Lenses and Optical Telescopes Flashcards
Explain the shape of:
i) biconvex lens
ii) piano-convex lens
iii) piano-concave lens
iv) biconcave lens
i) 2 curved sides where the thickest area is through the principle axis
ii) 1 curved side and a flat side where the thickest area is through the principle axis
iii) 1 curved side and a flat side where the thinnest area is through the principle axis
iv) 2 curved lines where the thinnest area is through the principle axis
Describe the axis of symmetry called?
The principle axis
Define the principle focus
The point on the principle axis where rays cross which were originally parallel to the principle axis
What is the distance between the centre of the lens and the principle focus called?
The focal length
Name the 6 types of converging lens
1) biconvex
2) pianoconvex
3) positive meniscus
4) negative meniscus
5) pianoconcave
6) biconcave
Describe the image for an object that is beyond 2F
Real
Inverted
Diminished
Describe the image for an object that is at 2F
Real
Inverted
Same size
Describe the image for an object that is between F and 2F
Real
Inverted
Magnified
Describe the image for an object that is between F and the lens
Virtual
Upright
Magnified
State the lens formula and state the rules about the signs for focal length and the nature of the image
1 / f = 1 / u + 1 / v
where u is distance from object to lens, and v is from lens to image
- Converging lenses have +ve focal lengths, diverging lenses have -ve focal lenghts
- Distances to real images are +ve, and to virtual images are -ve
Define angular magnification and give its units
The ratio of the angle subtended at the eye by the image formed by an optical instrument to that subtended at the eye by the object when not viewed through the instrument.
M = Angle subtended by image of eye (α) / angle subtended by object of unaided eye (β)
Units: Radians (rad)
Describe the ray diagram for an astronomical telescope consisting of two converging lenses
The telescope is in normal adjustment because the image is formed at infinity (i.e. light rays leave the telescope parallel)
The focal points of the two lenses coincide, making the distance between them equal to the sum of the focal points
Give the equation and units for the angle subtended by an object of height h, distance d away
θ = h /d
units: radians
Give the equation relating the focal lengths of the objective and eyepiece lenses to the angular magnification
M = fₒ / fₑ
Describe and explain the shape of reflecting telescopes
A parabolic mirror is used to create one focal point, to prevent spherical aberration
Define Cassegrain telescope
A reflecting telescope in which light reflected from a convex secondary mirror passes through a hole in the primary mirror.
The focal point is at the hole in the primary mirror and an eye piece is used to make the rays from the secondary mirror parallel
Define Newtonian telescope
A reflecting telescope in which the light from the main mirror is deflected by a small flat secondary mirror set at 45°, sending it to a magnifying eyepiece in the side of the telescope.
State why reflective telescopes can be larger than refractor telescopes
Because a mirror can be supported from behind, where as a lens must be supported at the edge. Therefore a large lens is likely to break under its own weight
State 2 reasons why the size of the telescopes is important
- A larger objective lens means a much greater collecting area (it is proportional to diameter²), meaning fainter objects can be seen
- The larger the diameter of the aperture, the greater the resolving power (the image will show more detail)
State 2 reasons why reflecting telescopes are often preferred over refracting telescopes
- Reflectors can be made larger
- Mirrors don’t refract light, therefore don’t suffer from chromatic aberration
Define chromatic aberration
The effect produced by the refraction of different wavelengths of light through slightly different angles, resulting in a failure to focus.
State and explain 2 common problems with reflecting telescopes
- The secondary mirror and the ‘spider’ holding it in place both diffract the light as it passes, leading to poorer quality images
- There is some refraction eventually in the eyepiece used to view the final image
Describe the diffraction pattern produced by a circular aperture
A central maxima surrounded by light and dark rings
Give the equation for the angle of the minima for a single slit diffraction pattern
sinθ = nλ / a
where a is the slit width and n is the order of the minima
Define Rayleigh Criterion
Two objects will be just resolved if the centre of the diffraction pattern of one image coincides with the first minimum of the order
Give the equation for the angular separation for 2 objects that are just resolved and an assumption made for this equation
θ ≈ λ / D
θ = 1.22 λ / D
where θ is in radians, and D is the diameter of the objective
The aperture of the telescope is assumed to be the same as the diameter of the objective
Define resolving power
The ability of an optical instrument to separate or distinguish small or closely adjacent images.
The better the resolution, the smaller the angle θ
Define CCD
A charge-coupled device (CCD) is a light-sensitive integrated circuit that stores and displays the data for an image in such a way that each pixel (picture element) in the image is converted into an electical charge the intensity of which is related to a color in the color spectrum.
Describe the structure of a CCD and how it works
- A silicon chip divided into picture elements (pixels)
- Incident photons cause electrons trapped in ‘potential wells’ to be released
- The number of electrons liberated is proportional to the intensity of light
- When exposure is complete, the charge is processed to give an image
- Quantum efficiency of pixel > 70%
