# Shouldn't the refracted ray be along the boundary here?

• B
• Physical_Fire
In summary: In between, you smoothly go from one to the other. In summary, the conversation discusses the concept of total internal reflection, where a light ray is reflected back into a medium rather than being refracted out of it. The diagram shows a glass cube in contact with a liquid, with a light ray directed at a 42° angle of incidence at the vertical face. The angle of refraction is 27°, and total internal reflection occurs for the first time at point P. The critical angle for the glass-liquid boundary is calculated to be 63°. The conversation also addresses a question about the refracted ray going along the boundary and the possibility of total internal reflection occurring at the same angle as the critical angle. It is clarified that
Physical_Fire
A glass cube is held in contact with a liquid and a light ray is directed at a vertical face of the cube. The angle of incidence at the vertical face is 42° and the angle of refraction is 27° as shown in the diagram. The light ray is totally internally reflected for the first time at P. Complete the diagram to show the path of the ray beyond P to the air and calculate the critical angle for the glass-liquid boundary.

Here is the image: .

In the answer scheme, the critical angle is given as 63°. If it is 63°, shouldn't the refracted ray travel along the boundary and not totally internally reflect, as total internal reflection occurs when the angle of incidence is greater than the critical angle? How is it possible when they are the same angle?

Last edited:
Welcome to PF.

Can we assume that the point P is the same as the point X ?

Yes, it was a typo from my part; I apologize. I fixed it.

Baluncore
your question is about the refraction of light from the glass to the liquid and for the critical angle you gotta get an angle of incidence that makes the refractive angle 90 so yes they are supposed to go with the boundry

But in the image, the ray doesn't go along the boundary, and I have trouble visualizing it. How should it be drawn?

Physical_Fire said:
...as total internal reflection occurs when the angle of incidence is greater than the critical angle?
Just make it "greater or equal" if that limiting case confuses you. When you approach it from below, it is when the refraction disappears. When you approach it from above, it is when the refraction appears.

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