Idon't understand how to find the frequency and period of wavelengths. can anyone me?

Acamcorder has a power rating of 20 watts. if the output voltage from its battery is 9 volts, what current does it use?

Which of the following is part of the explanation or description of electromagnetic waves? (select all that apply) a. electrically charged molecules in a medium transfer their energy to nearby electrically charged molecules. b. a changing electric field causes a changing magnetic field. c. magnetic molecules in a medium transfer their energy to nearby magnetic molecules. d. a changing magnetic field causes a changing electric field.

Why should the wavelength scale of spectroscope be calibrated