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Reflection and Refraction ofLight. Geometrical Optics – Using a Ray Approximation Light travels in a straight-line path in a homogeneous medium until.

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Presentation on theme: "Reflection and Refraction ofLight. Geometrical Optics – Using a Ray Approximation Light travels in a straight-line path in a homogeneous medium until."— Presentation transcript:

1 Reflection and Refraction ofLight

2 Geometrical Optics – Using a Ray Approximation Light travels in a straight-line path in a homogeneous medium until it encounters a boundary between two different media Light travels in a straight-line path in a homogeneous medium until it encounters a boundary between two different media The ray approximation is used to represent beams of light The ray approximation is used to represent beams of light A ray of light is an imaginary line drawn along the direction of travel of the light beams A ray of light is an imaginary line drawn along the direction of travel of the light beams

3 Wave Fronts and Rays, cont. “Near field” “Far field”

4 Reflection of Light A ray of light, the incident ray, travels in a medium A ray of light, the incident ray, travels in a medium When it encounters a boundary with a second medium, part of the incident ray is reflected back into the first medium When it encounters a boundary with a second medium, part of the incident ray is reflected back into the first medium This means it is directed backward into the first medium This means it is directed backward into the first medium

5 Specular Reflection Specular reflection is reflection from a smooth surface Specular reflection is reflection from a smooth surface The reflected rays are parallel to each other The reflected rays are parallel to each other All reflection in this text is assumed to be specular All reflection in this text is assumed to be specular

6 Diffuse Reflection Diffuse reflection is reflection from a rough surface Diffuse reflection is reflection from a rough surface The reflected rays travel in a variety of directions The reflected rays travel in a variety of directions Diffuse reflection makes the road easy to see at night Diffuse reflection makes the road easy to see at night

7 Specular and Diffuse Reflection Specular Diffuse

8 Law of Reflection The normal is a line perpendicular to the surface The normal is a line perpendicular to the surface It is at the point where the incident ray strikes the surface It is at the point where the incident ray strikes the surface The incident ray makes an angle of θ 1 with the normal The incident ray makes an angle of θ 1 with the normal The reflected ray makes an angle of θ 1 ’ with the normal The reflected ray makes an angle of θ 1 ’ with the normal

9 Law of Reflection, cont. The angle of reflection is equal to the angle of incidence The angle of reflection is equal to the angle of incidence θ 1 =θ 1 ’ θ 1 =θ 1 ’ 11 1’1’ Incident and reflected ray are in the same plane.

10 Refraction of Light When a ray of light traveling in a transparent medium encounters a boundary leading into a second medium, part of the ray is reflected and part of the ray enters the second medium When a ray of light traveling in a transparent medium encounters a boundary leading into a second medium, part of the ray is reflected and part of the ray enters the second medium The ray that enters the second medium is bent at the boundary The ray that enters the second medium is bent at the boundary This bending of the ray is called refraction This bending of the ray is called refraction

11 Refraction of Light, cont. The incident ray, the reflected ray, the refracted ray, and the normal all lie on the same plane The incident ray, the reflected ray, the refracted ray, and the normal all lie on the same plane The angle of refraction, θ 2, depends on the properties of the medium The angle of refraction, θ 2, depends on the properties of the medium

12 Following the Reflected and Refracted Rays Ray  is the incident ray Ray  is the incident ray Ray  is the reflected ray Ray  is the reflected ray Ray is refracted into the lucite Ray is refracted into the lucite Ray  is internally reflected in the lucite Ray  is internally reflected in the lucite Ray is refracted as it enters the air from the lucite Ray is refracted as it enters the air from the lucite

13 Refraction, cont. Speed of light=constant? Speed of light=constant? Yes, but only in ONE medium! Yes, but only in ONE medium! v=c (light velocity) v=c/n Water (optically denser than air) Air Index of refraction

14 The index of refraction defines the velocity of light in the optically denser medium  c/n. The index of refraction defines the velocity of light in the optically denser medium  c/n. Speed of light in vacuum (air) Speed of light in a medium (e.g. water) Index of refraction

15 Index of Refraction, cont. For a vacuum and air, n = 1 For a vacuum and air, n = 1 For other media, n > 1 For other media, n > 1 n is a unitless ratio n is a unitless ratio

16 Frequency Between Media As light travels from one medium to another, its frequency does not change As light travels from one medium to another, its frequency does not change Both the wave speed and the wavelength do change Both the wave speed and the wavelength do change The wavefronts do not pile up, nor are created or destroyed at the boundary, so ƒ must stay the same The wavefronts do not pile up, nor are created or destroyed at the boundary, so ƒ must stay the same

17 Change of Wavelength n=  / n Vacuum wavelength Wavelength of a medium with the refractive index n

18 Refraction Details Light may refract into a material where its speed is lower Light may refract into a material where its speed is lower The angle of refraction is less than the angle of incidence The angle of refraction is less than the angle of incidence The ray bends toward the normal The ray bends toward the normal

19 Refraction Details, cont. Light may refract into a material where its speed is higher Light may refract into a material where its speed is higher The angle of refraction is greater than the angle of incidence The angle of refraction is greater than the angle of incidence The ray bends away from the normal The ray bends away from the normal

20 Snell’s Law of Refraction n 1 sinθ 1 =n 2 sinθ 2 n 1 sinθ 1 =n 2 sinθ 2 θ 1 is the angle of incidence θ 1 is the angle of incidence 30.0° in this diagram 30.0° in this diagram θ 2 is the angle of refraction θ 2 is the angle of refraction

21 SubstanceRefractive index Air Water1.33 Ethyl alcohol1.36 Fused quartz1.46 Glycerine1.47 Glass Oil1.50 Zircon1.92 Diamond2.42 Some indices of refraction for various substances at 590 nm:

22 Example and Application A digital information on a DVD consists of a series of pits that are “read” by a laser beam. The surface of a DVD is shown on the right side. A digital information on a DVD consists of a series of pits that are “read” by a laser beam. The surface of a DVD is shown on the right side.


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