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Telescopes (AQA A Level Physics): Flashcards

Exam code: 7408

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  • Define focal length.

    The focal length is the distance from the centre of a lens to its focal point, where parallel rays of light converge.

  • What are the three rules used to construct ray diagrams for thin converging lenses?

    • Rays through the optical centre pass through undeviated

    • Rays parallel to the principal axis refract through the focal point

    • Rays through the focal point emerge parallel to the principal axis

  • State three differences between the properties of a real image and a virtual image formed by a lens.

    • A real image forms where light converges; a virtual image forms where light appears to diverge

    • A real image is always inverted; a virtual image is always upright

    • A real image can be projected onto a screen; a virtual image cannot

  • In the lens equation, if the image formed by a lens is virtual, the value of v is ...........

    In the lens equation, if the image formed by a lens is virtual, the value of v is negative.

  • Define magnification as a ratio of heights.

    Magnification, M, is the ratio of the image height to the object height: M = \frac{h_{i}}{h_{o}}

  • Give the equation for magnification in terms of the image and object distances from the lens.

    M = \frac{v}{u}, where v is the distance from the lens to the image and u is the distance from the lens to the object

  • True or False?

    The thicker and more curved a lens is, the longer its focal length becomes.

    False.

    The thicker and more curved a lens is, the shorter its focal length becomes, making it a more powerful lens.

  • Define normal adjustment.

    Normal adjustment is when a refracting telescope is arranged so the final image forms at infinity, with both lenses' focal points meeting at the same point.

  • For a refracting telescope to be in normal adjustment, how must the focal lengths of the two lenses compare?

    The focal length of the objective lens must be longer than the focal length of the eyepiece lens: f_{o} > f_{e}

  • A refracting telescope uses two .......... lenses, called the objective lens and the eyepiece lens.

    A refracting telescope uses two converging lenses, called the objective lens and the eyepiece lens.

  • Define angular magnification.

    Angular magnification is the ratio of the angle subtended by the image at the eye to the angle subtended by the object at the unaided eye: M = \frac{\beta}{\alpha}. It has no units.

  • What is the equation for the angular magnification of a refractor in normal adjustment?

    M = \frac{f_{o}}{f_{e}}, where f_{o} is the objective focal length and f_{e} is the eyepiece focal length

  • How can a refracting telescope be designed to achieve a greater angular magnification?

    • Use a longer objective focal length

    • Use a shorter eyepiece focal length

  • True or False?

    The length of a refracting telescope in normal adjustment is equal to fo minus fe.

    False.

    The length of the telescope is equal to f_{o} + f_{e}, the sum of the two focal lengths, not their difference.

  • Define Cassegrain telescope.

    The Cassegrain telescope is the most common type of reflecting telescope, using a primary mirror and a secondary mirror to collect and focus light.

  • Describe the shape and role of the primary mirror in a Cassegrain telescope.

    The primary mirror is large and concave. It reflects incident light towards a focal point located behind the secondary mirror.

  • Describe the shape and role of the secondary mirror in a Cassegrain telescope.

    The secondary mirror is smaller and convex. It reflects light again to form a real, magnified image at the eyepiece.

  • In a Cassegrain telescope, rays entering parallel to the principal axis do not cross before the secondary mirror — they only cross in the .......... of the primary mirror.

    In a Cassegrain telescope, rays entering parallel to the principal axis do not cross before the secondary mirror — they only cross in the aperture of the primary mirror.

  • State two features examiners look for in a correctly drawn Cassegrain ray diagram.

    • The primary mirror drawn as one continuous concave curve, with a correctly convex secondary mirror

    • Rays entering parallel to the principal axis and crossing only in the aperture of the primary mirror

  • True or False?

    The secondary mirror in a Cassegrain telescope blocks light such that the central portion of the final image is missing.

    False.

    The secondary mirror causes a slight reduction in the amount of light reaching the aperture, but light from a distant source arrives as parallel rays, so the image is not missing any part — it is just slightly less bright.

