# Total Internal reflection is Glass Prism Question

#### Mohd95

My textbook says that for total internal reflection to occur 2 conditions need to be met. Light is traveling more slowly in the first medium than the second medium and no.2 is the angle of incidence must be large enough for no refraction to occur.

My question is about the first condition why does total internal reflection occur when a light ray is shined through a Glass Isoceles Right Angle Prism like the one in old submarine periscopes. I mean isn't light traveling from air to glass therefore entering from a less dense medium into a more dense medium meaning light is faster in the first medium than the second medium. I don't understand it

I need a good answer plz and I am in Grade 10 so dont make it very complex

Also Sources would be appreciated. Thank You very much

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#### Andy Resnick

The first condition is backwards- total internal reflection can occur when light travels from a more dense to a less dense medium (e.g. glass to air, or water to air), and 'density' refers to the index of refraction: a more dense medium has a higher index.

Does this help?

#### Mohd95

Tyvm it definetly helpe

#### Doc Al

Mentor
My textbook says that for total internal reflection to occur 2 conditions need to be met. Light is traveling more slowly in the first medium than the second medium
Sounds right to me.
I mean isn't light traveling from air to glass therefore entering from a less dense medium into a more dense medium meaning light is faster in the first medium than the second medium.
Sure, light travels from air to glass, but that's not where the total internal reflection is taking place. The total internal reflection takes place where the light is going from glass to air (or trying to, at least). (Just as Andy Resnick explained.)

I think you just misinterpreted your textbook a bit. In your example, the 'first medium' would be the glass, not the air. (I hope the text went on to explain things a bit more clearly.)

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