IF Period: T = 0.5 s [one-half second per cycle] QUESTION: What is the frequency of the electromagnetic wave illustrated below? A. 1.2x106 Hz, B. 1.2x1015 Hz, C. 6.0x1014 Hz  E 1000 nm 500 nm Example: IF Period: T = 0.5 s [one-half second per cycle] THEN Frequency:  = 1/(0.5 s) = 2.0 s-1 [two cycles per second] Wavelength,  = length of one cycle = 250 nm. SCRIPT: What is the frequency of the electromagnetic wave illustrated below? A. 1.2x106 Hz, B. 1.2x1015 Hz, C. 6.0x1014 Hz PAUSE Wavelength is easier to determine from the graph shown. We’ll determine wavelength first, then calculate frequency from that. CLICK Wavelength, which is usually represented by the greek letter lambda, is the length of on cycle and is also the distance traveled by the wave in one cycle. We can determine it from the graph by determining the distance between consecutive crests CLICK CLICK or the distance between consecutive troughs CLICK or the distance between two nodes that have one node between them CLICK Looking at the graph, we can see that the wavelength is 250 nanometers. HIGHLIGHT 250 nanometers. An electromagnetic wave is a traveling wave. It’s moving at a speed of 3.00x10^8 meters per second. HIGHLIGHT Remember that speed is distance traveled per unit time…. CLICK Therefore,… The speed of the wave c, HIGHLIGHT c Is equal to Distance traveled by the wave during one cycle HIGHLIGHT lambda in c=lambda/T Divided by the time it takes for the wave to go through one cycle. HIGHLIGHT T T, here, is the time it takes for the wave to go through one cycle, and is called the period…. We can rearrange the equation “c equals lambda over T” to solve for T. The period T, is equal to lambda over c. HIGHLIGHT equation The frequency of the wave is defined as the number of cycles that pass through a point per unit time. Frequency is usually represented by the greek letter nu, which looks like the letter v. HIGHLIGHT nu, CALLOUT “nu” BY definition, frequency is just the reciprocal of the period. For example, if it takes a one cycle to 0.5 seconds to pass a point, then we expect two cycles to pass every second. Therefore, since period is equal to lambda over c. Frequency, which is the reciprocal of the period, is equal to c over lambda. CLICK CONTINUED ON NEXT SLIDE EM wave is a traveling wave: c = speed 3.00x108 m/s Period, T = time it takes for one cycle to pass through a point. Therefore: c = /T, or T = /c Frequency: number of cycles that pass through a point per unit time:  = 1/T = c/.

QUESTION: What is the frequency of the electromagnetic wave with a wavelength of 250 nm? A. 1.2x106 Hz, B. 1.2x1015 Hz, C. 6.0x1014 Hz  = 1/T = c/ = (3.00x108 m/s) / (250x10-9 m) =1.2x1015 /s = 1.2x1015 s-1 = 1.2x1015 Hz SCRIPT: Earlier, we determined from the graph that the wavelength is 250 nanometers. The problem, them essentially becomes this: What is the frequency of the electromagnetic wave with a wavelength of 250 nanometers? CLICK So, we use the formula… The frequency, nu… HIGHLIGHT nu Is equal to The speed of the electromagnetic wave, c… HIGHLIGHT c Which is 3.00 x 10^7 meters per second HIGHLIGHT number Divided by the wavelength lambda HIGHLIGHT lambda Which is 250 nanometers. HIGHLIGHT 250x10^-9 m Note that a nanometer is 10^-9 meters. So 250 nanometers is 250x10^-9 meters. Punching these numbers into a calculator, we find that the frequency is 1.2x10^15 cycles per second. The unit is usually written as seconds raised to the negative one power, HIGHLIGHT s^-1 Or as Hertz HIGHLIGHT Hz CALLOUT “Hertz” pointing to Hz The correct answer is B. PAUSE CLICK END RECORDING.

Video ID: 2-1-4 © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08