# Lenses and Telescopes

• Created by: syaqub18
• Created on: 23-05-15 18:17
how do lenses change the direction of light?
by refraction
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what are the two types of images formed by lenses?
real and virtual
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what is the principal focus?
point where axial rays pass through after refraction
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what is the principal axis?
construction line which is perpendicular to the lens and passes through the centre of the lens
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what is the focal length?
the distance from the lens to the principal focus
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what is meant by the power of a lens and what is its unit?
how quickly it causes the initial parallel beam of light to converge to the focus it is measured in dioptres
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what is the equation for the power of a lens?
1/ focal length
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what is a CCD?
silicon chips divided in to a grid of million of identical picture elements (pixels)
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how does a CCD work?
photons incident on silicon chip, electrons are released, electrons proportional to the intensity of the photons, well under chips traps released electrons, they build up creating pattern, this is converted in to a digital signal
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what is quantum efficiency of a pixel?
the percentage of photons of a pixel that liberate an electron
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what are the advantages of using a CCD?
more sensitive to light than photographic film, record changes of an image, sensitive to a wider range of wavelengths, constant quantum efficiency
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how is a diffraction pattern occur?
when light passes through a circular apperature
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what is the central circle of a diffraction pattern called?
the airy disc
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when can two objects be distinguished?
when the centre of the airy disc from one source is at least as far away as the first minimum of the other source (i.e. airy discs don't touch)
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what does the rayleigh criterion state?
The Rayleigh criterion states that the resolution of two point objects is NOT possible if any part of the central maximum of either image lies inside the first minimum ring of the other image.
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what is meant by the resolving power of a telescope?
the ability to show detail
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## Other cards in this set

### Card 2

#### Front

what are the two types of images formed by lenses?

real and virtual

### Card 3

#### Front

what is the principal focus?

### Card 4

#### Front

what is the principal axis?

### Card 5

#### Front

what is the focal length?