Lenses and Optical telescopes

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AQA Physics A ­ Option A
Astrophysics Revision notes.
Lenses and Optical Telescopes
Principal focus
Rays parallel to the principle axis of the lens converge onto a point called the principle
focus parallel rays that aren't parallel to the principle axis converge somewhere else on
the focal plane.
Focal length of converging lens
Focal length, f,
Is the distance between the lens axis and the principle focus.
Converging lens
Brings light rays together
Changes direction of light rays by refraction
A more powerful (thicker) lens converges the rays more strongly and will have a
shorter focal length.
Meters, m
Power of lens
(diopmeters, D)
Formation of images by a converging lens.
A real image is formed when light rays from an object are made to pass through another point in
space. The light rays are actually there, and the image can be captured on screen.

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O = object
I = image
U = distance between object and lens axis
V = distance between image and lens axis
F = focal length.
A virtual image is formed when light rays from an object appear to have come from another point
in space. The light rays aren't really where the images appear to be, so the images can't be
captured on a screen.…read more

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Ray diagrams
Lens equation
Astronomical telescope consisting of two converging lenses…read more

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A telescope, in normal adjustment, is set up so that the principal focus of the
objective lens is in the same position as the principal focus on the eye lens, so the
final magnified image appears to be infinity.
Angular magnification in normal adjustment
The objective lens converges the rays from the object to form the real image
The eye lens acts as a magnifying glass on this image to form a magnified virtual image.…read more

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M = f0
If you assume the object is an infinity, then the rays from it are parallel, and the real image is formed
on the focal plane magnification in terms of focal length.
Telescopes magnifying power is a ratio of apparent size of an object seen through a
telescope to its size when seen through the eye alone.
Value of the magnifying power depends on eyepiece.…read more

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Schmidt-cassegrain telescope combines an extremely short focus primary mirror at the back end
of a sealed tube with a thin lens at the front.
Small convex mirror, a secondary mirror, to reflect light back through a hole cut in the primary
mirror at the bottom end of the tube
More compact than a refractor or Newtonian reflector of the same aperture.
Relative merits of reflectors and refractors including a qualitive treatment of spherical and chromatic
aberration.…read more

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Requires longer amount of time to cool to ambient temp
Some chromatic aberration in bright images
Commonly manifests itself as faint fringes of colour around objects like mars or Jupiter.
Expense of producing a large achromatic lens
A chromatic triplet lens has 6 surfaces which must be figured
Lost ratio between a 150mm apochromatic lens and high quality 150mm mirror is at least
10-to-1.…read more

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To counter defect an achromatic lens is used
o Combination of two or more lenses made of different glass
A properly curved mirror reflects all the starlight at the same point
o Image formed by a reflector had no blurred colours.…read more

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Diffraction limits resolving power.
Resolving power of a telescope, Rayleigh criterion
A telescopes resolving power is its ability to produce sharp, detailed images under ideal
observing conditions.…read more

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Charge coupled device.
Use of CCD to capture images.
Monochrome CCD to obtain hi-res images
Red, green and blue filters
Data merged electronically when chips are dumped
Pixels of CCD are dumped, or unloaded, to a computer with software able to turn images into a
picture or into a raw count.…read more


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