P. J. Ouseph

University of Louisville

 

Physics Department

Louisville, KY  40292

502-852-0918

pjouse01@louisville.edu

 

 

 


CD Rainbows

 

Abstract (50-75 words)

This demonstration shows rainbow like spectra produced by a clear CD which is a ÔprotectiveÕ CD with data spirals on it but no dye or coating. When light falls on a section of a CD at an angle of incidence of about 600 interference patterns on the reflected and transmitted sides can be seen. Circular spectra can be seen on a poster board when light falls normally on a CD kept in front of the poster board.

 

Construction of Apparatus:

 

Construction of this apparatus is rather simple. It involves making a CD holder. A clear CD with data spiral but with no coating, found in some CD packages is used for this demonstration. These CDs have spirals spaced 1.6 µm apart and therefore, they make good gratings. The CD is then glued to a rectangular (13.7 cm x 30 cm x 6 mm) black plastic with a central hole of 11-cm diameter. The black plastic is fixed to an aluminum base. To reduce the light falling on the poster board a black paper is glued to the center of the disc covering the clear central part of the disc without the data spirals.  Pasco light source (model SF-9366) is used in these demonstrations. In trying these light sources we found the focal lengths of the lenses vary from source to source. To obtain a broad light beam we added a divergent lens to one of the light sources. Using this system spectacular spectra can displayed on poster boards or white boards in classrooms.

Use of Apparatus: 

The CD rainbow demonstration is appropriate for high school and college physics classes. The CD spectra may be shown on a white board in properly darkened classrooms or on poster boards. Two types of spectra can be demonstrated with the apparatus:

 

1.    Circular Spectra

    

         Place the CD about 40 cm in front of the white board or poster board and illuminate the

         CD with light from a Pasco light source. The plane of the CD and the plane of the board are   

         parallel in this set up. Circular spectra seen are shown below.

                             

                              File written by Adobe Photoshop¨ 4.0   

 

      Fig.1. Circular spectra produced by white light falling normally on a CD.

 

2.    Spectra of light falling on CD at large angle of incidence 

 

             For this demonstration the center of the CD is at a distance of 10 cm from the poster board and the plane of the CD is 900 to the plane of the board. Light is falling on the CD at an angle of incidence of about 600 and less than half of the CD close to the board is illuminated. Rainbows can be seen on the transmitted light side as well as on the reflected side. In addition, two rainbows, one close to the CD and the other away from the CD, are seen on both sides.

 

 

              File written by Adobe Photoshop¨ 4.0

 

             Fig.2. Spectra produced by light falling on a CD at large angle of incidence.

 

 

              The line drawing illustrates the positions of the rainbows.

 

 

 

                          

 

 

                  Fig.3. Line drawing illustrates the positions of spectra of different orders on the

                              transmitted light (right) side reflected light (left) side.