Lenses and Telescopes (Unit 5) Flashcards Preview

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Flashcards in Lenses and Telescopes (Unit 5) Deck (34):

Definition of Principal focus (Focal Point)

The point through which all light parallel to the axis of the lens passes through.


Definition of Focal length

The distance between the centre of the lens and the focal point.


Constructing ray diagrams through converging lenses

See sheet


Letters in Lens makers formula
1/u + 1/v = 1/f

u is the Object distance
v is the Image distance
f is the focal length of lens
u comes before v in the alphabet, object comes before image in lens
RIP – Real Is Positive
(Real image has positive distance, Virtual image has negative distance)


Description of images formed with different object distances

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Formation of images by converging lenses

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Ray diagram of a refracting telescope in normal adjustment

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Angular magnification in normal adjustment

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Ray diagram of a Cassegrain telescope

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Definition spherical aberration – lens

Where parallel rays at different distances from principal axis are brought to focus at different points (on principal axis).


Definition spherical aberration - mirror

Where parallel rays furthest from the principal axis are brought to focus closer to reflector than rays closest to principal axis.


How can spherical aberration be prevented in a mirror

Use a parabolic mirror.


Diagram of spherical aberration of a mirror

See sheet


Effect on image of spherical aberration

Image is blurred


Definition chromatic aberration

Where different wavelengths are refracted by different amounts resulting in different focal lengths for different wavelengths (colours).


Effect on image of chromatic aberration

Images have multicoloured, blurred edges


Diagram of chromatic aberration from a lens

See sheet


Resolving power of telescopes

the smallest angular separation that two point objects can still be discerned as separate


Definition Rayleigh criteria

two point objects can be just be resolved when the central maximum of the diffraction pattern of one object lies over the first minimum of the diffraction pattern of the other.


Definition of Airy Disc

The bright central maximum (spot) of the diffraction pattern produced when light from a point source passes through a circular aperture.


Sketch of diffraction pattern from a circular aperture

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Sketch illustrating Rayleigh criteria

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Structure and operation of a CCD

• A CCD is silicon chip divided into picture elements (pixels).
• Incident photons cause electrons to be released.
• The number of electrons liberated is proportional to the intensity of the light.
• These electrons are trapped in ‘potential wells’ in the CCD.
• An electron pattern is built up which is identical to the image formed on the CCD.
• When exposure is complete, the charge is processed to give an image.


Quantum efficiency of a CCD

Quantum efficiency of pixel >70%
a measure of the proportion of the incident photons that release electrons


Definition of quantum efficiency

The percentage of the photons incident on the CCD that release electrons and so are detected


Why are telescopes put into space?

• Electromagnetic waves are absorbed by the atmosphere
• Light pollution and other interference at ground level
• The effect the atmosphere has on the path of the light as it passes through


Non-optical telescopes positioned in space

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Non-optical telescopes positioned on the ground

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What does collection power of a telescope depend upon?

Collection power of a telescope (rate at which energy is collected) is proportional to the cross-sectional area of the objective (lens or mirror).
As cross-sectional area = pi x r2 = pi(d/2)^2 = (pi x d^2)/4
Where d is the diameter of the objective, therefore
Collecting power of a telescope is proportional to d^2


What absorbs X-rays, UV and infra-red in Earth’s atmosphere?

• X-rays are absorbed by Ozone (and Oxygen)
• UV are absorbed by Ozone (and Oxygen)
• Infra-red are absorbed by water vapour (and carbon-dioxide)


Similarities and differences between radio and optical telescopes

• Radio telescopes have a similar structure in that a concave reflecting surface reflects electromagnetic radiation to a detector at the focal point (of the reflecting surface).
• Radio telescopes are much larger – radio wavelengths are much longer than optical wavelengths
• Radio telescopes have a lower resolving power – the wavelengths of radio waves are very much larger than optical wavelengths (even though the diameters of radio telescopes are larger).
• Radio telescopes have a greater collecting power – collecting power depends on the area of the objective which is much larger for radio telescopes (depends on the square of the diameter).
• Radio telescopes are not as affected by atmosphere so their positioning is less critical
• Radio telescopes have only 1 reflecting surface not 2


Advantages of mirrors in telescopes rather than convex lens

• No chromatic aberration – mirrors do not refract light
• no spherical aberration – use of parabolic mirror
• no distortion – mirrors can be supported across its whole surface (lenses can only be supported around their edge)
• better resolving power and/or greater brightness – mirrors can be made larger
• more light gets through (image is brighter) – lens absorbs more light


Disadvantages of mirrors in telescopes rather than convex lens

• they have a frame holding the central mirror (secondary), that:
o reduces the light intensity (as it blocks some light from hitting the primary mirror)
o creates diffraction effects around the edges causing blurring of the image


Advantage of larger objective diameter (mirror or lens)

• It collects more light – so can observe fainter objects
• It has a higher resolution – possible to distinguish (resolve) two objects close together.