Physics P7.2 How Does A Telescope Work

The title is pretty self-explanatory.

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A convex lens is thicker in the middle than at A two-convex lens telescope:
Ayesha Khanom 11L
the edges. Parallel rays entering these lenses are The objective lens has a long focal point so that
refracted and come to a point called the focus ­ it produces a large image inside the telescope.
it is said they converge. The eyepiece has a short focal length and is
A fat convex lens, with surfaces that are more used to magnify the real image.
curved, has a shorter focal length than a thin The distance between the two lenses is equal to
lens made of the same material. The fat lens the sum of the two focal lengths.
refracts the light more, so it is said to be more
powerful. Angular magnification makes the angle between
The power of a lens is measured in dioptres: stars much larger than if looked at with the naked
eye. The equation needed:
P7.2 How does
Moving a lens back and forth until a clear image a telescope
can be seen is the method for measuring the work?
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 Most telescopes use concave mirrors rather
ruler and pencil; use arrows to show direction than lens as objective ­ these are called
that light is travelling; a ray through the centre of reflecting telescopes.
a lens does not change direction; a ray parallel to A concave mirror will bring parallel light (such as
the principal axis emerges to pass through the that from stars) to focus.
focus; and a ray through the focus emerges An eyepiece lens is then used to magnify the
parallel to the principal axis. image from the mirror.
The advantages: reflects all colours the same,
whereas a lens will reflect more blue light than
red (this is called `chromatic aberration'); it is
easier to make a large mirror than large mirror
because its shape can be supported from the
back; it is hard to make sure the glass in a lens
has no imperfections.
Disadvantage: eyepiece MUST be inside the
telescope, so obscures some of the image.


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