Home Lab 15 Diffraction Gratings

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1 Home Lab Lab 15 Diffraction Gratings Home Lab 15 Diffraction Gratings Activity 15 1: Diffraction from a Transmission Grating Objective: To accurately measure the wavelength of light from a laser pointer using a diffraction grating. Materials Included: Diffraction grating Optical Bench and accessories Laser pointer Materials Provided by You: Ruler Background: We have already used the diffraction grating in Lab06 but we used it as tool and did not study how it works. In this Lab we will do an activity to help us understand how it works. The diffraction grating consist of thousands of small parallel lines ruled on a piece of transparent material such as plastic. The one in your kit has about 4545 lines per cm. The value for d is the inverse of this number. Watch your units. Procedure: 1. Place the laser pointer on the wooden platform on the optical bench and hold it in position with two clothespins. One clothespin will also be used to keep the laser on but don t turn it on yet or you will kill the batteries. (Need new laser ponters) 2. Place the diffraction grating in the U- shaped black holder and clamp it in position on the optical bench as close as possible to the laser pointer. 3. Place the entire optical bench on a table and direct the laser beam through the diffraction grating without the glass rod attached at a large blank wall. The wall should be about 2 meters away from the diffraction grating. You should observe a few red spots on the wall. Describe in detail the spots on the wall and explain how they relate to the integer m in the formula, dsin =m described in the lecture notes for Lab Record the number d and measure the distance to the wall L and record it the Table Measure the distance y between the m = +1 and m = -1 red spots. Divide the number by 2 and record it in the Table PHYS 6251, PHYS 6252, PHYS

2 6. Calculate L 2 y m 2 and record it in Tale Using the distance to the wall L and the spacing between the m = +1 and -1 red spots calculate sin and record it in Table sin m 1 2 (y y ) m m L 2 y 2 m where m=0, 1, 2, etc 8. Calculate the wavelength of the light from d sin m and record it in Table m 9. Repeat steps 5, 6, 7, and 8 for m=2 and Compute the average value of the wavelength and record in Table m d L y +m + y -m 1/2(y +m + y -m ) L 2 y m 2 Sin m Ave = 11) How well does your average of the measured value agrees with the accepted value of the wavelength for this laser pointer, which I am assuming to be 660 nm. PHYS 6251, PHYS 6252, PHYS

3 12) For each m (exclude m=0) calculate how much each measurement differs from the average value of the wavelength. From these differences, comment on the preciseness of your measurement. PHYS 6251, PHYS 6252, PHYS

4 Activity 15 2: Reflective diffraction from a compact disk (CD) Objective: To observe and describe diffraction produced when white light strikes a CD Materials Provided by You: One Compact disk (CD) A small flashlight Adhesive Tape Procedure: 1. Tape a CD to a wall at about eye level. 2. Turn on your flashlight and place the light near your eye pointing toward the CD (as shown below). 3. Stand directly in front of the CD (eye about.5 meters away) and shine the light directly at the hole in center of the disk (you should be able to see the reflection of the light in the CD). 4. Slowly move toward the CD with your eye while keeping the reflection of the shining light in the center of the CD hole. As you move closer, circles of colors should be seen on the CD. 5. Move as close as you can to the CD. Now move back and forth to get a sense of the pattern of colors. 6. As you move from far away toward the CD What is the first color that appeared? What part of the CD did the ring of color first appear (center, edge, etc.)? 7. As you move even closer What is the last color in the spectrum (rainbow) you observed? PHYS 6251, PHYS 6252, PHYS

5 List all the colors you observe first to last: 8. As you move very close to the CD describe the pattern you observe: Activity 15 3: Determination of track separation distance on a compact disk (CD) Objective: To determine the distance between recording tracks on a CD The recording tracks of an audio or data CD have regular spacing distances and since the tracks make a reflective diffraction grating we can determine the distance between the tracks. Magnification of recording tracks on audio CD. Materials Provided by You: One Compact disk (CD) (a DVD cannot be used) Laser Pointer 2 meter sticks Materials Provided: Procedure: 1. Place 2 meter sticks in a T shape on a table or on the floor (as shown below). 2. Place an upright compact disk at the base of the T and a Laser Pointer, pointing at the CD anywhere along the line shown below. PHYS 6251, PHYS 6252, PHYS

6 3. Arrange the Laser and CD so that the Laser beam strikes the CD and reflects the beam back along a line parallel to the original beam. It is not important how far the Laser is from the CD, but the CD should be exactly 1 meter from the top of the T stick. 4. The beam must strike the part of the CD that is parallel to the table so that the central beam and first order interference fringes reflect back to from a line parallel to the ruler that makes the top part of the T (see picture below). Reflect of Laser beam back to ruler showing reflection of central beam and maxima at n = When the reflected beam and interference pattern can be seen on the ruler (as shown above) you are ready to make measurements. The distance from the ruler to the CD (L) should equal one meter. Next measure the distance from the central spot to the fringes on both sides (x). Calculate the angle, (tan = x/l). Record your measurements in the table below. Raw Data L (cm) x 1 (cm) x 2 (cm) x ave (cm) ( º) 6. Now you are ready to calculate the distance between tracts by using. Calculate d for m =1 (assuming or ) show your work. PHYS 6251, PHYS 6252, PHYS

7 Now determine your % error using: (the known value for or show your calculation. PHYS 6251, PHYS 6252, PHYS