A-Level Physics (Optics) exam question help

Please help with the question below. Ignore the rays drawn on the diagram as I'm not sure whether they are correct. How should the rays be drawn and also the mark scheme mentions:
ray through A links to ray in B AND ray in B horizontal by eye.
Why does the ray through B need to be horizontal?

Thanks!

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Reply 1

Eimmanuel

Study Forum Helper

Original post by janani1296

I assume that you know the Snell’s law of refraction. (If you need a reminder, you can follow this link.)
When a light ray exits at 90° from the surface, it means the angle of refraction is 0° and this also implies that the angle of incidence in prism B is also 0° according to Snell’s law.
So the light ray in B is horizontal.