# Video: Determining the Angle of Incidence of a Light Ray

What is the incident angle of the light ray shown in the diagram?

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### Video Transcript

What is the incident angle of the light ray shown in the diagram?

Now this is a short question, and it wants us to work out the incident angle. So we’ve been given a light ray going in this direction. And it’s meeting some sort of surface or boundary here at an angle of 20 degrees. So if we wanna work out the incident angle, we need to know what that means first.

The incident angle is defined as the angle between the incident ray and the line normal to the surface or boundary. In other words, it’s the angle between this ray, the incident ray, and the line normal to the surface which happens to be this line here, the blue dotted line, now the blue and orange dotted line, which looks straightly nice.

But anyway, how do we know that this dotted line is normal to the surface? Well, normal means perpendicular to our at 90 degrees to. And this dotted line is indeed at 90 degrees to the surface, which is this blue line. So we know that the dotted line is the normal. Therefore, the question wants us to work out this angle here.

Now, we know that the angle between the surface and the normal is 90 degrees. And this is true on both sides of the normal. So we know that this total angle here from the surface to the normal is 90 degrees. That’s the definition of how we find the normal. And we know that this angle here has been given to us as 20 degrees.

So the angle that we’re looking for is simply 90 degrees minus 20 degrees, which happens to be 70 degrees. And that is the answer to our question. The incident angle of the light ray shown at the diagram is 70 degrees.