PHYSICS OPTICS. Mr Rishi Gopie

Transcription

1 OPTICS Mr Rishi Gopie

2 Ray Optics II Images formed by lens maybe real or virtual and may have different characteristics and locations that depend on: i) The type of lens involved, whether converging or diverging ii) The focal length, f, of the lens involved iii) The location of the object with respect to the lens, i.e. the object distance, u Consider the formations, locations and characteristics of images produced by lenses for various object locations: 1) Convex or Converging lens i) Object is at infinity (i.e. far away from the lens) Diag. 20 Page 2 of 11

3 ii) Object is just beyond 2F Diag. 21 Page 3 of 11

4 iii) Object is at 2F Diag. 22 Page 4 of 11

5 iv) Object is between 2F and F Diag. 23 v) Object is at F Diag. 24 Page 5 of 11

6 vi) Object is between F and the lens (i.e. within f) Diag. 25 Note that as the object approaches the lens, the image moves further away from the lens on the opposite side of the lens (except when the object is within the focal length). Note also that the object and image positions can always be interchanged by applying the principle of reversibility of light. 2) Concave or Diverging lens. For all locations of the object Page 6 of 11

7 Diag. 26 Experiments to determine f for a converging lens i) Accurate method Diag. 27 Page 7 of 11

8 Consider the object and image finally formed: ii) Diag. 28 Approximate method Diag. 29 Page 8 of 11

9 Defects of lenses include: 1) Spherical aberration due to the curved surfaces of a lens bringing incident light rays to more than one focus (because of differing curvatures) and therefore producing several images, close to one another and overlapping one another, which combine to give a single blurred image. 2) Chromatic aberration due to each colour in incident polychromatic light (i.e. light consisting of more than one colour being brought to a different focus by the lens and therefore producing several images close to one another and overlapping one another (each of a different colour), which combine to produce a single, blurred, coloured image. Page 9 of 11

10 Dispersion of Light Dispersion is the breaking up of polychromatic light (i.e. light make- up of more than one colour of wavelength, e.g. white light) into its component colours due to refraction (or diffraction) of the light. Refraction can occur as the light passes through a triangular glass prism, for example. Diag. 30 Each colour in the incident polychromatic light is refracted by a different amount and so separation of the colours occur. For instance, red light is refracted the least and violet light is refracted the most (of the rainbow colours ROYGBIV). A pure spectrum (i.e. one in which each colour is more distinct as there is less overlapping of colours) can be obtained by employing the use of two converging lenses (one before and one after the prism with the source of light in the focal plane of the one before the prism and the screen in the focal plane of the one after the prism). Newton investigated dispersion by making two hypotheses and testing them. These hypotheses are: 1) White light is polychromatic (i.e. a mixture of different colours of light) 2) The prism separates the colours since the refractive index of glass is different for each colour and so each is refracted to a different extent. T To test these hypotheses, newton performed two further experiments 1) He added a second prism and allowed the emergent component colours (from the first prism) to be incident on it. No more colours were produced and the spectrum (red to violet) simply became wider. Page 10 of 11

11 Diag. 31 Also he used the second prism in such a way that it was inverted relative to the first prism and the colours were recombined by the second prism showing that the colours did not originate from either prism. This confirmed his first hypotheses. 2) He only allowed one of the colours (emerging from the first prism to be incident on the second prism (i.e. in effect he used monochromatic light on the second prism). Again, no other colours appeared. This confirmed his second hypothesis. Diag. 32 Page 11 of 11