Mammoth Memory

Why total internal reflection is always slow to fast

You don't need the following in an exam, it is just interesting to know.

Total internal reflection only occurs when travelling from a slow medium to a fast medium, and when the ray of light going towards the boundary is greater than the critical angle.

Total internal reflection between glass and air diagram.

They call this "Total internal reflection". So does this mean that at other angles the ray of light is not all reflected?

The answer is yes. 

In nearly all questions in exams at basic level the reflection is not taken into account until total internal reflection occurs. In reality, depending on the angle different amounts of refraction occur as follows:

Only about 3% is reflected at no angle when light travels between glass and air.

When the light is exactly in line with the normal line the light beam that passes into the air is very bright but 3% is reflected backwards into the glass.

At 15° the reflected ray will also be at 15° while the refracted ray will be at 23°

At 15° angle of incidence, there is a reflected ray of 3% strength of the original ray leaving 97% to pass into the air.

Light ray in = 15° to the normal, Reflected ray = -15° to the normal, dim reflection (3%), refracted ray = 23° to the normal, bright refraction

At 30° 4% is reflected while the refracted ray will be at 48°.

At 30° incidence there is a reflected ray of 4% strength.

Light ray in = 30° to the normal, Reflected ray = -30° to the normal, still low reflection (4%), refracted ray = 48° to the normal, bright refraction

At 40° 25% is reflected while the refracted ray will be at 75°.

At 40° incidence there is a reflected ray of 25% strength leaving 75% of the strength of the ray to pass into the air.

Light ray in = 40° to the normal, Reflected ray = -40° to the normal, brighter ray reflection (25%), refracted ray = 75° to the normal, still bright refraction but not as bright as it was

At 45° 100% is reflected and there is no refracted ray.

When total internal reflection occurs 100% gets reflected.

Light ray in = 45° to the normal, Reflected ray = 45° to the normal, total internal reflection, refracted ray = Zero

(Ray of light going towards the boundary is greater than the critical angle)

 

If you were to draw a graph of the amount of light reflected at the boundary when light travels from glass to air you would get the following:

Graph showing percentage of light reflected and refracted as it travels from glass to air.

 

 

 

 

If we plot a similar graph for water to air and diamond to air you get the following graphs.

Graph showing percentage of light reflected and refracted as it travels from water to air.

 

 

 

 

 

Graph showing percentage of light reflected and refracted as it travels from diamond to air.

 

 

 

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