1. Refraction, lenses and image formation In cameras, microscopes, telescopes and prescription glasses, images are being formed with the help of lenses, that is thin curved pieces of plastic or glass. 1.1 Convex and concave lenses The figure below shows you two different lenses: a plano-convex lens (with one planar and one convex surface) and a plano-concave lens (with one planar and one concave surface). The optical axis (axis of symmetry) is indicated by a dashed line, and for each lens three incident rays of green lights are shown (green arrows). The lenses are placed in air (refractive index: n = 1) and made of glass (refractive index: n' = 1.52). 1.1.a. Draw the path of each ray of light, for both lenses, paying special attention to what happens at each interface between air and glass. Are the lenses converging or di- verging? 1.1.b What is the name of the point the initially parallel light rays converge to or seem to diverge from? 1.2 Chromatic aberrations The refraction index of a material changes with the wavelength of the refracted light. 1.2.a. Look at the picture at the right, which shows white light refracted through a glass prism. Does the refractive index of glass increase or decrease with wavelength for visible light? Explain your reasoning. 1.2.b. Draw the paths for both a red and a blue horizontal collimated light beams arriv- ing on a plano-convex lens. 1.2.c. Explain how your drawing is related to the issue of chromatic aberrations.