One viewer has sent in his tip that he can "see" the studs on his wall for hanging pictures by just holding a lamp close to the wall.
It sounds fishy but it does work on some wall but not on others.
It will not work on freshly painted, or well insulated walls.
Actually he is not really seeing the stud at all but seeing a collection of dust that runs right over the stud.
All walls slowly get dirty. Colder walls get dirty faster because they tend to collect a bit of moisture, minute unnoticed condensation, that traps dust faster than the rest of the wall. Walls, or ceilings like many cathedral ceilings, have little or no insulation on the other side of the 2x3 or 2x4 structural member. This makes the drywall colder in front of the stud or ceiling joist than it is where there is insulation. Hence the dust mark develops over the stud.
Sometimes this dust, called ghosting, can be seen easily, but the use of a lamp makes it stand out.
Click on this link if you would like to see an animation explaining "Thermal Bridging".