Lenses and Telescopes (Unit 5) Flashcards
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
See sheet
Formation of images by converging lenses
See sheet
Ray diagram of a refracting telescope in normal adjustment
See sheet
Angular magnification in normal adjustment
See sheet
Ray diagram of a Cassegrain telescope
See sheet
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
See sheet
Sketch illustrating Rayleigh criteria
See sheet
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
See sheet
Non-optical telescopes positioned on the ground
See sheet
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
Similarities
• 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).
Differences
• 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.
