When light travels from one material to another it usually changes direction The bending of light that occurs at the borderline of two materials is called.

REFRACTION

• When light travels from one material to another it usually changes direction• The bending of light that occurs at the

borderline of two materials is called refraction

Geometrical concepts:-incident ray-refracted ray-normal to the point of incidence-incident angle-refracted angle

Refraction at the air-glass boundary

Conclusion

• When light waves travel from a less dense (air) to a more dense medium (glass), the ray is bent towards the normal.

What happens during refraction?

glass

air

The speed of light depends on the material through which the light is travelling.

Why is light refracted?

When light enters a different material (e.g. from air into glass), the speed of light changes.

This causes the light to bend or refract.

The speed of light is affected by the density of the material through which it is travelling. When light enters a more dense medium (from air into glass), its speed decreases and this is why refraction occurs

7

RefractionBecause the frequency of

the wave is determined by the source, if the wave speed slows down, its wavelength must decrease. (And vice versa) Fast Medium -

low refractive index like air

Slow Medium - higher refractiveindex like glass

Frequency stays the same

Refraction causes visual effects such as this ruler appearing to be bent.

Effects of refraction

This happens because light from the part of the ruler in the water is refracted as it travels from the water into the air.

Light rays from the stone are refracted as they leave the water. The brain assumes that these rays have travelled in straight lines and is fooled into forming an image where it thinks the light rays came from.

image

How does refraction make this stone look closer to the surface of the water than it really is?

actual location

9

Willebrord Snellius (1580-1626)

• Dutch astronomer and mathematician

• Most famous for the law of refraction (Snell’s Law)

• Found a new method for calculating the radius of the earth as well as π

Index of refraction • the amount of bending depends on the optical

properties of the two materials --> characterized by their index of refraction: n

• n is a number: n=1 for vacuum, n=1.33 for water, n=2.42 for diamond, n=1.5-1.9 for different types of glass …….

• when the amount of bending is bigger, the difference in n is bigger for the two materials

The laws of refraction: Snell’s laws• If light travels from material 1 with index of

refraction n1 to material 2 with index of refraction n2 the following laws determine the direction of the refracted ray:

2211 )sin()sin( nn

1

2

1

2

The incident ray, the normal to the incidence point and the refracted ray are all in one plane

12

Absolute Refractive Index (n)

• is a measure of how much the speed is slowed when entering a medium from a vacuum (or air)

• n = speed of light in a vacuum speed of light in a mediumValues include: n alcohol = 1.36 n diamond = 2.41 n glass = 1.52 n water = 1.33

Which substance does light travel the fastest?Why will you never get a number less than 1?