Reflecting on light addition                        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 visible spectrum and through the demonstration be allowed to manipulate the colors to discover properties of color addition.



Š      3 small mirrors

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 come up front and use mirrors to “pick off” colors and reflect them to a common spot in the room. I find my white ceiling works best. You can clearly see the effects of adding the primary colors of light.






How the physics is demonstrated


This is a fun introduction into color and light. Many students do not know the difference between pigment addition and light addition. Any combination of colors that can be picked off can be added together.



Construction  and Tips Regarding the Demonstration





Sources & References

Sheet of diffraction grating and plexi mirrors were purchased from Educational Innovations .