Students will learn about Refraction.

Diffraction

• Diffraction is the bending of a wave around a barrier/obstacle.

• Waves spread out when going through a small opening.

• The size of the opening must be small, typically on the order of the wavelength of the waves applied.

Law of Reflection

• The angle of incidence of a light ray is equal to the angle of reflection.

• (incident waves are incoming; reflected are those that “bounced” off the smooth surface)

Refraction

• When light is incident upon the surface of a transparent medium, some of the light may be reflected and some of it passes through the medium.

• The angle between the new direction of propagation and the normal to the surface is the angle of refraction. This angle depends on the indices of refraction of the media that contain the incident and refracted rays.

Index of refraction

• The index of refraction n = c/v

• c is the speed of light in a vacuum, 3 x 10^8 m/s

• v is the speed of light in the medium (m/s)

Indices of Refraction

• Air• Alcohol• Benzene • CCl4• Corn oil• Diamond• Glycerol• Lucite• water

• 1.00• 1.36• 1.50• 1.46• 1.47• 2.42• 1.47• 1.50• 1.33

Example

• The index of refraction for water is 1.33• What is the speed of the light in water?

• N = c/v

• 1.33 = 3 x 10^8/v• V = 2.26 x 10^8 m/s

Example

• The speed of light in a medium is 2.4 x 10^8.• What is the index of refraction?

• n = c/v= 3 x 10^8 / 2.4 x 10^8= 1.25

Frequency remains the same

• When light travels from air into water or glass, the speed of light decreases. However, the frequency of the light remains the same because the water or the glass does NOT affect the frequency.

• Therefore, if the speed decreases going into the medium, the frequency stays the same, but the wavelength decreases.

Snell’s Law (The Law of Refraction)

• The product of the index of refraction of a medium, n1, and the sine of the angle of incidence theta 1 equals the product of the index of refraction of the second medium, n2, and the sin of the angle of refraction theta 2.

Example

• A monochromatic ray of light with frequency 5 x 10^14 Hertz traveling in air is incident upon medium A at an angle of 45 degrees. If the angle of refraction is 29 degrees, what is the index of refraction of the medium?

• n1 sin theta1 = n2 sin theta2• 1 x sin 45 = n2 sin 29• 0.707 = n2 (.48) n2 = .707/.48 = 1.47

Example

• A ray of light traveling through air is incident upon a sheet of glass at an angle of 30 degrees. What is the angle of refraction?

• n1 = 1.00; n2 = 1.52

• N1sin30 = n2sin theta• 1 x 0.50 = 1.52 sin theta• .50/1.52 = sin theta 0.329 = sin 19.2 degrees.

Group Activity

• Calculate the angle of refraction for a beam of light in air incident into fresh water. n for water is 1.33.

• theta1 = 10 degrees theta2 ?• theta1 = 20 degrees• theta1 = 30 degrees• theta1 = 40 degrees• theta1 = 50 degrees

• Theta1 = 10; theta 2 = 7.47• Theta1 = 20; theta2 = 14.89• Theta1 = 30; theta2 = 22.1• Theta1 = 40; theta2 = 28.9• Theta1 = 50; theta2 = 35.2