  • Define chromatic aberration.

    Chromatic aberration is a distortion, seen only in refracting telescopes, where different wavelengths of light are refracted by different amounts, causing the edges of an image to appear coloured.

  • Define spherical aberration.

    Spherical aberration is a distortion, affecting both refractors and reflectors, where rays of light come to focus at different points due to the spherical curvature of a lens or mirror, blurring the image.

  • How can chromatic aberration be reduced in a refracting telescope?

    By using a second diverging lens, which refracts light in the opposite direction, bringing red and blue light to the same focal point.

  • How can spherical aberration be entirely eliminated in a reflecting telescope?

    By using a parabolic mirror.

  • Chromatic aberration occurs because .......... light has a shorter wavelength than red light, so it is refracted more by a lens.

    Chromatic aberration occurs because blue light has a shorter wavelength than red light, so it is refracted more by a lens.

  • State two advantages of reflecting telescopes over refracting telescopes relating to size and weight.

    • A mirror can have a much larger diameter than a lens, giving greater magnification, and reflectors can be made much shorter than refractors

    • Large single mirrors can be made light and easily supported from behind, allowing a more rapid response to astronomical events

  • Which type of telescope, refractor or reflector, is limited to observing only visible wavelengths of light?

    Refracting telescopes can only observe visible wavelengths of light; reflecting telescopes can be designed to observe wavelengths outside the visible spectrum.

  • True or False?

    Both refracting and reflecting telescopes suffer from chromatic aberration.

    False.

    Chromatic aberration only occurs in refracting telescopes, since it results from the refraction of light by a lens; mirrors in reflecting telescopes only reflect light, so cannot produce it.

  • Define Airy disc.

    The Airy disc is the large central maximum of the circular diffraction pattern produced when light passes through a circular aperture. It is twice as wide as the surrounding maxima.

  • Define the Rayleigh criterion.

    The Rayleigh criterion states that two sources will be resolved if the central maximum of one diffraction pattern coincides with the first minimum of the other.

  • State two ways of increasing the resolving power of a telescope.

    • Increasing the diameter of the aperture

    • Operating at a shorter wavelength of light

  • Give the equation for the angular separation, θ, between two sources, and define the symbols used.

    \theta = \frac{s}{d}

    • θ = angular separation (rad)

    • s = distance between the two sources (m)

    • d = distance between the sources and the observer (m)

  • For a circular aperture, the value of θ = λ/D is multiplied by a factor of ...........

    For a circular aperture, the value of θ = λ/D is multiplied by a factor of 1.22.

  • According to the Rayleigh criterion, when are two sources resolvable, just resolvable, and not resolvable?

    • Resolvable when \theta > \frac{\lambda}{D}

    • Just resolvable when \theta \approx \frac{\lambda}{D}

    • Not resolvable when \theta < \frac{\lambda}{D}

  • True or False?

    The larger the value of θ, the greater the resolving power of a telescope.

    False.

    The smaller the value of θ, the greater the resolving power of the telescope.

  • Define collecting power.

    The collecting power of a telescope is a measure of the amount of light energy it collects per second.

  • How does the collecting power of a telescope relate to the diameter of its objective?

    Collecting power is directly proportional to the square of the diameter of the objective: \text{collecting power} \propto D^{2}

  • State two advantages of a telescope with a larger aperture diameter.

    • Greater collecting power, so images are brighter

    • Greater resolving power, so images are clearer

  • The collecting power of a telescope is equivalent to the power per unit area, or .........., of the incident radiation collected.

    The collecting power of a telescope is equivalent to the power per unit area, or intensity, of the incident radiation collected.

  • Give the equation used to compare the collecting power of two telescopes in terms of their objective diameters.

    \frac{\text{collecting power of telescope 1}}{\text{collecting power of telescope 2}} = \left(\frac{D_{1}}{D_{2}}\right)^{2}

  • Give the equation used to compare the resolving power of two telescopes operating at the same wavelength, in terms of their objective diameters.

