2.  If an object is beyond 2F, the image from a converging lens will be inverted, real, and smaller, and will always lie somewhere between F and 2F on the other side of the lens  As the object changes position, its image will change location, but will always be between F and 2F  Since you cannot move the film to the location of the image, you focus by moving the lenses so the image falls perfectly on the film

3.  A movie projector works almost opposite to a camera  A projector takes a small object (the film) and projects a large, inverted, real image on a screen  Because the image is larger than the object, the film must be located between F and 2F  Also, because the image is inverted, the film must be loaded into the projector upside down so the final image appears upright

4.  A magnifying glass is a converging lens  The object must be located between F and the lens, so no real image is produced – it is virtual  The refracted rays diverge, but the brain extends these rays backwards and produces an enlarged, upright, virtual image located on the same side of the lens as the object

5.  The earliest telescopes were designed by two of history’s first astronomers ◦ Galileo Galilei (1564-1642) ◦ Johannes Kepler (1571-1630)  Each had their own design, using different combinations of lenses ◦ Galileo’s had two lenses – one converging and one diverging ◦ Kepler’s used two converging lenses, but this also inverted the image (not a problem for astronomy, but annoying to use on the ground!)  Sir Isaac Newton later refined both designs using mirrors as well as lenses

6.  Two lenses in a telescope:  Light enters through the objective lens  Observer looks through the eyepiece  F 1 is the focal point of the objective lens, and F 2 is the focal point of the eyepiece  The two lenses are positioned so that the focal points to the right of both lenses are at the same place Galileo’s Design:

7.  Rays (shown in red) from a distant star or planet pass through the objective lens  If an eyepiece were not present, the objective lens would focus the image to the far right of the diagram  The rays reach the eyepiece before an image is formed, refracting the rays (shown in green) and creating a virtual, upright image between the two lenses Galileo’s Design:

8.  Kepler’s telescope used two converging lenses, positioned so that their focal points are at the same point between the lenses  Light from distant stars or planets (shown in red) enters through the objective lens and forms an image between the two lenses  The image formed by the objective lens becomes the object for the eyepiece lens Kepler’s Design:

9.  Light from the first image (shown in green) then passes through the eyepiece lens and forms a virtual image that appears to come from just beyond the objective lens  The final image formed is inverted and larger than the image from the objective lens Kepler’s Design:

10.  Galileo’s and Kepler’s designs suffered from a defect known as chromatic aberration, which affected focusing around the edges of the lenses  Newton overcame this problem by using a concave mirror as the objective  The objective in a telescope does not have to be a lens; it only has to be able to collect and focus light on a point  Today we call this a reflecting telescope; ones that use only lenses are called refracting telescopes Newton’s Design:

11.  Light enters the telescope and travels to the concave mirror objective  The mirror reflects the light towards focal point F1  Before the rays reach F1, a plane mirror reflects them to F2, the focal point of the eyepiece lens  Rays from F2 pass through the eyepiece, magnifying the image Newton’s Design:

12.  Similar to Kepler’s telescope design  Rays from the eyepiece (shown in red) pass through the objective lens, and the refracted rays form an inverted, real image between the lenses  Rays from the image (shown in green) pass through the eyepiece, which again refracts the rays, and then forms the final inverted, virtual image  The image produced by the objective lens basically becomes the object for the eyepiece