What are some things you could say or do to convince a younger sibling that light is a form of energy that travels in straight lines?

Answer 1

Perhaps the most obvious first action to demonstrate that light has energy and travels in a straight line is to bring the person outside into the sunshine and point out the physical sense of warmth on the skin in bright sunlight. Step into a shaded area and there is a noticeable absence of the feeling of warmth.

The same process can be done with infrared light from a fireplace. Most fireplaces have a metal screen in front. (It is more convincing with a screen than a glass door on the fireplace. ) First, with the screen opened completely, sit close enough to the fire that you can feel the heat modestly on your skin. (You do not need to get real close; a few feet safely away works with a hot fire.) You then close the screen and that feeling decreases markedly. What is happening here is the the screen is reflecting the infrared and microwave light when it is closed. The visible light still gets through but the visible light is only a small part of the electromagnetic energy given off by the fire. You can tell that you are not blocking heated air currents by taking a small smoking ember and holding it near your observation point and seeing that the air currents do not change appreciably when the screen is closed. You may be able to get some directional sense of the radiation from the fire by moving to different locations and perhaps closing half the screen if that kind of manipulation is possible.

Of course, plants need energy to grow and that comes from the sun too, but the doubting student may raise arguments about plants growing in the shade.

If you want to get dramatic, go find a laser or at least a video of laser. The little pen light lasers give off too little energy, so you need one that might be used in a research lab. There are numerous examples of heating and blowing things up that you can find. An internet search should turn up many examples.

As a final thought, one might point out that it is wise to park your car in the shade on a hot sunny day. The car interior is heated by the greenhouse effect due to the sunlight entering through the glass. Two cars a few feet apart, one shade and one not, offer a fairly dramatic difference that is hard to explain without invoking the the energy in light and its directional character.