cahier physique - physics workbook

DISCUSSION

word bank: stronger, index of refaction, focal lenght,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 decribe 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 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 .

PROCEDURE

step1:

step3: Bring the cardboard, and glass plate near the edge of the table so you can sight through the glass plate toward the two pins. Position a ruler so that one edge aligns with the 2 pins as shown. Draw a line along the ruler and label the line B'R. Move the glass plate aside for a second time.

step4: Draw a line B to B ' , showing the path of the light ray through the glass. Overall, the path of the light ray is from IB to BB'to B'R, showing that the light ray was bent twice.

step5: Draw normals to the surface of the glass at B and B'. Show the angle of incidence and the angle of refraction with curved arrows at both boundaries.

step6: Find the angles of incidence and refraction for both boundary. The angle are from the normals. Use a protractor.

angle of incidence 1 : air/glass	angle of refraction1 : air/glass	Angle of incidence 2: glass/air	angle of refraction 2: glass/air

ANALYSIS

1) Describe what happens to a light ray as it travels (a) from air to glass and (b) from glass to air.

2) Light travels faster through air than it does through glass, make a generalized statement about what happens to a light ray with respect to the normal as it moves from a faster speed in one material to a slower speed in another.

3) You will learn that there is a law (called Snell's law) that relates the angle of incidence to the angle of refraction
from air to glass: sin(i) = n sin (r). i is the angle of incidence. r is the angle of refraction. n is the index of refraction and it only depends on the medium the light has to go through. n is greater than 1. Greater n is, more bent the light will be. The index of refraction of glass is usually around 1.5. Find the index of refraction by using the first 2 columns of the TABLE1. If you did a good job, you should find a value close to 1.5.
sin(i) = n sin (r), solve for n.

4) Was the purpose of this lab accomplished? Why or why not ?