General and practical optics / by Lionel Laurance.

  • Laurance, Lionel.
Date:
[1908]
Catalogue details

Licence: In copyright

Credit: General and practical optics / by Lionel Laurance. Source: Wellcome Collection.

Provider: This material has been provided by UCL Library Services. The original may be consulted at UCL (University College London)

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    In Fig. 23 k B is an object placed in front of the mirror M M. Rays diverging from A, after reflection, enter the eye E, and are projected to a virtual focus at A', from which point they appear to diverge. Those from B are projected to B' and A' IV is tlie image of A B. A' is apparently as far behind M M as A is in front of it; so also B and B' are equally distant from M M. The complete image is erect and corresponds exactly as regards shape, distance, and size to the object itself, the relative directions of tlie rays from each point on the object being unchanged by the reflection. Lateral Displacement by Reflection.—The image is, however, laterally displaced, the right hand of a person becomes the left of his image in the mirror and vice versa. If the eye regards A B (Fig. 23) directly, A is to the right of A B, but looking into the mirror A' is seen to the left of A' B'. If the top of a page of printed matter be held obliquely down- wards against the mirror the letters will be in the same order from the left to right, only they will be upside down, and at the same angle to the mirror as the page, thus resembling a case of type. Engravers sometimes use a mirror in front of the letters or objects they wish to draw on a wood-l)lock and copy the image they see in the mirror. On taking an impression of the block the letters or objects arc in their riglit position. Distance of Imag-e. —If a person stands at, say 2 feet, in front of a looking-glass and looks into it he sees an image of himself at a distance of 4 feet. The light has travelled 2 feet to the mirror and then 2 feet to his eyes, and is mentally projected backwards through a distance of 4 feet. If an object is placed in contact with a glass mirror its image appears behind the silvered surface, and only twice the thickness of the glass itself separates object and image. The image appears rather nearer owing to vertical dis- placement by refraction. If the mirror is of polished metal the two are in contact. Fig. 25. Position of Imag-e.—Since the angle O C M, between the mirror M and the object 0 C (Fig. 24), and the angle I C M, between the mirror and the image C I, are e(|ual, it follows that the angle 0 C I between the object and the image is twice as large as either; so if the mirror be placed at an angle of 45° with the object, the object and image are at right angles to each other, as is shown in Fig. 25.