As is shown in the pictures below it is easy to show students how radiation stretches on the surface of an inflating balloon.
Simply have them draw wavelengths (simple sine curves) onto the balloons with markers along with tick marks showing scale measured in millimeters. As they inflate the balloon in stages they can re-measure the wave from peak to peak to find the expansion rate and use the tick marks to help calibrate their scale.
Sometimes this balloon model is used to show the expanding universe by painting galaxies on with markers and inflating the balloon. This model is problematic for two reasons. First, it may confuse students since it is only expanding along the surface of the balloon and not actually three-dimensionally, even though the object is three-dimensional. Second, the galaxies in this model stretch unlike real galaxies which are subject to the (stronger) forces of gravity which hold them together so they do not expand with universal expansion.
You may want to mention the stretched radiation from the Big Bang in the form of the Cosmic Background Radiation, both because many students have heard of these terms in popular media and are interested in finding out about them and in preparation for future lessons (following) which discuss these concepts more fully. For more information on this information and an animated version of the balloon stretching activity see http://www.sns.ias.edu/~whu/physics/physics.html.
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