refraction of light quantum physics chapter...

REFRACTION OF LIGHT

Quantum Physics – Chapter 3

Refraction refers to the change of direction that light undergoes when it passes from one transparent medium to another of a different nature. (Surface between the two media is referred to as the interface.)

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To quantify a medium’s refractivity, its index of refraction (n) can be calculated.

The index of refraction (n) is the ratio between the speed of light in a vacuum (c) and the speed of light in a transparent medium (v).

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NOTE: n is always greater than 1

Example 1: Calculate the index of refraction (n) for diamond if light travels through it at a speed of 1.24 x 108 m/s.

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c = 3.00 x 108 m/s v = 1.24 x 108 m/s n = ?

n = 3.00 x 108 m/s = 2.42 1.24 x 108 m/s

Indices of refraction for various media:

NOTE: the differences for nvacuum and nair are negligible so both use n = 1.

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The incident ray is the light ray that travels toward the interface.

The normal is an imaginary (dotted) line drawn perpendicular to the interface which crosses the point of incidence and travels through the two media.

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The angle of incidence (θi) is the angle the light ray makes with the normal.

The angle of refraction (θR) is the angle the refracted light ray makes with the normal inside the material.

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n1 < n2

Light travels more slowly in medium 2 and the refracted ray bends toward the normal = θR < θi.

n1 > n2

Light travels more slowly in medium 1 and the refracted ray bends away the normal = θR > θi.

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note: when a light ray travels from a less dense medium to a denser medium, it bends towards the normal(and vice versa).

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When the index of refraction refers to light passing between two transparent media other than a vacuum, the relative index of refraction can be defined as the ratio between n2 (containing the refracted ray) n1 (containing the incident ray.)

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Example 2: A light ray passes from water to glass. What is its index of refraction?

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nwater = 1.33 nglass = 1.50

nwaterglass = 1.50 = 1.13 1.33

Since the light ray travels from a less refractive medium (nwater = 1.33) to a more refractive medium (nglass = 1.50), it makes sense that the relative index of refraction from water to glass is greater than 1 and that n = 1.13.

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QUANTUM PHYSICS - Pg. 83 # 1-3

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1) The incident ray, the refracted ray and the normal at the point of entry are all in the same plane.

2) The ratio of the sine of the angle of incidence (θi) to the sine of the angle of refraction (θR) is a constant. (This leads us to Snell's Law.)

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QUANTUM PHYSICS - Pg. 86 # 3-13

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Total internal reflection is a phenomenon that occurs when a ray of light passing from a highly refractive medium to a weakly refractive medium is not refracted but completely reflected.

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The Critical Angle (θc) is the angle of incidence in a dense medium, such that the angle of refraction in the less dense medium is 90o .

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Looking at the following diagrams, as the angle of incidence in the dense medium is increased, the angle of refraction increases towards 90o. During this time a weak reflected ray is also observed.

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Only when the angle of incidence (θi) in the medium exceeds the critical angle (θc) does all the light become reflected internally.

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Only when the angle of incidence (θi) in the medium exceeds the critical angle (θc) does all the light become reflected internally.

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Thus, total internal reflection can occur when:

1. The light ray travels from a highly refractive medium to a weakly refractive medium. (n2 < n1)

2. The angle of incidence is greater than the critical angle. (θi > θc)

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We can formulate an equation for the critical angle using Snell's Law for two media of refractive index n1 & n2 .

When θ1= 90o and θ2= θc

but sin(90o) = 1, therefore:

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To calculate the critical angle (θc), use:

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QUANTUM PHYSICS - Pg. 88 # 1, 2, 4, 5, 6

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