Physics Help Forum Reflection after having reached the critical angle?

 Light and Optics Light and Optics Physics Help Forum

 May 20th 2008, 02:55 PM #1 Junior Member   Join Date: May 2008 Posts: 7 Reflection after having reached the critical angle? Is this equal to the angle of incidence or is it equal to angle of incidence-the critical angle. Judging by the simple laws of reflection I'm guessing that it's the first one, in fact I'm very sure. Some confirmation? Thanks, Rob
May 20th 2008, 05:01 PM   #2

Join Date: Apr 2008
Location: On the dance floor, baby!
Posts: 2,781
 Originally Posted by rsmith Is this equal to the angle of incidence or is it equal to angle of incidence-the critical angle. Judging by the simple laws of reflection I'm guessing that it's the first one, in fact I'm very sure. Some confirmation? Thanks, Rob
I'm not sure what you mean. Below is a nice diagram which I've shamelessly copied from here.

The incident ray reflects. Thus the angle of incidence is equal to the angle of reflection. The reflection angle is the critical angle by definition.

-Dan
Attached Thumbnails

__________________
Do not meddle in the affairs of dragons for you are crunchy and taste good with ketchup.

See the forum rules here.

 May 21st 2008, 02:53 AM #3 Junior Member   Join Date: May 2008 Posts: 7 Yeah that diagram helps. It just seemed strange that once the critical angle had been reached, that the TIR ray would suddenly jump to equal the incident ray. What I thought might happen is that once the critical angle had been reached, every degree past that critical angle would equal the IR ray if you get what I mean. It's kind of logical I suppose, but it doesn't obey the laws of reflection. Just thought I'd check. Thanks, Rob
 Nov 5th 2008, 03:28 AM #4 Member   Join Date: Nov 2008 Location: PAKISTAN Posts: 79 It is equal to angle of incidence when light is travelling from denser to lighter medium
 Dec 8th 2009, 06:31 AM #5 Member   Join Date: Nov 2008 Location: PAKISTAN Posts: 79 hi there , i think you are CORRECT
Dec 14th 2009, 02:06 PM   #6
Member

Join Date: Dec 2008
Posts: 65
 Originally Posted by rsmith Yeah that diagram helps. It just seemed strange that once the critical angle had been reached, that the TIR ray would suddenly jump to equal the incident ray. What I thought might happen is that once the critical angle had been reached, every degree past that critical angle would equal the IR ray if you get what I mean. It's kind of logical I suppose, but it doesn't obey the laws of reflection. Just thought I'd check. Thanks, Rob
There is no jump.
Some of the light is reflected at any angle (and the rest is transmitted/refracted). The amount that is transmitted (refracted) decreases as the angle of incidence approaches the critical angle and becomes zero at this specific value.
So what happens is that for incidence at an angle equal or larger than the critical angle there is only reflection and no refraction (that's why is called Total internal reflection).
The reflection part of the ray goes according to the usual law of reflection for any angle, below or above the critical angle.

Jan 16th 2010, 03:31 PM   #7
Senior Member

Join Date: Dec 2009
Posts: 209
perfect

 Originally Posted by mircea There is no jump.
 Originally Posted by mircea Some of the light is reflected at any angle (and the rest is transmitted/refracted). The amount that is transmitted (refracted) decreases as the angle of incidence approaches the critical angle and becomes zero at this specific value. So what happens is that for incidence at an angle equal or larger than the critical angle there is only reflection and no refraction (that's why is called Total internal reflection). The reflection part of the ray goes according to the usual law of reflection for any angle, below or above the critical angle.

This is the best answer I've seen for describing the critical angle. Any rays of light that strike a surface at angle that is greater than the critical angle are completely reflected back in the direction of travel at the same angle they struck the material at. Interestingly, if the incident ray is precisely at the critical angle, the refracted ray is tangent to the boundary at the point of incidence.

The actual formula for the critical angle is:

where n2 is the refractive index of the less optically dense medium, and n1 is the refractive index of the more optically dense medium.

Many Smiles,
Craig

 Tags angle, critical, reached, reflection

 Thread Tools Display Modes Linear Mode

 Similar Physics Forum Discussions Thread Thread Starter Forum Replies Last Post milan Kinematics and Dynamics 1 Apr 20th 2009 10:15 AM alchemist7 Light and Optics 0 Feb 9th 2009 08:04 AM DaveTheZombie Advanced Electricity and Magnetism 0 Dec 1st 2008 03:03 PM werehk Thermodynamics and Fluid Mechanics 0 Jul 18th 2008 10:04 PM Air Light and Optics 4 May 13th 2008 05:07 AM