Description:
Today, lenses are used in many optical instruments ranging from eyeglasses to the camera. The refracting optical telescope, refined by Galileo, is another important optical instrument that shows the usefulness of lenses in producing images. This lab allows us to explore how the telescope uses a combination of lenses to produce a magnified image.
Purpose:
One of the objectives for this lab is to understand the differences between real and virtual images and to be able to distinguish their characteristics. Another objective is to use what the previous labs have taught us about how real images are produced in order to build our own refracting telescope.
Preliminary Questions:
1) From the previous activity, "Real Images and the Thin Lens Equation, we observed that the lens must be in between the object and the screen in order to produce a real image.
2) From the previous activity, we also observed that the characteristics of real images is that they can be projected onto a screen and that rays of light pass through the location of a real image.
Set Up:
Investigating Virtual Images
1) By investigating the 15 cm double concave lens, we found that it wasn't possible to produce a real image like we were able to do with the convex lenses.
2) We found that it is possible to see an image (a virtual image) by looking "into" the lens. A virtual image is different from a real image in that it appears behind the lens and it originates from a location where light does not actually reach.
The Magnifying Lens
4) The image produced by a magnifying glass is virtual. This is because virtual images, like the images produced by a magnifying glass, appear to be behind the lens in a location though which rays of light don't actually pass.
5) The position where the image has maximum magnification while remaining in focus corresponds with the number written on it (10 cm) representing its focal length. Therefore, the position of where the image has maximum magnification while remaining in focus (at 10 cm) is at its focal length.
The Telescope
5) The image we saw when looking through the eyepiece was inverted, magnified, and virtual. We knew that it was virtual because the image was produced behind the lens in a location through which light rays don't actually pass.
6) The distance between the two lenses (30 cm) is special because it is the sum of the focal lengths (10 cm + 20 cm).
7) The image seen through the eyepiece was also virtual, inverted, and reduced.
8) By using two lenses of the same focal length at the special distance (the sum of the focal lengths), we found that the image was virtual, inverted and true to size.
We learned that, in a refracting telescope, a lens forms a real image of a distant object. This real image is then not projected onto a screen but into space to be examined by another lens. This second lens is the eyepiece and it acts as a magnifying lens, producing a virtual image. Therefore, when we look through a telescope, we are really seeing an image of an image.
Extension:
The Galilean telescope is the Galileo Galilei's original design of the refracting telescope. Designed in 1609, this telescope uses a convergent (convex) objective lens and a divergent (concave) eyepiece lens; the objective lens is the lens closest to the object while the eyepiece is the lens closest to the eye. The image produces by a Galilean telescope is magnified, upright, and virtual. The magnification of the telescope is determined by the ratio between the focal length of the objective lens and that of the eyepiece. The biggest disadvantage to the Galilean telescope is its small field of view, only allowing about one-fourth of the moon's surface to be seen at a time and making it usable only for magnifications of up to 30x.
Ray Diagram of Galilean Telescope:
