Telescope Purposes:
• Gather as much light as possible- this is done by using a large aperture lens or mirror.
Amount of light depends on AREA OF A LENS. So, lens with an aperture of 300mm
diameter gather 4 times as much as on with 150mm
• Resolve full details- larger aperture lens or mirror. Larger area, better detail.
• Magnify the image of a distant object- lens or mirror with a long focal length.
Refracting Telescope:
Made by using two convex glass lenses or curved mirrors, the
refracting telescope was designed by Galileo. He used two
convex lenses together to magnify two objects in the distance.
He was then able to observe four of Jupiter’s moon. All modern
earth and space- based telescopes are refracting. The telescope
has a small lens at eye piece to focus and magnifies the bright image and a large lens at end
of cylinder to gather and bend light:
1. Convex Lens: uses rays of light that converge to a point, because the rays of light
pass through it, the focal point is real. This lens are fatter in the middle:
focus
2. Concave Lens: The rays of light from this lens diverge away from a point. Because of
this the rays of light do not pass through it and so the focal point is virtual. This lens
is thinner in the middle:
Emma Padgett
,Practical Investigation 1: Measuring the focal length of a convex lens:
Equipment List:
• A3 piece of paper
• Ruler
• Table to record results
• Convex lens
• Window
• Pen
Method:
1. Hold a piece of A3 paper up to a window spa that light can reflect off it.
2. Place the lens in front of it and move forwards and back until there is a clear image
of the outdoor scenery on the paper.
3. Have another person measure the distance between the lens and the paper
4. Record results.
Practical Investigation 2: Ray tracing for convex and concave lens:
Equipment List:
• 3 A3 pieces of paper
• Pencil
• Ray box
• Convex lens
• Power supply
• Carboard slits
• Ruler
Method:
1. Plug in power supply and attach the ray box.
2. Switch it on to ensure it is working.
3. Place the cardboard slits into the ray box so 3 lines
appear.
4. Place the A3 paper on the desktop in front and place
the ray box at one side.
5. Place your concave lens in the middle of the paper and
draw around it.
6. Turn on the ray box and light should shine through the
lens.
7. Locate the focal point by seeing where three lights meet at one point.
8. Draw in pencil an X where this point is
9. Draw over the lines of light on the paper and then draw a line from the focus point
to the lens
10. Then change do the same convex lens
11. Join the two focus points with a pencil and a ruler.
Risk Assessment:
Emma Padgett
, • Overheating products: the ray box for example could overheat and cause burns to a
student. To stop this students are asked to turn off components when they aren’t in
use and keep the voltage as little as possible.
• Glassware: Lens could fall off the table and smash causing cuts and glass injuries. If
this happens, students should tell their peers and teacher to ensure no one goes in
that direction. It is then cleaned up and put into the glass bucket.
Magnification:
angle subtended by image at eye (ß)
angle subtended by an unaired eye (a)
OR:
focal length objective (fo)
focal length eye piece (fe)
Problem 1: Spherical Aberration
• A spherically shaped lens does not refract parallel rays of light
to one focus point so the image formed is distort.
• To avoid this, the lens should have a parabolic surface to
ensure rays meet at one focal point.
Problem 2: Chromatic Aberration
• Light from different frequencies bends by a
different amount when it passed through a lens.
Each frequency of light has a different refractive index. This means that
different rays focus at different points. Blue will be neared the lens and
red will be further away.
• This can be solved by using lenses that have longer focal lengths or
using a compound lens, which are made from different frequencies by
different amounts.
Disadvantages of a refracting telescope:
1. Lenses are made from glass so the light can travel through them, this means there
can be no bubbles of air in the glass.
2. The lenses can only be supported around their edges and this is where they are
thinnest and weakest.
Emma Padgett
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