Physics P7.2 How Does A Telescope Work

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A convex lens is thicker in the middle than at the edges. Parallel rays entering these lenses
are refracted and come to a point called the focus ­ it is said they converge.
· A fat convex lens, with surfaces that are more curved, has a shorter focal length than a thin
lens made of the same material. The fat lens refracts the light more, so it is said to be more
· The power of a lens is measured in dioptres:
· Moving a lens back and forth until a clear image can be seen is the method for measuring
the focal length. The distance of the lens from where the image is displayed is the focal point
of the lens.
· In order to draw a ray diagram: always use a ruler and pencil use arrows to show direction
that light is travelling a ray through the centre of a lens does not change direction a ray
parallel to the principal axis emerges to pass through the focus and a ray through the focus
emerges parallel to the principal axis.
P7.2 How does a telescope work?
Ayesha Khanom 11L Physics
A twoconvex lens telescope:

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The objective lens has a long focal point so that it produces a large image inside the
· The eyepiece has a short focal length and is used to magnify the real image.
· The distance between the two lenses is equal to the sum of the two focal lengths.
Angular magnification makes the angle between stars much larger than if looked at with the
naked eye.…read more


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