CHEMISTRY COMMUNITY

Which of the following happens when the frequency of electromagnetic radiation decreases? Explain your reasoning.
(a) The speed of the radiation decreases.
(b) The wavelength of the radiation decreases.
(c) The extent of the change in the electrical field at a given point decreases.
(d) The energy of the radiation increases.

I understand why a, b, and d are all incorrect answers, but I don't quite understand what option (c) means/ is referring to.

Could you explain why option a is incorrect? I was confused about options a and c.

Option A is incorrect because speed is constant! :D

C is correct because the lower the frequency, the longer the wavelength; therefore, causing it to have a decrease in the slope (extent of the change).

Option A is incorrect because the speed is constant. Option C is correct because the longer the wavelength is causes the slope to decrease.

I understand why C is correct in the ways that the people above have explained it, but the solution manual says that "The electrical field corresponds to the amplitude; as the frequency decreases the waves broaden and the extent of the change (the slope of the wave) decreases." Why does the electrical field correspond to the amplitude and what does this have to do with the radiation decreasing?

In addition to Madeleine’s question, could someone also explain why and how you know the slope of a wave decreases with an increasing wavelength? Did you just look at Figure 1.8 in the book or through a formula?

To answer your question Joanna, you can look at the formula wavelength x frequency = speed of light. Because speed of light is a constant, when the wavelength increases the frequency must decrease. In doing so, the slope must decrease in order to cover all areas of the x-axis.

A is incorrect because frequency does not affect the speed of light.
B is incorrect because wavelength should increase when frequency is decreased.
D is incorrect because Ephoton = h*frequency

For C, the "extent of the change in the electric field at a given point" must be referring to the slope of the electric field with time. If the frequency decreases, the electric field changes less per unit of time, and therefore the slope decreases. (This question could not be referring to the amplitude, which does not depend on frequency.)

I also could understand why A,B, and D were incorrect, but still have trouble understanding the logic behind why C is correct. Is there an equation that could be used that allows us to know that the extent of the change in the electrical field decreases, such as the one covered in class today?

Also, what does the electric field in this context refer to?

To follow up on the answers given above, how do you find the slope of the wave? Is this taken from the maximum (or minimum) point to the middle line?

The slope is proportional to the amplitude and the frequency. In this problem, the amplitude is not changing, but the frequency is decreasing, so the slope decreases.