Video Transcript
Which of the following statements is not a reason why when visible light is used for communications it must be transmitted through fiber cables rather than through the atmosphere? 1) visible light does not reflect from the ionosphere; 2) visible light is strongly absorbed by Earth’s atmosphere; 3) visible light travels faster through fiber cables than through Earth’s atmosphere; or 4) very bright visible light broadcasts would dazzle people and animals.
So in this question, we’re trying to work out the statement which is not a reason why visible light must be transmitted through fiber cables rather than through the atmosphere when we use the visible light for communications. In other words, three of the statements given to us are reasons why we need to use fiber cables and one of them is not. We need to find that one statement which is not a reason why. So let’s go through them each one by one and see which one is the answer to our question.
Number one: visible light does not reflect from the ionosphere. Well, let’s say here is the earth right and we’ve got a transmitter and a receiver on two different parts of the planet. Now, the following method is used when we use radio waves to send information from the transmitter to the receiver. Firstly, we send a radio wave along the ground. And secondly, we send one up into the air from the transmitter. Now, what happens in this case is that the radio wave that we’ve drawn in orange going up into the sky interacts with the ionosphere, which we’ve drawn in pink. Now, radio waves get reflected by the ionosphere. So the orange ray that we’ve sent up into the air goes back down again and it’s received by the receiver.
Now, obviously, the ground wave will reach the receiver first. And then after some time delay, the wave reflected by the ionosphere will also reach the receiver. This time delay — the time difference between the ground wave reaching the receiver and the reflected wave reaching the receiver — can be used to calculate how far apart the transmitter and receiver are. But this only works with radio waves. We cannot do this with visible light because visible light does not get reflected by the ionosphere. Some of it gets transmitted through and the rest of it gets absorbed by the ionosphere itself.
So we cannot use the same method of communication with visible light as we can with radio waves. Therefore, the fact that visible light does not reflect from the ionosphere is actually a reason why we cannot use visible light in this way. But we’re looking for a statement which is not a reason why. Therefore, option one is not the answer to our question.
So let’s move on to option two. But you might think “well, big deal you know. We could just send a visible light ray along the ground to our receiver and that still gets to some communication, doesn’t it?” Well, let’s look at option two and we’ll see that that’s not the case.
So option two: visible light is strongly absorbed by the Earth’s atmosphere. So let’s say we’ve got the Earth and here in orange is our transmitter, which is basically a massive light bulb. And it’s sending light along the Earth to our receiver here in blue. So what will happen? Well, the receiver will often struggle to see the light signal, especially if they’re a far away from the transmitter. Because as the light travels through the atmosphere, the atmosphere actually absorbs a lot of the light. So even though there’s a lot of power being radiated by the transmitter, not a lot of it actually gets to the receiver’s eyes.
And this is a genuine problem. Visible light is actually strongly absorbed by the Earth’s atmosphere. Therefore, just beaming along the ground is probably not a good idea and we need to use fiber cables and so option two once again is a reason why we need to use fiber cables. And hence, it’s not the answer to our question.
So option three then: visible light travels faster through fibre cables than through the Earth’s atmosphere. To see if this statement true, let’s see what fibre cables actually made of. Fibre cables are actually made of glass. So we need to compare the speed of light through air, which is how we’d be transmitting it if we weren’t using fibre cables and the speed of light through the glass, which is what fibre cables are made of.
Now to compare the speed of light, we can compare the refractive indices of the two objects. The refractive index 𝑛 of air is roughly one, whereas the refractive index of the glass is roughly 1.44. And of course, we can put little subscripts 𝑎 and 𝑔 to show that we’ve get refractive indices of air and glass. Now, the larger the refractive index of a medium, the slower light travels through it. So light must travel slower through glass because it has a larger refractive index than air. And hence, visible light actually travels slower through fibre cables than it does through Earth’s atmosphere.
Therefore, option three is not a reason why we must use fibre cables because what option three is trying to tell us is that we use fibre cables because light travels faster along them. So we get information down the other end quicker, but this is not true. And what we’re looking for in this question is the statement which is not true. So statement three is the answer to our question.
But just to be sure, let’s check option four. Option four says that very bright visible light broadcasts would dazzle people and animals. Now as we said earlier on option two, if you really want to transmit light through the Earth’s atmosphere, then we would need some very bright light sources because most of the light is gonna be absorbed. So in order to get any of it to the receiver, which we’ll draw over here, the light from the transmitter needs to be very very bright.
Now, let’s say some poor animal gets in the way of our transmission, it most certainly will be dazzled by the bright light and the same goes for people getting in the way of the transmission as well. So option four is a reason why we need to use fibre cables and hence not the answer to our question just as we expected.
And so our final answer is that the statement we’re looking for is that visible light travels faster through fibre cables than through the Earth’s atmosphere. This is not a reason why we must use fibre cables when we use visible light for communications.
