REFRACTION OF GLASS

PROBLEM: To calculate the index of refraction for glass.

MATERIALS: Four straight pins, piece of cardboard, protractor, a four inch by four or five inch square glass piece (preferable 1/4 inch thick; a good source would be the glass shelves that are inside a medicine cabinet) (or 1/4 inch thickness of plexiglass), paper.

PROCEDURE AND OBSERVATION:

1. Using cardboard backing, lay the square of glass in the exact center of a clean white sheet of paper. Using a sharp hard pencil, draw a clean outline around the glass square.

2. Remove the glass square, and draw a normal as shown in fig 1.

3. Draw a 20° angle of incidence with the normal ( the line forming the angle should be about 3 inches in length)

4. Place a pin at both ends of the line forming the angle of incidence ( pins A and B ).

5. Replace the glass square. Crouching down low enough, place the eye at table level; so that you will see the pins, through the edge of the glass square. Now take pin "C" and try to stick it on the other side of the glass square, when pins "A and B" look like one pin. That will only happen when pin "A" will be blocked by pin "B". Then place pin "D" so that all pins are lined up.

6. Remove the glass square and connect the holes left by "B", "C", and "D". Then measure the refraction angle formed by points "B" and "C".

7. For determining the index of refraction of the glass use the following formula;

SINE OF THE INCIDENT ANGLE
INDEX OF REFRACTION         = ---------------------------------------------
SINE OF THE REFRACTION ANGLE

8. Repeat steps # 3,4,5, and 6 using the following angles of incidence 25°, 30°, 35° and 40°.

Results

Angle of incidence

Sine of the incident angle

Angle of refraction

Sine of the refraction angle

Index of refraction

20 °

.3420

      

25 °

.4226

      

30 °

.5000

      

35 °

.5736

     

40 °

.6428

      

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Angle and Sine
Angle   Sine Angle   Sine Angle   Sine Angle   Sine Angle   Sine
0 °   .0000 11°   .1908 21°   .3584 31°   .5150 41°   .6561
1°   .01745 12 °   .2079 22 °   .3746 32 °   .5290 42 °   .6691
2 °   .03490 13 °   .2250 23 °   .3907 33 °   .5446 43 °   .6820
3 °   .05234 14 °   .2419 24 °   .4067 34 °   .5446 44 °   .6947
4 °   .06976 15 °   .2588 25 °   .4226 35 °   .5736 45 °   .7071
5 °   .08716 16 °   .2756 26 °   . 4384 36 °   .5878 46 °   .7193
6 °   . 10453 17 °   .2924 27 °   .4540 37 °   .6018 47 °   .7314
7 °   .12187 18 °   .3090 28 °   .4695 38 °   .6157 48 °   .7431
8 °   .13917 19 °   .3256 29 °   .4848 39 °   .6293 49 °   .7547
9 °   .15643 20 °   .3420 30 °   . 5000 40 °   .6428 50 °   .7660
10 °   .17360 51°   .7771
QUESTIONS:

1. What is a normal ?

2. At what angle can a beam of light strike a glass surface with out bending the beam of light ?

3. We see all objects by one of two ways; list them.

4. When a beam of light strikes a transparent object at an angle happens to the beam of light ?

5. What is the formula for finding the index of refraction ?

6. What does the index of refraction indicates ?

7. Define transparent. List three examples.

8. Define translucent. List three examples.

9. Define opaque. List three examples.

10. What is an angle of incidence ?

11. What is an angle of reflection ?

12. What is an angle of refraction ?

13. Compare rays striking a smooth surface and a rough surface. Explain what happens.

14. What will the angle of refraction be for a piece of glass, that has an index of refraction equal to 1.5 , when the incident ray forms a 30° angle ?

15. What is the relationship between the speed of light and angle of refraction ?

16. Complete the following diagrams.