(a) Explain, using Abbe's theory of image formation, the importance of the objective lens. [5 marks] (b) Below, is an expensive 'apochromatic' objective lens which corrects almost completely for all aberrations (left) and a cheaper lens (right). (i) With suitable sketches, explain two aberrations corrected in the apochromatic objective lens. (ii) Estimate the lateral resolution of both lenses. (iii) Estimate the depth of field of both lenses. (iv) What is the meaning of the 0.17,160 and symbols written on the lens? [12 marks] (c) (i) Describe the theory behind the contrast technique of dark field microscopy. (ii) How is this technique achieved for transmitted light microscopy? [6 marks] (d) (i) How may a microscope be modified to achieve polarized light contrast? (ii) Which materials, with examples, are suitable for examination with polarized light microscopy? [5 marks]