AQA · A-Level · Physics

    Course version: 7408 · Version from Sept 2016

    Relative merits of reflectors and refractors including a qualitative…

    Full specification wording

    Reflecting telescopes (A-level only): Relative merits of reflectors and refractors including a qualitative treatment of spherical and chromatic aberration.

    Refracting telescopes use a convex objective lens, which disperses white light because refractive index varies with wavelength.

    Checked against the AQA 7408 specification (opens in a new tab) on .

    • Point 3 of 3
    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Refracting telescopes use a convex objective lens, which disperses white light because refractive index varies with wavelength. Blue light focuses closer to the lens than red light, producing coloured fringes called chromatic aberration. A reflector uses a concave mirror, and reflection does not disperse light, so chromatic aberration is absent. However, spherical mirrors bring marginal parallel rays to a slightly different focus than paraxial rays, causing spherical aberration; this is reduced by using a parabolic mirror or an aperture stop. Large refractors also sag under their own weight and absorb some light, while large mirrors are easier to support and can be made larger. Refractors may need a long tube and have a practical aperture limit.

    What earns the marks

    • Chromatic aberration arises in refractors because the focal length of a lens depends on wavelength, so different colours focus at different points.
    • Reflectors avoid chromatic aberration because reflection at a mirror does not depend on wavelength.
    • Spherical aberration arises when a spherical mirror brings marginal rays to a different focus from paraxial rays.
    • Spherical aberration can be reduced by using a parabolic mirror or by restricting the aperture.
    • Large mirrors are easier to support than large lenses, which may sag, and mirrors can be made in larger apertures.
    • A refractor may absorb some light in the lens and requires a long tube, whereas a reflector can be more compact for a given aperture.

    Common mistakes and how to put them right

    • Claiming that mirrors suffer chromatic aberration.

      Put it right by stating that reflection does not disperse light, so chromatic aberration is absent in reflectors.

    • Confusing spherical aberration with chromatic aberration.

      Put it right by linking spherical aberration to mirror or lens shape and chromatic aberration to wavelength-dependent focal length.

    • Stating that a parabolic mirror removes all aberrations.

      Put it right by noting that it removes spherical aberration but other aberrations can remain.

    Exam tips

    • Compare reflectors and refractors point by point rather than describing each separately.
    • Use the terms chromatic aberration and spherical aberration precisely, linking each to its cause.
    • Support each merit or drawback with a brief physical reason, such as wavelength-dependent refraction or support of a large mirror.

    Try the lesson

    The start of the Reflecting telescopes (A-level only): Relative merits of reflectors and refractors including a qualitative treatment of spherical and chromatic aberration. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which advantage does a reflecting telescope have over a simple refracting telescope of similar aperture?

    Listen

    Refracting telescopes use a convex objective lens, which disperses white light because refractive index varies with wavelength. Blue light focuses closer to the lens than red light, producing coloured fringes called chromatic aberration. A reflector uses a concave mirror, and reflection does not disperse light, so chromatic aberration is absent. However, spherical mirrors bring marginal parallel rays to a slightly different focus than paraxial rays, causing spherical aberration; this is reduced by using a parabolic mirror or an aperture stop. Large refractors also sag under their own weight and absorb some light, while large mirrors are easier to support and can be made larger. Refractors may need a long tube and have a practical aperture limit. A strong answer earns marks for these points. Chromatic aberration arises in refractors because the focal length of a lens depends on wavelength, so different colours focus at different points. Reflectors avoid chromatic aberration because reflection at a mirror does not depend on wavelength. Spherical aberration arises when a spherical mirror brings marginal rays to a different focus from paraxial rays. Spherical aberration can be reduced by using a parabolic mirror or by restricting the aperture. A common slip: Claiming that mirrors suffer chromatic aberration. Instead, stating that reflection does not disperse light, so chromatic aberration is absent in reflectors. Exam tip: Compare reflectors and refractors point by point rather than describing each separately.

    See it

    A top-down flowchart starting from a node labelled Telescope, which splits into two branches. The left branch is Refractor: convex objective lens, leading to Refractive index varies with wavelength, then Blue focuses closer than red, then Chromatic aberration: coloured fringes. The right branch is Reflector: concave mirror, which splits into two paths. One path goes to Reflection does not disperse light, then No chromatic aberration. The other path goes to Marginal and paraxial rays focus differently, then Spherical aberration, then Reduced by parabolic mirror or aperture stop.
    It shows how refractors and reflectors differ in the aberrations they suffer, with chromatic aberration arising from wavelength-dependent refraction in a lens and spherical aberration from the shape of a mirror.

    Key words

    Say what each word means before you turn the card.

    Chromatic aberration

    Try answering aloud or in your head before turning the card.

    Spherical aberration

    Try answering aloud or in your head before turning the card.

    Parabolic mirror

    Try answering aloud or in your head before turning the card.

    Aperture stop

    Try answering aloud or in your head before turning the card.

    See every definition
    Chromatic aberration
    Arises in refractors because the focal length of a lens depends on wavelength, so different colours focus at different points.
    Spherical aberration
    Arises when a spherical mirror brings marginal rays to a different focus from paraxial rays.
    Parabolic mirror
    Reduces spherical aberration, which arises when a spherical mirror brings marginal rays to a different focus from paraxial rays.
    Aperture stop
    Reduces spherical aberration, which arises when a spherical mirror brings marginal rays to a different focus from paraxial rays.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart starting from a node labelled Telescope, which splits into two branches. The left branch is Refractor: convex objective lens, leading to Refractive index varies with wavelength, then Blue focuses closer than red, then Chromatic aberration: coloured fringes. The right branch is Reflector: concave mirror, which splits into two paths. One path goes to Reflection does not disperse light, then No chromatic aberration. The other path goes to Marginal and paraxial rays focus differently, then Spherical aberration, then Reduced by parabolic mirror or aperture stop.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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