    \frac{\text{resolving power of telescope 1}}{\text{resolving power of telescope 2}} = \frac{D_{2}}{D_{1}}

  • True or False?

    The collecting power of a telescope depends on the focal length of its objective lens.

    False.

    Collecting power depends on the diameter (aperture) of the objective, since it is proportional to the surface area collecting light, not the focal length.

  • Define non-optical telescope.

    A non-optical telescope detects wavelengths of the electromagnetic spectrum outside the visible region, such as radio, infrared, ultraviolet and X-ray telescopes.

  • What are the three main advantages of placing a telescope in space rather than on the ground?

    • No absorption of electromagnetic waves by the atmosphere

    • No light pollution or other sources of interference at ground level

    • No atmospheric effects, such as scattering or scintillation (twinkling)

  • Which wavelength ranges can ground-based telescopes observe, despite atmospheric absorption?

    • All visible wavelengths (although there is often some distortion)

    • Very narrow ranges of infrared wavelengths

    • Most microwave and radio wavelengths

  • State the approximate wavelength range of radio, infrared, ultraviolet and X-ray/gamma telescopes.

    • Radio: 1 mm to 10 m

    • Infrared: 700 nm to 1 mm

    • Ultraviolet: 10 nm to 400 nm

    • X-ray: 0.01 nm to 10 nm, gamma: less than 10 nm

  • Radio telescopes have a typical resolution of .......... rad.

    Radio telescopes have a typical resolution of 10-3 rad.

  • Give one structural difference between radio and optical telescopes.

    Radio telescopes use a single primary reflector, whereas optical telescopes use two mirrors; the radio dish also does not need to be as smooth as an optical mirror.

  • Why must the mirrors in infrared telescopes be kept very cold?

    To avoid interference from heat radiated by the surrounding equipment and structure, which would otherwise swamp the faint infrared signal.

  • Why must ultraviolet telescopes be located in space?

    All UV wavelengths are strongly absorbed by ozone in the atmosphere, so UV telescopes must be space-based; this makes them inconvenient to maintain.

  • Name objects that X-ray and gamma-ray telescopes can detect that are not visible at other wavelengths.

    • Neutron stars

    • Black holes

    • Pulsars

    • Gamma-ray bursts (GRBs)

  • True or False?

    Radio telescopes have a higher resolving power than optical telescopes of a similar size.

    False.

    Radio waves have a much longer wavelength than optical waves, so radio telescopes have a much lower resolving power (~10-3 rad) than optical telescopes.

  • Define charge-coupled device (CCD).

    A CCD is a detector that is highly sensitive to photons; incident photons release electrons, the number of which is proportional to the intensity of the incident light, forming a digital image.

  • Define quantum efficiency (QE).

    Quantum efficiency is the percentage of incident photons that cause an electron to be released.

  • The quantum efficiency of a CCD is typically .......... %.

    The quantum efficiency of a CCD is typically 70–90 %.

  • Give the equation for quantum efficiency (QE).

    \text{QE} = \frac{\text{number of electrons produced per second}}{\text{number of photons absorbed per second}} \times 100\%

  • What is the typical resolution of a CCD, and how does this compare to the human eye?

    • CCD: typically about 10 μm

    • Human eye: typically about 100 μm

    • The smaller the pixel size, the better the resolution

  • Why does the better resolution of a CCD often not affect the final image observed through a telescope?

    The overall resolution of a telescope is usually limited by the diameter of the objective, not by the detector, so the CCD's resolution rarely makes a difference to the final image.

  • State three ways in which a CCD is more convenient to use than the human eye.

    • Exposure time and number of images captured can be easily adjusted

    • Information can be accessed remotely

    • Images can be stored and analysed digitally

    • CCDs can detect a wider range of wavelengths, including beyond the visible spectrum

  • True or False?

    Photographic film has a lower quantum efficiency than the human eye.

    False.

    Photographic film has a higher quantum efficiency (4–10%) than the human eye (1–4%), although both are far lower than a CCD (70–90%).

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