get -6 as the final answer? I didn't quite catch what he meant by "flip it over"..