PHYS 1112 Module 13.3

The Electromagnetic Spectrum

You’ve all probably seen this picture, or one like it, before. Now let’s make sense of it.

Pause & Predict 13.3.1
For x rays in vacuum, what is the wavelength, λ, if the frequency of the x rays is 9.44 × 10¹⁷ Hz?

Pause & Predict 13.3.2
What is the wavelength of this orange light when it travels through diamond (n = 2.42)?

Practice!

Practice 13.3.1
An electromagnetic wave with an electric field that oscillates with a period of 8.43 ps is travelling in a vacuum. What is the wavelength of this electromagnetic wave?

Check your answer: B. 2.53 mm

Practice 13.3.2
An electromagnetic wave with a frequency of 4.72 x 10¹⁴ Hz has a speed of 1.71 x 10⁸ m/s when it travels through a medium. What is the index of refraction of that medium?

Check your answer: D. 1.75

Practice 13.3.3
An electromagnetic wave with a frequency of 4.72 x 10¹⁴ Hz has a speed of 1.71 x 10⁸ m/s when it travels through a medium. What is the wavelength of this electromagnetic wave when it is in the medium?

Check your answer: D. 362 nm

Could Superman really use x-ray vision? How would it possibly work?

First, Superman would need to be able to “see” x-rays in the way that we “see” visible light. This might be possible since he is from Krypton and not human. His biology will be different and he might have x-ray sensors in his eyes. Second, the objects that he observes with his x-ray vision would have to emit x-rays (in a manner similar to how the blue circle emits blue light). This means there would have to be a source of x-rays to illuminate the object, in order for Superman to actually “see” the object with x-ray vision. Regular light bulbs don’t emit EM waves in the x-ray region, so they can’t be a source of x-rays. The Sun emits x-rays, but those are absorbed in our atmosphere. If Superman could emit x-rays, say from his eyes, he could illuminate objects with x-rays, and then the illuminated object would “reflect” x-rays back at him and he would “see” the object.