Simple home experiment to show that light travels in a straight line 

The fact that light is a wave causes it to travel in straight lines. When light strikes specific obstructions, though, it might vary its route away from a straight line. Diffraction is a term used to describe this phenomenon. Since diffraction is such a minor effect, light is assumed to travel in a straight line. The concept of light travelling in a straight line is termed as rectilinear propagation of light. Here is a simple experiment to explain this concept to your kids.  

What you’ll need   

• Three old CDs 
• Thread of any colour of your choice 
• A candle 
• Cello tape 
• Six clips (the ones used for drying clothes would suffice) 

How-to: 
Get these on a table, call your kids and start the experiment. Here’s a step-by-step guide for them to follow.  

• First of all, you would need to make a stand for the CDs. Thus, begin by holding the CDs upright and then fix the clips. Ensure that the CDs remain upright or perpendicular to the plane of the table. Now, all the three CD stands are ready. 
• Next, align all the three CD stands in a straight line. Make sure that you align the CDs such that the holes of the three CDs lie in a straight line. 
• Take a thread and tie it straight in the middle of the table to be doubly sure that the holes of the CDs are actually forming a straight line. You can use cello tape to fix the thread on both sides of the table for convenience. 
• Now, place the CDs one after another on the thread. All the CDs would appear in a perfect straight line as desired. 
• Take the candle, light it and place it at one end of the CDs. You can also place it on the thread. The tip of the candle and the holes of the CDs must lie in a straight line. 
• The height of the candle and the height of each CD hole should be the same. Try to observe the flame of the candle from the other side.  
• Your line of sight should be the same as the straight line through which light is passing via the holes. You can see the flame of the candle now because light travels through these CD holes and reaches your eyes. 
• Now, displace the CD in the middle of the stack a bit. You are unable to see the flame now because the light got blocked and is unable to reach you. 

What exactly happened in this experiment? 
Since light always travels in a straight line, the flame of the candle couldn’t be seen when the middle CD was removed. This is how the experiment proves that light can pass through small holes and apertures and change direction, but it always travels in straight lines.