You have an open air column of length 1.2 m in air at 20°C. Calculate the frequency of the second harmonic.

What are similarities and differences between refraction, reflection, diffraction and absorption?

Give an example in which the electrical energy changes to light energy

Aresistor with an unknown resistance is connected in parallel to a 9 ω resistor. when both resistors are connected in parallel to an emf source of 21 v, the current through the unknown resistor is measured with an ammeter to be 4 a. what is the resistance of the unknown resistor? answer in units of ω.