OpticsReflection, Refraction & Total Internal Reflection

Revision on Reflection• Quiz No 1 Six students are arranged in front of a mirror. Their positions are shown

below. The image of each student is also drawn on the diagram. Make the appropriate line of sight constructions to determine which students each individual student can see.

Ans

• Al can see Ed and Fred

• Bo can see Ed, and Fred

• Cy can see Cy, Di, Ed, and Fred

• Di can see Cy, Di, and Ed,

• Ed can see Al, Bo, Cy, Di, and Ed

• Fred can see Bo, Cy, Di, and Ed

Quiz No. 2

A man 2.0 m tall wishes to see his full length in a plane mirror. The minimum length of mirror that he must buy is

• A 2.5 m • B 2.0 m • C 1.25 m • D 1.0 m

Ans• D. 1:2 ratio between portion of mirror required to view

the image and the height of the object is observed.

What’s Wrong Here?• Can you spot 3 subtle distortions of reality in this painting?

(Painting: A Bar at the Folies-Bergere)

3 Subtle Distortions1) Note the bottles at the left. Manet painted their reflections in the

mirror but misplaced them, painting them farther toward the front of the bar than they should be.

2) Note the reflection of the women. Since your view is from directly in front of the woman, her reflection should be behind her, with only a little of it ( if any) visible to you. Yet Manet painted her reflection well off to the right.

3) Note the reflection of the man facing her. He must be you! Because reflection shows that he is directly in front of the woman, and thus he must be the viewer of the painter. You are looking into Manet’s work and seeing your reflection well off to your right. The effect is errie because it is not what we expect from a painting or from a mirror.

Refraction

Magic or Physics?

Revision on Refraction

• Quiz No. 2

• When light passes from an optically denser medium into an optically less dense medium, it will bend ______________ the normal.

• When light passes from a medium with a low refractive index into a medium with high refractive index, it will bend ____________ the normal.

Quiz No. 3• Complete the diagram til the light ray enters air again.• Label the

normal / incident ray / refracted ray / angle of incidence / angle of refraction

 

 

glass

air

Quiz No. 4• Indicate on the diagram the approximate location where Arthur

observes the fish to be. Must Arthur aim above or below where the fish appears to be in order to strike the fish?

The story of the Archer Fish

If you were that “disabled” Archer Fish

• If you are an Archer Fish, born without the natural talent of “solving the physics of refraction”, what would you do to compensate your lack of talents?

• Hint…think physics….you do have a way out…

Just Apply Power of Physics!

Optical Density

• Like any wave, the speed of a light wave is dependent upon the properties of the medium.

• In the case of an electromagnetic wave, the speed of the wave depends upon the optical density of that material.

• The optical density of a medium is not the same as its physical density.

• The more optically dense which a material is, the slower that a wave will move through the material.

Refractive Index (η)

• One indicator of the optical density of a material is the refractive index (η) of the material.

• It is a number which indicates the number of times slower that a light wave would be in that material than it is in a vacuum.

• A vacuum is given an η value of 1.0000.

Refractive Index• Material• Index of Refraction• Vacuum 1.0000 <--lowest optical density• Air 1.0003 • Ice 1.31 • Water 1.333 • Ethyl Alcohol 1.36 • Plexiglas 1.51 • Crown Glass 1.52 • Light Flint Glass 1.58 • Dense Flint Glass 1.66 • Zircon 1.923 • Diamond 2.417 • Rutile 2.907 • Gallium phosphide 3.50 <--highest optical density

Snell’s Law

• There exist a relationship between the angle of incidence and refraction and the refractive indexes of the two mediums:

Quiz No. 5

• Determine the angle of refraction for the following problems.

Ans

• A : 53.9o

• B: 28.4o

• We usually leave angles to 1 d.p

Quiz No. 6

• Perform the necessary calculations at each boundary in order to trace the path of the light ray through the following series of layers. Show all the calculated angles.

Ans

• air - flint glass: 18 degrees • flint glass - water: 22 degrees• water - diamond: 12 degrees

• diamond - zirconium: 13 degrees• cubic zirconium - air: 30 degrees

Quiz No. 7

• A ray of light is traveling through air (n = 1.00) towards a lucite block (n = 1.40) in the shape of a 30-60-90 triangle. Trace the path of the light ray through the lucite block shown in the diagram below.

Ans

Refractive Index

Absolute Refractive Index

Relative Refractive Index

Relative Refractive Index

Quiz No. 8

Quiz No. 9

Real and Apparent Depth

Quiz No. 10What is the refractive index of the pond water?

World Above from a fish’s eyes

Total Internal Reflection

Total Internal Reflection

• When light ray hits the glass block at a certain angle of incidence, the eye cannot see the light ray.

Total Internal Reflection

Total Internal Reflection

• The maximum possible angle of refraction is 90-degrees.• Total internal reflection (TIR) is the phenomenon which

involves the reflection of all the incident light off the boundary. TIR only takes place when both of the following two conditions are met:

1. the light is in the more dense medium and approaching the less dense medium.

2. the angle of incidence is greater than the so-called critical angle.

Quiz No. 11

• For each combination of media, which light ray (A or B) will undergo total internal reflection if the incident angle is gradually increased?

Ans

• Practice A: Light ray A is in the more dense medium and it will be the one which will undergo TIR.

• Practice B: Light ray A is in the more dense medium and it will be the one which will undergo TIR.

Critical Angle (c)

Quiz No. 12

• Suppose that the angle of incidence of a laser beam in water and heading towards air is adjusted to 50-degrees.

• Use Snell's law to calculate the angle of refraction?

Ans

• Good luck!

• This problem has no solution. The angle of incidence is greater than the critical angle, so TIR occurs. There is no angle of refraction.

Quiz No. 13

• Some optical instruments, such as periscopes and binoculars use trigonal prisms instead of mirrors to reflect light around corners. Light typically enters perpendicular to the face of the prism, undergoes TIR off the opposite face and then exits out the third face.

• Why do you suppose the manufacturer prefers the use of prisms instead of mirrors?

Ans

• A prism will allow light to undergo total internal reflection whereas a mirror allows light to both reflect and refract.

• So for a prism, 100 percent of the light is reflected. But for a mirror, only about 95 percent of the light is reflected. For these reasons, a prism will produce a brighter image due to the greater percent of light being reflected.

Applications of T.I.R

• Optical fibres

applet

Optical Fiber

• Used T.I.R to transmit light rays/information• Telephone signals, internet, cable television

Rainbow Formation –ACE (3 points)

• Homework groups of two• Presentation in class• Questions:

1.What are the conditions for a rainbow to be formed?

2.With the conditions existing, how does a rainbow forms?

3.Why is the top of a rainbow always red and bottom violet?

Extra Challenge:

Make a rainbow, either in class or on video!

Rare: Full-circle rainbow