3.9.1 Telescopes Flashcards
What is a lens
Equipment that firms an image by refracting light
What are the 2 types of lenses
- convex, converging lens
- concave, diverging lens
What is a converging lens
Parallel rays of light brought to a focus along the principal axis
What is the principal axis
An imaginary line that passes through the centre of a lens
- a lens is constructed so that it is symmetrical about its principal axis
What is a focal point
The point which rays parallel to the principal axis of a lens are brought to a focus
What is the focus length
Distance from the centre of the lens to the focus point
What is does the focal length dependent on
depends on how curved or how thick the lens is
- the more curved or thick the lens the shorter the length
- the shorter the focal length the more powerful the lens
What is th
What are characteristics of a real image
- light converges towards a focal point
- always inverted
- can be projected onto a screen
- intersection of 2 solid lines
What is an example of real image
An image from a projector into a screen
What are the characteristics of. Virtual image?
- light diverges away from a focal point
- always upright
- cannot be projected onto a screen
- the intersection of dashed lines
What is an example of a virtual image
An image in a mirror
What is assumed when constructing a ray diagram
- the lenses used are very thin
What are rhe 3 main rules for constructing ray diagrams
- rays passing through the principal axis pass through the optical centre of the lens undeviated
- rays parallel to the principal axis will be refracted and pass through the focal point
- rays through the focal point will emerge parallel to the principal axis
How are images formed with converging lenses described by
- nature, real or virtual
- orientation, inverted or upright
- size, magnified diminished or the same size
What is the lens equation?
1/ the focal length = 1/distance of the image from the lens + 1/distance of the object from lens
- only works for converging or diverging lenses
How does the distance from the object determine if it is real or virtual?
- if the image is real, the value of v is positive
- if the image is virtual, the value of v is negative
What is the magnification as a ratio of heights
Magnification = image height/ object height
What is magnification
The ratio of 2 opposite sides ( image height and object height) must be the same ratio must apply to 2 adjacent sides v and you
What lens does a refracting telescope use
Uses 2 converging lenses to project images of distant objects
What are the 2 lenses in a refractor called?
-Objective lens
- eyepiece lens
What is the function of an objective lens
Collects light from stars and brings it to a focus at its focal length
What is the purpose of an eyepiece lens
Lens is placed at a distance of its focal length away from the image and produces parallel rays of lights to be analysed
What is a normal adjustment?
A simple refractor is adjusted so that the final image is at infinity
What conditions must occur for a refractor to be in normal adjustment
Both lenses must be arranged so that their focus, points meet in the same place
- the focal length of the objective lens must be longer than the focal length of the eyepiece lens
What is the ratio equation for angular magnification?
Angle subtended by image at eye/ angle subtended by an object at unaided eye
Telescopes magnify the angular size of distant objects, suggesting:
- the telescope produces an image which subtends a larger angle than the object
- when viewed by the naked eye, the angle subtended by the object is much less than the angle subtended by their image when viewed through a telescope
what is a Cassegrain telescope
Reflecting telescopes or reflectors utilise parabolic curved mirrors to collect and focus light from distant objects
What is the law of reflection?
When the angle of incidence is equal to the angle of reflection
How do reflecting telescopes work through the law of reflection
- the primary mirror is large and concave, incident light reflects towards a focal point which is behind the secondary mirror
- the secondary mirror is smaller and convex light reflects again to form a real magnified image at the eyepiece
- the rays are directed through an aperture towards an eyepiece lens which is located behind the primary mirror
What are the important features of the ray diagram for a cassegrain telescope
- the rays enter the telescope parallel to the principal axis
- the curvature of the mirrors does not have to be the same
- the rays do not cross before the secondary mirror, they only cross in the aperture of the primary mirror
- shading indicated the non reflective side of the mirror
What is aberration?
A distortion in the image produced by optical apparatus, such as lenses and mirrors
- which is caused by a defect that causes light to spread out rather than being brought to a focus at a single point
What are the 2 types of aberration that affect the quality of images produced by refractors and reflectors
- chromatic aberration
- spherical aberration
What is chromatic aberration
- image distortation seen only in refracting telescopes
Different wavelengths of light are refracted by different amounts causing the edges of an image to appear coloured - this is because blue light has a shorter wavelength than red light, meaning blue light is refracted more by a lens than red light
- because blue light has a bigger refractive index.
