GCSE Physics P7 (OCR) - Light, Telescopes, and Images

Flashcards by Josh Price, updated more than 1 year ago
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GCSE Physics (P7) Flashcards on GCSE Physics P7 (OCR) - Light, Telescopes, and Images, created by Josh Price on 04/23/2015.

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 Question Answer What is the speed of waves affected by? The medium they are travelling through. How does refraction lead to the formation of an image by a convex/converging lense? Because the lense is fatter in the middle than at the edges, the bends at different speeds. The light which enters at the bottom, crosses to the top. Will a more powerful lens have a curvy surface of a less curvy surface? More curvy because a lense with a short focal length bends the rays of light more. What is the equation for calculating the power (in dioptres) of a lens? 1 --------------------------- focal length (m) Why does light from astronomical objects reach Earth as effectively parallel sets of rays? Because the objects are so far away. What are the features of a simple optical telescope? They have two converging lenses of different powers, with the more powerful lens as the eyepiece. What are the optical elements of a telescope? a) An objective lens or mirror to collect light from the object being observed and form an image of it. b) An eyepiece which produces a magnified image of the image from the object that is viewed. What is the equation for calculating the angular magnification of a telescope? Focal length of objective lens ----------------------------------------------------- Focal length of eyepiece lens Why do most astronomical telescopes have concave mirrors, not converging lenses, as their objectives? - A mirror reflects rays of all colours in exactly the same way. - Its weight can be supported from the back as well as the sides, so that it doesn't sag. - A mirror can be made very smooth so that the image is not distorted. - Reflectors can be made to focus most types of electromagnetic radiation. How do concave mirrors bring a parallel beam of light to a focus? The angle of incidence equals the angle of reflection, so rays parallel to the axis of the reflector are reflected to the focus. Why are large telescopes needed to collect weak radiation from very distant sources? Because the larger the telescope, the more radiation can be detected. This radiation can help astronomers to find out more about its source. What is diffraction? The effect of waves spreading out from a narrow gap. When doing so, they curve a little at their edges. How can diffraction be increased? - By increasing the wavelength of the wave. - By narrowing the aperture (gap). Why does the aperture of a telescope have to be very large? Because radiation is diffracted by the aperture of a telescope. The aperture must be very much larger than the wavelength of the radiation detected to produce sharp images. How can a spectrum be produced by refraction in a prism? Because the colour of light depends upon its frequency (and therefore wavelength). Red has the lowest frequency, and violet has the highest (shortest wavelength). Dispersion happens through a prism because the light travels through the glass at different speeds, which splits up the light so that different colours are refracted through different angles. How else can a spectrum be produced? By a diffraction grating, which is a set of very narrow evenly spaced parallel lines ruled on a thin sheet of glass. When light shines on the grating, different colours emerge at different angles to produce several spectra.

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