Filtering the rainbow    Presenter: Andy Jackson, Harrisonburg City Schools,        


Va. SOL:


PH.10  The student will investigate and understand that different frequencies and wavelengths in the electromagnetic spectrum are phenomena ranging from radio waves through visible light to gamma radiation. Key concepts include

a)     the properties and behaviors of radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays; and

b)    current applications based on the wave properties of each band.


National Standards:


Electromagnetic waves result when a charged object is accelerated or decelerated. Electromagnetic waves include radio waves (the longest wavelength), microwaves, infrared radiation (radiant heat), visible light, ultraviolet radiation, x-rays, and gamma rays. The energy of electromagnetic waves is carried in packets whose magnitude is inversely proportional to the wavelength.



Students will be introduced to the effect of colored filter on the spectrum of visible light.



Š      set of primary and secondary color filters

Š      1 overhead projector

Š      1 diffraction grating for projector

Š      two pieces of posterboard or file folder


Safety Considerations






Set up the overhead projector facing a white screen. Mount the diffraction grating over the top lens. Place two pieces of posterboard with smooth edges near each other in the center of the overhead projector’s stage. Darken the room. A clear visible spectrum will be projected. Adjust spacing of the slit between the two pieces of posterboard to maximize the clarity and brightness of the spectrum. Have students predict and observe what will happen when a red filter is placed over the slit. Proceed with the other colored filters – blue, yellow, green, etc. Students will note that some filters allow only the color of the filter through (Red and green) while for others this is mostly true (blue) and for others entirely untrue (yellow). Allow students to make suggestions about different combinations.






How the physics is demonstrated


This is a fun introduction into color and light. Students often can’t decide whether a red filter should block or transmit red light. Students can deduce that yellow light is the combination of red and green light after seeing red, green, and yellow filters demonstrated.


Construction  and Tips Regarding the Demonstration





Sources & References

Sheet of diffraction grating was purchased from Educational Innovations .


Filters were obtained from

Vincent lighting systems

Po box 931141

Cleveland, OH 44193


L106 primary red

L139 primary green

80S  R80 Primary Blue

010S L010 Medium yellow

113S L113 Magenta

115S L115 Peacock Blue