Chapter 28, Problem 39A

To determine

To Explain: The reason for not seeing a laser beam going across the room unless there is fog, chalk dust, or a mist in the air.

Explanation of Solution

Introduction: 

The human eye can see the objects when the light reflected from the object enters the eyes. The laser light that passes in a vacuum never reaches the eye because the light never scatters. 

Explanation: 

When traveling through the vacuum, laser light beam is invisible unless it strikes directly into the human eye. When the laser strikes on any surface and scatters then only the light is visible to the human eye. 

If the room is filled with fog, chalk dust or mist in the air, the human eye can see the laser beam because the fog or chalk dust scatters/reflects the light. If laser light can’t hit anything, there is nothing that will reach the eye. 

Conclusion:

The human eye can’t see the laser light until it hits and reflects from any medium.