### 1. Unit analysis

Sometimes you want to to express a measure in different units. For example, when talking about how far away something is, occasionally it might be helpful to say it is a certain DISTANCE (New York is 300 miles from here), and also sometimes it is more useful to usage TIME to express how far away it is (New York is a 6 hour journey from here). Of food miles space not equal to hours, for this reason there should be some method to transform from one come the other. In this case, the switch is speed: if a vehicle drives an mean of 50 miles/hour, then it have the right to drive 300 mile in 6 hours. Because that this consistent speed, 300 miles equates to 6 hours. Problem 1. If friend walk in ~ a rate of 4 miles an hour, and your friend stays two miles away, how much away is she house a. In miles b. In minutes, if you space walking c. In minutes, if you space driving in ~ an median speed of 25 mile an hourIn much the exact same way, various units can be used to characterize light. We have the right to refer to irradiate by the wavelength, the frequency, or that energy. This is comparable to talking about distance in systems of mile or hours.

### 2. Wavelength --> Frequency

Light waves take trip at a consistent speed. Therefore there is a one to onerelationship in between light"s wavelength and its frequency. If waves are short,there need to be more of lock in a collection amount of time to take trip the exact same distancein that time (the exact same speed). Problems 1. The rate of light is 186,000 miles per second. What is the frequency of light that has actually a wavelength of 3 feet? 2 inches? 1/1,000,000 inches? one mile? 2. What is the wavelength of the radio waves of her favorite radio station? (HINT: the frequency of radio station is equal to the station number times 1,000,000 Hz. So WAMU - nationwide Public Radio - in ~ FM 88.5 - is 88,500,000 Hz. Now, use the reality that the wavelength is same to the rate of light, a constant, divided by frequency.)

### 3. Frequency --> Energy

In 1900, Planck uncovered that there to be a direct relationship between aphoton"s frequency and also its energy:E = h nu

The higher the frequency of light, the higher its energy. We recognize from theproblems above that higher frequencies mean shorter wavelengths. We have the right to alsosay the E = h c / lambda. High frequency irradiate has short wavelengths and high energy. X-rays or gamma-rays are instances of this. Radio tide are instances oflight through a long wavelength, short frequency, and low energy.In much the same way, the gallons of gas you placed in her car and the price of the gas space proportional:the exact same value multiplied by a continuous (the price the a gallon of gas). If youknow the continuous (the price every gallon) and you know the number of gallons,you deserve to calculate exactly how much the gas costs. Or, if you know how much the gascost, you deserve to calculate how much gas to be bought.Problems1. Planck"s constant is 4.136 x 10-15 eV sec. What is the frequency of light that has anenergy of 12.5 keV? (Hint: 1 keV = 1000 eV)2.

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What is the energy corresponding to the frequency of her favorite radiostation? (see trouble 3 for the frequency of her favorite radio station). Howdoes that compare to the power given off by a 50 Watt light pear in an hour? Back