Reflection vs. Refraction

Refraction zRefraction of Light: Bend or change direction z1. As light rays enter a new medium the cause light to bend z2. The denser the medium – the slower the light travels z3. Index of Refraction: a measure of how much a medium bends the light that travels through it. z4. The faster the beam, it will bend away from normal, the slower the beam, it will bend toward the normal.

The ray model says… Light has four different ways in which it can interact with matter. Medium 1Medium 2 Reflection

Light has four different ways in which it can interact with matter. Medium 1Medium 2 Refraction

Light has four different ways in which it can interact with matter. Medium 1Medium 2 Scattering

Light has four different ways in which it can interact with matter. Medium 1Medium 2 Absorption

Reflective Objects

Reflection Reflection off PaperReflection off a Mirror Regular Reflection Diffuse Reflection

Law of Reflection Normal - an imaginary line drawn perpendicular to the surface

Reflections in a Mirror

Q

Refraction and Lenses

Refraction Refraction is based on the idea that LIGHT is passing through one MEDIUM into another. The question is, WHAT HAPPENS? Suppose you are running on the beach with a certain velocity when you suddenly need to run into the water. What happens to your velocity? IT CHANGES! Refraction Fact #1: As light goes from one medium to another, the velocity CHANGES!

Refraction Suppose light comes from air, which in this case will be considered to be a vacuum, strikes a boundary at some angle of incidence measured from a normal line,and goes into water. The ratio of the two speeds can be compared. The denominator in this case will ALWAYS be smaller and produce a unitless value greater or equal to 1. This value is called the new medium’s INDEX OF REFRACTION, n. All substances have an index of refraction and can be used to identify the material.

Refraction Suppose you decide to go spear fishing, but unfortunately you aren’t having much luck catching any fish. The cause of this is due to the fact that light BENDS when it reaches a new medium. The object is NOT directly in a straight line path, but rather it’s image appears that way. The actual object is on either side of the image you are viewing. Refraction Fact #2: As light goes from one medium to another, the path CHANGES!

Refraction What EXACTLY is light doing when it reaches a new medium? We don’t want you to think ALL of the light refracts. Some of the light REFLECTS off the boundary and some of the light REFRACTS through the boundary. Angle of incidence = Angle of Reflection Angle of Incidence > or < the Angle of refraction depending on the direction of the light

Refraction – Going from Air to Water The index of refraction, n, for air (vacumm) is equal to 1. The index of refraction for water is If you are going from a LOW “n” to a HIGH “n”, your speed DECREASES and the angle BENDS TOWARDS the normal

Refraction – Going from Water into Air The index of refraction, n, for air (vacumm) is equal to 1. The index of refraction for water is If you are going from a HIGH “n” to a LOW “n”, your speed INCREASES and the angle BENDS AWAY the normal Note: If the angles are EQUAL, then the “n” must be equal for each. The ray will pass straight through.

Refraction – Snell’s Law A scientist by the name of Snell discovered that the ratios of the index’s and the ratio of the sine of the angles are the same value!

Example The refractive index of the gemstone, Aquamarine, is Suppose a ray of light strikes a horizontal boundary of the gemstone with an angle of incidence of 23 degrees from air. Calculate the SPEED of light in Aquamarine Calculate the angle of refraction within Aquamarine 1.90 x 10 8 m/s degrees

Total Internal Reflection There is a special type of refraction that can occur ONLY when traveling from a HIGH “n” medium to a LOW “n” medium. Suppose we are traveling FROM water and going into air. Should the ANGLE OF INCIDENCE get TOO LARGE, the angle of refraction will EQUAL 90 DEGREES. We call this special angle of incidence the CRITICAL ANGLE,  c, for that material (water in this case)

Total Internal Reflection If we EXCEED the critical angle, for that material, the ray will reflect INTERNALLY within the material. We call this idea TOTAL INTERNAL REFLECTION. In this figure, the angle of incidence EXCEEDS the critical angle for water and the ray reflects according to the law of reflection at the boundary.

The Critical Angle So the question is, how can you calculate the critical angle? Remember, it is when the refracted ray is equal to 90 degrees cc

Example Suppose a light ray is traveling in heavy flint glass( n = 1.65) and once it strikes the boundary, enters air. Calculate the critical angle for flint glass degrees

Lenses – An application of refraction There are 2 basic types of lenses A converging lens (Convex) takes light rays and bring them to a point. A diverging lens (concave) takes light rays and spreads them outward.

Converging (Convex) Lens Much like a mirror, lenses also take light rays from infinity and converge them to a specific point also called the FOCAL POINT, f. The difference, however, is that a lens does not have a center of curvature, C, but rather has a focal point on EACH side of the lens.

