TBR light that hits an interface in a perpendicular fashion

[deleted783484]

We are given a figure of a half-cylinder with a point a at the top of the half cylinder. Light is transmitted through a half cylinder and the beam was allowed to strike a semi-circular screen.

Question asks : Light that initiates from point a in the figure:
A- will always strike the middle of the screen
B- doesn't leave the half-cylinder disk
C- splits to strike the screen at multiple points
D- slows when leaving the disk and entering air.

Answer is A which does not make sense to me. If Light is hitting the cylinder of different material than air then n is bigger than air. so if it hits the cylinder with an angle of 90 degrees then

n*sin90= nair* sin angle hits the screen

so then for them to be equal the angle can't be 90!


Their explanation is " for light that strikes an interface in a perpendicular fashion it's speed will change but there will be no change in the orientation of the rays"


Thanks a lot!

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[aldol16]

You're forgetting the definition of theta. Theta is always defined as the angle relative to the normal. Therefore, Snell's law becomes n1*sin(0) = n2*sin(theta2). Sin(0) = 0. Thus, no matter what n1 and n2 are, the only possible results for theta 2 are 0 degrees and 180 degrees. One of those doesn't make sense, which leaves you with the correct answer.

 

[deleted783484]

You're forgetting the definition of theta. Theta is always defined as the angle relative to the normal. Therefore, Snell's law becomes n1*sin(0) = n2*sin(theta2). Sin(0) = 0. Thus, no matter what n1 and n2 are, the only possible results for theta 2 are 0 degrees and 180 degrees. One of those doesn't make sense, which leaves you with the correct answer.

Thank you so much!

 

[DaAlienist]

You're forgetting the definition of theta. Theta is always defined as the angle relative to the normal. Therefore, Snell's law becomes n1*sin(0) = n2*sin(theta2). Sin(0) = 0. Thus, no matter what n1 and n2 are, the only possible results for theta 2 are 0 degrees and 180 degrees. One of those doesn't make sense, which leaves you with the correct answer.

What is the normal exactly? Can you draw it? Thank you

 

[aldol16]

What is the normal exactly? Can you draw it? Thank you

It's the angle measured from the perpendicular to the surface. https://en.wikipedia.org/wiki/Snell's_law

 

[DaAlienist]

It's the angle measured from the perpendicular to the surface. https://en.wikipedia.org/wiki/Snell's_law

Thank you!