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Lesson 3: Lenses

# Object image height and distance relationship

The video explores the relationship between the size of an object and its image when viewed through a convex lens. It demonstrates that the ratio of the distance from the object to the lens, to the distance from the image to the lens, is the same as the ratio of the height of the object to the height of the image. Created by Sal Khan.

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• In terms of Magnification, doesn't hi/ho = -di/do, the negative sign matters no?
• Yes (the next video talks about the positive and negative conventions)
(1 vote)
• Where can i find problems for optics online or in which books?
• Hi,
This wasn't very clarified in the video so can someone plz help me out:
Say you have an image of 30 cm tall flower and it is captured by a camera with a 28mm lens. If the image formed behind the lens is 29mm behind the lens, how far Is the lens from the rose? how high is the image?
• Our key formula is 1/do + 1/di = 1/f
f = focal distance = 28 mm
di = image distance = 29 mm
do = object distance = what we want to find
So, that gives us:
1/do + 1/29 = 1/28
1/do = 1/28 - 1/29
1/do = 1/ 812
do = 812 mm (has to be mm since 28 and 29 were mm!)
do = 81.2 cm
Then, for the height of the image, we use the formula Sal finds in this video:
hi/ho = di/do
hi/300mm = 29mm/812mm (change EVERYTHING to mm to be safe!)
hi = (29/812)*(300mm)
hi = 10.7mm
• the correct lens formula is 1/f = 1/v - 1/u , isn't it or 1/f = 1/di - 1/do
(1 vote)
• See, what u r talking about is called 'sign convention'......where u assume a Cartesian plane in which the optical centre of the lens acts as the origin and the principal axis as the x-axis......so any distance on the left side of lens is taken as negative (u= do= distance of the object from the lens) and any distance on the right is taken as positive(v= di= distance of image from the lens)......also, when the image is inverted the height of the image is taken as negative for the same reason....
• How can I calculate the object or image height if I have the focal length, object distance and image distance? Thank you :)
• Does this mean that you could also write the equation found in the previous video as (1/f)=(1/ho)+(1/hi) ? Also, can this formula also somehow be used to find the focus of a convex/concave parabolic mirror?
• no,as only ho/hi=u/v for a lens .it doesn't implies that ho=u & hi=v.
• Does this formula apply to parabolic convex and concave mirrors and to convex and concave lenses?
• Can anyone explain me what is angular magnification?