Applications of Converging Lenses Obviously, converging lenses play an important role in our lives as our eyes are these types of lenses. Often times we need additional corrective lenses to fix our vision. In figure A, we see an eye which converges what we see on the retina. In figure B, we see an eye which converges too LATE. The eye itself is often too short and results in the person being far sighted. In figure C, we see an eye which converges too SOON. The eye itself is often too long and results in the person being near sighted In the later 2 cases, a convex or concave lens is necessary to ensure the image is on the retina.

Applications of Converging Lenses A camera uses a lens to focus an image on photographic film.

Ray Diagrams The rules for ray diagrams are the SAME for lenses as they were for mirrors except you go THROUGH the lens after refraction and instead of going through, C (center of curvature) you go through the actual center of the lens. ff Rule #1: Draw a ray, starting from the top of the object, parallel to the principal axis, then through “f” after refraction. Rule #2: Draw a ray, starting from the top of the object, through “f”, then parallel to the principal axis, after refraction. Rule #3: Draw a ray through the center of the lens.

Ray Diagrams As before, you draw the image down to the intersection as shown. ff Since this image could be projected on to a screen it is a REAL IMAGE and real images ALWAYS are found on the OPPOSITE side of the lens from the object. Likewise, virtual images would appear on the SAME SIDE as the object. The characteristics in this case are still inverted and reduced.

Lenses – The Mirror/Lens Equation To CALCULATE the image’s position and characteristics you use the same equations you used for mirrors. An object is placed 35 cm in front of a converging lens with focal length of 20 cm. Calculate the image’s position relative to the lens as well as the image’s characteristics cm-1.33x This image is REAL (since the object distance is positive) and on the OTHER side of the lens. The image is INVERTED and ENLARGED.

An observer observes a light source S in a mirror. Where does he perceive the mirror image of S to be located? S ●1●1 ●2●2 ●3●3 ●4●4

How big does a mirror have to be in order to see your entire length in it? Half His Height!

To see more of her head in the mirror, she … 1. should hold the mirror closer 2. should hold the mirror farther away 3. needs a bigger mirror

Refraction

ii RR As light travels from one medium to another, the speed of light changes and the light bends accordingly.

n 1 sin  I = n 2 sin  R Snell’s Law: The Law of Refraction Index of Refraction of the media in which the angle of incidence is Index of Refraction of the media in which the angle of refraction is

Index of Refraction Cannot be less than 1.00 Air = 1.00 Glass = 1.5 Water = 1.33

Q

Bruno wishes to “spear” a fish with a laser. Should he aim the laser beam… 1.above 2. below 3. directly at the observed fish to make a direct hit?

Light is incident on a block of glass as shown. Which ray is the refracted ray?

Calculate the critical angle θ from glass (n=1.5) to air. θ o o o o o

Calculate the critical angle θ from glass (n=1.5) to air. θ o o o o o n 1 sin  I = n 2 sin  R 1.5 sin  I = 1.0 sin 90 sin  I =.667  I = 41.8

Real or Virtual? zImage: a copy of an object formed by reflected or refracted light zVirtual image: right side up appears to be coming from behind the mirror. zReal Image: is formed when reflected light rays actually meet at a point. The image is upside down (inverted),

Fiber Optics

Fiber optics uses total internal reflection

Optics Lab 1.Set up your optics kit with a 1 slit filter in front of your light source. Use your mirror over graph paper, trace the line you created in your reflection and then measure your angle of incidence and angle of reflection. Repeat for two other angles. 2.Use a prism, pass your light beam through the prism and trace out your light ray and your incident ray. Measure your angle of incidence and angle of reflection, then using Snell’s Law, calculate the index of refraction going from air to glass and then from glass to air. Repeat the above procedure passing through a beaker of water. When the beam goes from a high index to low, does it bend toward or away from the normal? When the beam goes from low to high index, does it bend toward or away from the normal? 3.Shine your beam into the long side of your prism. Observe what happens as you change the angle of the incidence by rotating the prism. Try to identify the angle at which the laser beam makes the transition between refracting and reflecting when it reaches the boundary between the glass and the air. 4.Draw an arrow onto your graph paper. Place the mirror on the graph paper and draw a line there. Move your head until you see the reflection of the arrow. Shine a beam of light so it passes right through the arrow and hits the mirror. Trace the incident and reflected beams. Move the laser so the beam passes over the tip of your arrow from a different angle. Trace the incident and reflected beams of this second angle. Move the mirror and using a ruler, extend the two reflected rays until they meet at a point where the image of the tip of the arrow would be on your graph. The image forms where all rays that leave the same point on an object meet together. 5.Find your converging lens. Shine a single beam straight thought the middle of the lens. Draw the resulting beam. Move your optics light until the beam hits one outside area of the lens. Draw the resulting beam. Move your optic light to the other side of the lens, draw the resulting beam. Find the focal point for the converging lens. Repeat the entire procedure for the diverging lens on the other side of your graph paper. Once you have some line, remove the lens and trace back to before the lens and again find your focal point of the diverging lens.