Question

Part A: Investigation of lens behaviors using a distant object

Go to a room that has an outside window. With only the windows cracked open and all other lights off, use the reading glasses (or, more easily, the magnifying glass) and see if you can capture the light of something outside the window (perhaps a tree, or a light post or anything you can see outside the window) and “project” the captured light on to a screen (this screen can be made using a white clean sheet of paper). Move the glasses (or the magnifying glass) back a forth, until the projection appears in focus.

Let’s summarize the terminology:

Object Distance is defined to be the distance from the lens to a bright object. We use do to represent object distance in equations.

Image Distance is defined to be the distance from the lens to the focused image produced by the lens of a bright object. We use di to represent image distance in equations.

Focal Length is defined to be the distance to the image that the lens produces of a very distant object (your tree outside the window, for instance). The focal length is a property of a given lens. We use f to represent the focal length in equations. You try to measure (at least have an idea how big it it) of the focal length of the reading glasses (or of the magnifying glass). Write it here:

f = ________________________________

You will not be able to measure the focal length for the glasses of your nearsighted friend.

This because those glasses are diverging (and that is why, looking through them, you should have seen the print getting smaller).

The Lens Equation: The lens equation gives the approximate relationship between these three quantities. It is given below:

(1) In the case of the tree outside, do is very large, so 1/do is very small and = 0, so we get that 1/f = 1/di or di = f (this is what you did when you estimated the focal length before.

Answer 1

Generally focal length of the lenses that we used for practical purposes vary and have values 15 cm, 20 cm, 25 cm. Although while doing experiment (calculating focal length experimentally) we don't get the exact value like you are using the lens of focal length of 15 cm you may get exoerimentally as 15.5 cm or 16 cm or 16.5 cm. Therefore the focal length is in cm range and generally varies from 15 cm to 25 cm.

f = +20.3 cm