cahier physique - physics workbook

DISCUSSION

word bank: stronger, index of refraction, focal length,image distance, object distance, magnification, focal point, shape
A convex lens can be used as a "burning glass" by moving the lens back and forth until the sunlight is focused into a small bright spot - an image of the sun. This image is hot enough to scorch the paper, perhaps setting it on fire. The lens is moved back and forth to refract the parallel rays of light from the sun to the point where the image is formed on the paper. THe place where the image forms is called the ________ _________ of the lens. The distance from the focal point to the lens is called the _________ __________. The ________ ________ is determined by its _______ ___ ______ and the _______of the lens. The _______ ________ is an indication of the refraction ability, or strength, of a lens. A lens with short focal length is considered to be _________ lens than one with a longer focal length.

There are three important measurements that are used to describe how lenses work as optical devices. There are (1) the _____ ______ (f), (2) the__________ __________(di), the distance from the lens to the image formed, and (3) the ________ __________ (do), the distance from the object being imaged to the lens. The relationship between these measurements is given in the lens equation, which is:

1/f = 1/do + 1/di

The ____________ produced by a lens is defined as the ratio of the height of the image (hi) to the height of the object (ho).
THis is also equal to the ratio of the image distance (di) to the object distance (do), or:

Magnification = - di/do

In this investigation (PART I)you will compare measuring the focal length of a lens directly with using the lens equation equation to calculate the focal length of a convex lens. Magnification of a lens will also be investigated by comparing direct measurement of magnification with theoretical magnification as calculated from the focal length .
In PART II you will try to build a telescope.

PROCEDURE

Step1: Measure the focal length of a lens directly. FIrst, place a lens in a lens holder and secure it t the 50cm mark of a meter stick. Second, point the meter stick at some distant object such as a tree about a block away. Move a cardboard screen in a holder back and forth until you obtain a harp image on the screen. Finally, measure the distance between the sharp image and the lens, which is the focal length (f) of the lens. Record the focal length in TABLE 1. see figure above

Step2: Find the focal length of the lens used in step1 by use of the lens equation. Set up a meter stick in holders, a screen in a holder, and a luminous object in a holder (usually a lamp). See figure below. With the screen and the object fixed in place, slowly move the lens along the meter stick to obtain the sharpest image possible. Note that the object and image should lie on a straight line along, and perpendicular to the principal axis of the lens. Measure and record in TABLE1 the object distance (do) and the image distance (di) to the nearest 1 mm. Measure and record to the nearest 0.5mm the height of the object (ho) and height of the image (hi). Record other observations here:

step3: Now place the lens less than one focal length from the bulb (use the value found in step1), then move the screen back and forth to see if you can obtain an image on the screen. Remove the screen and l ook through the lens at the bulb. Can you see the image? THis is called a virtual image, it can't be recorded on a screen but you can see it. (your eye has a lens built in and will use the virtual image as an object. A real image is then formed on your retina (you screen).
REcord Observations:

step4: REpeat step2 to step 3 for several lenses. REcord everything in TABLE 1.

TABLE1:

LENS	from step1: focal length (cm)	from step2:do object distance (cm)	from step2: di image distance (cm)	from step2: ho Object size (cm)	from step2: hi Object size (cm)	focal length from lens equation (cm)	Magnification - hi/ho
1
2
3

ANALYSIS

1) Find the focal length using the lens equation. You don't need to convert. 1/f = 1/do + 1/j
Did you find a close value in step1 ? FInd the % difference.

2) If available , ask your instructor the true focal length ftrue = ________________. Find the % error for both methods (direct measuring and lens equation). WHich method gives the best result ?
%error method 1 = _____________
%error method 2 = __________________

3) Discuss the advantages, disavantages, and possible sources of errors in the 2 ways of finding the focal length of lenses.

4) If you compute the magnification using the lens equation you get: M = - di/do = ___________.
Did you find that using hi/ho ? _________ What is your % error ? ____________
What there is negative sign in the formula?

PART II

Make a telescope by mounting a short a longer focal length lenses on a meter stick as shown in the figure. Focus the telescope by adjusting he shorter focal length lens until it magnifies the image from the longer length lens. Show the telescope to your instructor for full credit.
see figure first.

The object is far away. The image from the objective is real and inverted. You must set the distance between the lenses such as the real image forms between the center of the eyepiece and its focal length. The real image is now an object for the eyepiece. Because this object is place at a distance di < feuepiece, the image from the eyepiece will be virtual , erect and magnified. Only your eye can see it. The virtual image can't be seen on a screen.