Physical Chemistry : Photons and Photon Energy

Study concepts, example questions & explanations for Physical Chemistry

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Example Questions

Example Question #1 : Photons And Photon Energy

Which of the following quantities is zero for photons?

Possible Answers:

Both mass and speed

Mass

Neither mass nor speed

Speed

Correct answer:

Mass

Explanation:

Photons are fundamental particles that make up electromagnetic waves (light). The key aspect of photon is that it has very high speed  and no mass; therefore, mass for photons is always zero.

Example Question #2 : Photons And Photon Energy

Which of the following is/are the same for photons in gamma rays and in visible light? 

I. frequency

II. wavelength

III. speed

Possible Answers:

III only

I only

None of these

I and II

Correct answer:

III only

Explanation:

Electromagnetic spectrum is a spectrum of different types of light. Each type of electromagnetic wave differs from one another based on the frequency and wavelength; however, the speed of every electromagnetic wave in air is . The wave equation relates the speed of an electromagnetic to its wavelength and frequency as follows.

Here,  is speed of light,  is wavelength, and  is frequency. Since  is always constant, wavelength and frequency are inversely proportional to each other. This means that if the wavelength increases the frequency decreases.

Example Question #3 : Photons And Photon Energy

Photons in ultraviolet rays have a wavelength of 10nm. What is the energy of photons in radio waves?

Possible Answers:

Correct answer:

Explanation:

To solve this question, we need to use the equation relating energy of a wave to its wavelength.

Here,  is the energy,  is Planck’s constant,  is speed of light (for us it’s always ), and  is the wavelength (in meters). The energy of the ultraviolet rays is

If we look at the electromagnetic spectrum we will notice that the wavelength of radio waves is a lot higher than ultraviolet rays. Since energy is inversely proportional to wavelength we can conclude that the energy of radio waves is lower than ultraviolet's. The only answer choice with an energy lower than ultraviolet rays energy is 1.24neV.

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