REFRACTION!!!! The bending of light

• Aaah! We said before that light travelled in a straight line, so why does this bending of light happen?!?!?

• It happens because light travels at different speeds in different materials or MEDIUMS

• Medium: any physical substance through which energy can be transferred.

• Still…why does this cause light to “bend”?

A car will turn towards the slower wheel!!

Refraction: the change in direction of light when it travels from one medium to another

Angle of Refraction: the angle that a refracted ray makes with the NORMAL!!!

SummaryIf light travels from a faster medium to a slower medium, it will “bend” towards the normal i.e.

r i

• medium

#1: Fast• medium #2: Slow

• normal

• incident ray

• refracted ray

• qi

• qr

• If light travels from a slower medium to a faster medium, it will “bend” away from the normal i.e.

r i

medium #1: Slow

medium #2: Fast

normalincident ray

refracted ray

qi

qr

Phenomenon Resulting from Refraction

a) Apparent Depth – objects will appear to be less deep in water than they actually are.

• Light rays from the object bend away from the normal as they exit the water. The eye then backtracks the refracted ray in a straight line and assumes the light came from that direction (i.e. assumes object is there/sees an image there)

b) Bent pencil

Similar idea to apparent depth

c) Twinkling Stars • Different layers of the atmosphere have different temperatures which makes them like different mediums

• This means light bends as it travels through different layers of the atmosphere

• Hot air = slower medium

• Air closer to Earth is warmer (hence slower)

• This can cause stars to appear in a slightly different position than they actually are

• Because the air is constantly moving, light from a star won’t take exactly the same path as the light from that same star a second earlier.

• This makes it look like the stars position is constantly changing

• This causes the “twinkling” effect of the stars

• For picture: think of each colour as light from the same star at different times

How fast is light???

• Light travels fastest in a vacuum.

• Speed of light in a vacuum is

• This means it can travel around the Earth 7.5 times in one second!!!• (or in other words it takes about 0.13 s to travel around

the Earth)

83.00 10 /m s

Other materials

• Light travels more slowly in all other mediums• The more “optically dense” the material, the

slower light travels through it

• e.g. 8

8

8

2.26 10 /

1.97 10 /

1.24 10 /

water m s

crown glass m s

diamond m s

Index of Refraction

• A different way to talk about the speed of light in a certain medium

• It compares the speed of light in that medium to the speed of light in a vacuum.

cn

v

Where c is the speed of light in a vacuum (i.e. )v is the speed of light in the other medium (in m/s)n is the index of refraction of the other medium (no units associated with this)

83.00 10 /m s

• Example: Calculate the index of refraction of a sapphire if the speed of light in it is 1.70 x 108 m/s.

n = ?c = 3.00 x 108 m/s = 3.00 x 108 m/sv = 1.70 x 108 m/s 1.70 x 108 m/s

= 1.76

The index of refraction in sapphire is 1.76.

cn

v

Example: Calculate the speed of light in amethyst if its index of refraction is 1.54. v = ?

n = 1.54

c = 3.00 x 108 m/s

= 3.00 x 108 m/s 1.54 = 1.95 x 108 m/s

Light travels at a speed of 1.95 x 108 m/s in amethyst.

cn

vc

vn

Which material would have the biggest index of refraction?

8

8

8

2.26 10 /

1.97 10 /

1.24 10 /

water m s

crown glass m s

diamond m s

cn

v

n for air = 1.00

• Diamond : because it is the slowest!

1.33

1.52

2.42

water n

crown glass n

diamond n

Summary

• Slower mediums = higher index of refraction• Faster mediums = low index of refraction

– BUT : index of refraction can never go below 1!!!– Why?

• That would mean it was going faster than in a vacuum!!!

Homework

• Worksheet on index of refraction