REFLECTION AND REFRACTION
Part C: Images Formed by a Convex Lens/Concave Mirror and Focal Length of a Convex Lens/Concave Mirror
Objective
a) To describe images formed by a convex lens and a concave mirror;
b) To record changes in the image and image distance when the object distance is changed;
c) To determine focal length and the conditions under which light is focused at the focal point;
d) To determine the focal length of a convex lens and concave mirror.
Theory
You will see that when light comes from an object close to the lens or mirror, the distance of the image from the lens or mirror depends on how far the object is from the lens/mirror. The following diagram shows how an image of an object (that is not a very great distance from the lens) is formed using a simple convex lens. Note that the light rays coming from the object (the incident rays) are spreading out in different directions; not parallel, since this object is close to the lens.
The diagram below shows how an image is formed using a simple concave mirror.
When light comes from infinity (or very far away compared to the size of the lens/mirror), then the incident rays of light are essentially parallel. Whenever this is the case, the light is focused at a particular point called the focal point (at the point called “Focus” in the figure below). The distance of the focal point from the lens/mirror centre is called the focal length. Below is a diagram showing how parallel light is converged to a focus by a convex lens.
The focal point and focal length of a curved mirror are found the same way, with some obvious differences.
The focal length of a lens or mirror is a fixed property once the lens/mirror has been fabricated. It depends on the curvature of the surfaces and, for lenses, also on the material (index of refraction, n) from which the lens was made.
As stated above, when light comes from far away (i.e. from a distance that is large compared to the size of the lens/mirror itself), then the image is focused at the focal length of the lens/mirror.
Apparatus Reflection and Refraction simulation
Procedure
0) If you haven’t already, open the Reflection and Refraction simulation located at the following web address, and use the dropdown menu in the upper left-hand corner to change to Part C.
http://www.jjdebenedictis.com/Reflection-and-Refraction/
1) Place the lens in the middle of the track, but not too close to the object screen. Placing the lens too close to the object makes the image too large to fit on the screen – feel free to check this.
Now move the screen until the sharpest focus is obtained. Record the object distance and the image distance in a well-labelled table in your lab notebook.
2) In increments of roughly 10 cm at a time, move the lens further away from the object. Each time, adjust the position of the screen until you have a clear image. Record the object distance and the image distance each time.
What happens to the image distance as the object distance changes? How does its appearance (both size and orientation) change as the object distance changes?
3) Keep changing the object distance until you have covered the full length of the track. You should find that as the object distance gets larger, the image distance stops changing. Take enough data to show that you have found this (you can use increments less than 10cm if needed).
4) Determine the focal length of the lens.
5) Repeat using the concave mirror. In this case, the screen must go between the object and the mirror (place the screen low enough so that it doesn't block all the light coming from the object). The diagram below shows the placement of the mirror and screen.