Galileo’s telescope consisted of two lenses, an objective and an eyepiece. Unlike the telescope in Figure 26.12, however, Galileo used a diverging lens as his eyepiece (Fig. P26.30). This telescope was designed for the object (i.e., the Moon) to be very far away (at infinity) and produced an image that was also at infinity. In contrast to the case with a converging lens for an eyepiece, Galileo’s telescope produced an upright image. 

(a) Construct a ray diagram using Figure P26.30, showing how an object very far to the left will produce an upright image that is also very far to the left. For an object at infinity, the objective lens in Galileo’s telescope produces an image to the right of the eyepiece. That image then acts as the object for the eyepiece lens. Figure P26.30 shows how two rays produced by the objective lens form an image at the focal point F₂ of the eyepiece. That point is also a focal point F₁ of the objective lens. Draw in additional rays from the objective lens to help you find the rays produced by the eyepiece. 

(b) Suppose f_obj = 1.5 m and f_eyepiece = 21.0 cm. What is the approximate total length of the telescope? 

(c) What is the total magnification? The magnification of Galileo’s telescope is given by the same expression as found for the refracting telescope in Figure 26.12.