Four spherical refracting surfaces (a, b, c and d) are separating two media, as shown in the diagram below. Identify each of the following refracting surfaces as positive or negative retracting power. Assume if the radius of curvature of these surfaces is 15 cm. (4 marks) Type your answer here in the tex box below. Alternatively, you can upload photos or documents with hand-written answers in the file dropbox below the text box