Video: Eq17S1-Physics-Q06



Video Transcript

Compare line spectrum, continuous spectrum. Point of comparison, definition.

Okay, in this case, we’re comparing these two terms line spectrum and continuous spectrum, and the point of comparison is their definitions. So, what are the definitions of these terms? We notice that both terms involve the word spectrum. So, we know it will have something to do with wavelengths of light. For example, our eyes are sensitive to light within the visible spectrum, which doesn’t look quite like this, but you get the idea. A spectrum is a collection of wavelengths of light.

In particular, a spectrum refers to a collection of radiation which has a starting wavelength and an ending wavelength. We could say that the span from the start to the end is the spectrum we’re considering. Let’s say that we define our own spectrum that has a starting wave length 𝜆 one and an ending wavelength 𝜆 two. The spectrum of light we’re considering then is any wavelength that falls between these two end points.

Turning back to the two terms we want to define, let’s first consider the term line spectrum. To better understand this term, let’s consider the structure of an atom. We know that an atom consists of a nucleus at its center and electrons that orbit that nucleus. And what’s more, each electron in this atom occupies its own energy level.

Now the particular level of energy at which an electron resides isn’t fixed. It can change. For example, if incoming electromagnetic radiation interacts with an electron, that electron can gain energy and move to a higher energy level. And in a similar way, an electron can emit electromagnetic radiation in the form of a photon. In both of these processes, absorption as well as emission, the electron is constrained to only absorb or only emit certain particular wavelengths of light. And that’s due to the fact that the electron’s energy levels are quantized. That is only certain energy levels are allowed.

Here’s what all this has to do with line spectrum. If we were to look at all the possible wavelengths of light that the electrons in this atom could emit, let’s say that all of these wavelengths fit between our minimum and maximum values 𝜆 one and 𝜆 two, then if we were to plot radiation intensity on our vertical axis at each particular emitted wavelength of light, we would see a spike on this graph. But in between the spikes, we wouldn’t see any light at all. That is to say that this particular atom emits light at these wavelengths but not at other wavelengths. So, the question is, do we have a spectrum here?

And the answer is, we do have a spectrum, but it’s a special type. Because of the many possible wavelengths on the spectrum which are not represented, this spectrum is called a line spectrum. That is, in this spectrum, certain wavelengths are represented but many are not. Incidentally, when we look at the emission spectrum of any particular atom, it’s normal to see a line spectrum, lots of gaps between our intensity peaks. So, if that’s what a line spectrum is, then what about a continuous spectrum?

In this case, again, considering our 𝜆-one-to-𝜆-two spectrum, a continuous spectrum would mean that we have nonzero intensity of light at every wavelength within this range. There are no gaps or spaces. Every possible wavelength within this waveband is represented. One way to create a continuous spectrum is to take a ray of white light, say sunlight, and then put a glass prism in front of that beam. The prism then spreads out the wavelengths of light into their constituent colors. And even though this drawing doesn’t completely show it, the spectrum we get on the other side of the prism is continuous.

So, here’s what we can write for the definitions of these two types of spectrum. We can write that a line spectrum consists of specific frequencies and is not continuously distributed. While for a continuous spectrum, we can say that this is a spectrum consisting of all wavelengths in a continuous manner. This is how the definition of these two different terms compares.

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