How can red light be brought to the same focal point as blue light
Chromatic aberration cannot be eliminated entirely, but it can be significantly reduced by using a second diverging lens which refracts the light in the opposite direction
Chromatic aberration in reflecting telescopes
Mirrors can only reflect not refract
What is spherical aberration
Rays of light come to focus at different points due to the spherical curvature of a lens ir mirror, causing the image to become very blurred
- the further the ray is from the principal axis, the shorter it’s focal length
How does spherical aberration affect both lens and mirrors
- cannot be eliminated entirely in a refracting telescope but can be reduced by using a pareabolic lens.
- however, there is a limit to how much this improves the image quality, a this further increases the size and weight of the lens
- in a reflecting telescope, spherical aberration can be entirely overcome by using a parabolic mirror
What are advantages of refracting telescopes
- requires less maintenance
- not as sensitive to temperature changes
What are the disadvantages of refracting telescopes
- size
- weight
- construction
- image quality
- ability to observe non-visible wavelengths
How is ‘size’ a disadvantage of refracting telescopes?
- it is difficult to make large- diameter glass lenses which are completely free from defects
- large magnifications require large objective lenses and very long focal lengths
How is ‘weight’ a disadvantage of refracting telescopes?
- larger dimeter lenses are heavy and tend to distort under their own weight
- refractors are heavy and difficult to manoeuvre so they have a slower response to astronomical events
- observing equipment and electronics must be mounted to the telescope which adds to the weight
How is ‘construction’ a disadvantage of refracting telescopes?
- mirrors only use the front surface for reflection, which eliminates many problems associated with lenses
- mirror surfaces can be made very thin which allows for greater image detail
How is ‘image quality’ a disadvantage of refracting telescopes?
- mirrors cannot produce chromatic aberration
- reflectors don’t suffer from spherical aberration use if parabolic mirrors are used
How is ‘image quality’ a disadvantage of refracting telescopes?
- mirrors cannot produce chromatic aberration
- reflectors do not suffer from spherical aberration use if parabolic mirrors are used
How is ‘ability to observe non visible wavelengths ’ a disadvantage of refracting telescopes?
- reflectors can be designed to observe wavelengths of light outside of the visible dpectrum
- can be sent into space eliminating light absorption from the atmosphere
What are disadvantages of refracting telescopes
- secondary mirror can block light from primary mirror
- secondary mirror and it’s supports will cause some diffraction which can affect the clarity of the image
- mirrors reflecting telescopes are exposed to air so they require regular maintenance
- light is refracted in the eyepiece lens and some chromatic aberration may be introduced at this stage
Hat is the resolving power of a telescope
A measure of how much detail you can see
- a huge magnification telescope may have a low resolving power this the image will be blurred
What is the resolving power of an image dependent on
Minimum angular resolution
What is minimum angular resolution
- smallest angular separation at which the instrument can distinguish 2 points
- the smaller the minimum angular resolution, the better the resolving power of the telescope
How is an airy disc formed
When waves pass through an opening or aperture of a telescope
- they will diffract and interfere constructively or destructively to produce an interference pattern
What is an airy disc
A diffraction pattern with a central circle
What is the Rayleigh criterion
2 light sources can be the centre of the airy disc from one source is at least as far away as the first minimum of the other source
What is the equation for the Rayleigh criterion
Minimum angular resolution in radians = wavelength in metres/ diameter of the aperture in metres
What are radio telescopes
How is ‘construction ’ a disadvantage of refracting telescopes?
Lenses can only be mounted and supported around their edges however, this is where they are thinner and weakest
How is ‘image quality’ a disadvantage of refracting telescopes?
Refractors suffer from both chromatic and spherical aberration
How is ‘ability to observe non- visible wavelengths’ a disadvantage of refracting telescopes?
Are only able to observe wavelengths of visible light
What are the advantages of reflecting telescopes
- size
- weight
- construction
- image quality
- ability to observe non-visible wavelengths
How is ‘size’ a advantages of reflecting telescopes?
- the diameter of a mirror can be much larger than that of a lens so greater magnifications can be achieved
- the arrangement of the mirrors allows reflectors to be much shorter than refractors
- several small mirrors can be used to form a large composite objective mirror
How is ‘weight’ a advantages of reflecting telescopes?
- large single mirrors can be made, which are light and easily supportable from behind
- reflectors are lighter which allows for a more rapid response to astronomical events
How is ‘construction ’ a advantages of reflecting telescopes?
- mirrors only use the front surface for reflection, which eliminates many of the problems associated with lenses
- mirror surfaces can be made very thin which allows for greater image detail
How is ‘bility to observe non-visible wavelengths ’ a advantages of reflecting telescopes?
- reflectors can be designed to observe wavelengths of light outside the visible spectrum
- reflectors can be sent into space which eliminates light absorption due to the atmosphere
