Refraction of Refraction of LightLight

Section 11.1SNC2D

Light travels in straight lines and at a constant speed as long as it’s travelling in the same medium. Medium – (plural:media) is a material that is being

used.

But what happens when another medium is introduced?

For example ... What happens if you are riding in a golf cart on pavement, and then you hit some mud or gravel?

- The front wheels suddenly slow down, but the back wheels keep going and the golf cart twists around.

- This is similar to when light travels from one medium into a different medium ... Both its speed and direction may change.

What is Refraction?What is Refraction?Refraction is the bending or change in direction of light when it travels from one medium to another◦Example: from air to water.

Refraction is used in designing and building lenses.

The bending of light is due to different media slowing light down by different amounts. The more light slows down, the more it is refracted.

How Light RefractsHow Light RefractsAs light enters a slower medium,

the light waves are compressed. If light strikes a medium at an angle, the part of the light wave that enters the medium first will slow down first.

RefractionRefractionThis effect is commonly seen

when looking at something in the water. It appears like the light is coming straight to us when really it is bending.

What Causes What Causes Refraction?Refraction?Light travels at different speeds in

different media.

It is impossible for light to move at top speed when particles get in the way. The particles in a medium slow down the passage of the waves, which results in light travelling more slowly through a block of glass than through air.

The Speed of LightThe Speed of Light

Speed of light in different materials:

◦Vacuum (empty space where there are

very few particles) = 3.00x108 m/s or 300 million m/s

◦Air – slightly less than vacuum but use the same speed = 3.00x108 m/s

◦Water – 2.26x108 m/s

The Speed of LightThe Speed of Light

Example: The light ray strikes the Plexiglas at an angle.

(a) As the light enters the Plexiglas, it slows down and refracts.

(b) When the light leaves the Plexiglas and enters the air, it speeds up and refracts again.

The Speed of LightThe Speed of Light

Notice that light does not refract inside the block. Light refracts only at the boundary when it is entering or leaving a medium.

Reflection and Reflection and RefractionRefraction

incident ray reflected ray

NOTE: The angles of the refracted light rays are usually measured from the normal, drawn at 90° to the surface where the light ray crosses between the two media.

So, when light travels between two media at angles other than 90°, it bends due to its change in speed.

RULES FOR REFRACTIONRULES FOR REFRACTION1. Incident ray, refracted

ray and the normal all lie in the same plane. Incident ray and refracted ray are on opposite sides of the line that separates the two media.

Note: Angle of refraction is defined as the angle between the refracted ray and the normal.

Medium 1

Medium 2

incident ray

RULES FOR REFRACTIONRULES FOR REFRACTION2. If the change in medium causes a

decrease in speed, then the light bends towards the normal.

3. If the change in medium causes an increase in speed, then the light bends away from the normal.

Refer to diagram in handout.

The Index of RefractionThe Index of RefractionLight travels fastest in a vacuum.

The refractive index of the speed of light in a vacuum is assigned a value of 1.00.

The larger the refractive index, the more the medium decreases the speed of light.

The Index of RefractionThe Index of Refraction

So, the index of refraction for a medium is defined as the ratio of the speed of light in a vacuum to the speed of light in that medium.

Mathematically, it is written as:◦n is the index of refraction

◦c is the speed of light in a vacuum (in m/s)

◦v is the speed of light in a given medium (in m/s)

n has no units (it’s a dimensionless quantity) since m/s in c and v cancel each other out.

Index of Refraction ValuesIndex of Refraction ValuesMedia Index of refraction

(n)

Vacuum 1.00 (exactly) Air 1.00029CO2 gas 1.00045

Water 1.3333Ethyl Alcohol 1.362Glycerol 1.470Carbon disulfide 1.632Glass (crown) 1.52Ruby 1.54Glass (flint) 1.65Zircon 1.92Diamond 2.42

The speed of light in sodium chloride (NaCl)1.96x108 m/s. The speed of light in a vacuum is 3.00×108 m/s. Calculate the index of refraction.

G S

R P

A

Example Problem 1Example Problem 1

vNaCl = 1.96 × 108 m/s

c = 3.00 × 108 m/s

Refractive index n = ?

n = c v

n = 3.00 x 108 m/s 1.96 x 108 m/sn = 1.53

Therefore the index of refraction for NaCl is 1.53

Example Problem 2Example Problem 2Calculate the speed of light in

olive oil, if the index of refraction of olive oil is 1.48.

G S

R P

A

c = 3.00x108 m/sn olive oil = 1.48

v olive oil = ?

n = c v

v = c n

v = 3.00x108 m/s 1.48v = 2.03x108 m/s

Therefore, the speed of light in olive oil is 2.03x108 m/s.

DispersionDispersionThe refraction of

white light into separate wavelengths, or colours.

Recall that white light includes all the wavelengths of visible light.

The order of the colours of light in a spectrum are: red, orange, yellow, green, blue, indigo, violet [ROY G BIV].

Note that blue light bends more than red. So blue light travels slower than red light.

HomeworkHomeworkRead pg. 449-456Answer # 1-4 on pg. 455Answer # 1-8 on pg. 456