The Doppler effect applies to both sound and light waves. 

The focal length of a lens is the distance from the lens to the image.  

Ultrasound frequencies are below 20 kHz and can be heard by humans.  

Alternating current (AC) is used in homes because it is safer than direct current (DC).  

A thermos bottle minimizes heat transfer by radiation.

Significant figures are used to express the precision of measurements.

Water expands when heated at all temperatures.

Objects in free fall accelerate at different rates independent of their mass.

A hypothesis is a proven scientific fact.

The speed of sound increases as temperature decreases in air.