Total Internal Reflection in Glass prism Question?

[Mohd95]

Question about Total Internal Reflection?
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.

My second question is it also asks me : Why does total internal reflection occurs only when light travels slowly through the first medium than in the second and not the other way around. Explain

I need a good answer please and I am in Grade 10 so don't make it very complex

Heres the link of the prism I am alking about: http://www.gcse.com/waves/images/periscope.gif

Also Sources would be appreciated.

 

[zorro]

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.

In your link, you can see that the total internal reflection takes place only at the hypotenuse face of the prism (both figures). You are right about light entering from rarer to denser medium (air to glass), but at the site of reflection, you can observe that light tends to go from glass to air (denser medium to rarer medium). If the angle of incidence there is greater than the critical angle, then total internal reflection takes place. The periscopes are manufactured keeping all these conditions in mind.

My second question is it also asks me : Why does total internal reflection occurs only when light travels slowly through the first medium than in the second and not the other way around. Explain

When light travels from denser to rarer medium (slowly through the first than in the second), it bends away from the normal. With increasing angle of incidence, angle of refraction also increases. There comes a time when the refracted ray just grazes the plane of separation. Any angle of incidence greater than this will lead to total internal reflection.

If light travels from rarer to denser medium, it bends towards the normal. Even if you increase the angle of incidence, there is no case possible when the light grazes the plane of separation. So no total internal reflection is possible here.

I hope it is clear.

 

[Mohd95]

Thanx a lot man