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

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

8

Try answering these questions

1. What is refraction of light?2. Does the light bend towards or away from

the normal when the light enters at an angle into a more dense medium?

3. What happens to the speed of the light when it enters a more dense medium?

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?

Total internal reflection – simulation

Passing white light through a prism

A prism splits a ray of white light into the colours of the rainbow. This process is known as dispersion.The colours that make up white light are called the spectrum. The order of the colours in the spectrum is always the same.

Splitting white light

• Which colour is diffracted the most through the prism?

Violet is the most violently diffracted wavelength. Red is only slightly diffracted.

Violet has the highest frequency, energy and smallest wavelength.

Why does the sky appear blue?

• The atmosphere is densest (thickest) at the bottom, near the Earth. It gradually thins out as you go higher and higher up.

• The blue colour of the sky is due to Rayleigh scattering. As light moves through the atmosphere, most of the longer wavelengths pass straight through. Little of the red, orange and yellow light is affected by the air.

………..continued

• However, much of the shorter wavelength light is absorbed by the gas molecules. The absorbed blue light is then radiated in different directions. It gets scattered all around the sky.

WHY IS THE SUNSET RED?

• As the sun begins to set, the light must travel farther through the atmosphere before it gets to you. More of the light is reflected and scattered. As less reaches you directly, the sun appears less bright. The colour of the sun itself appears to change, first to orange and then to red. This is because even more of the short wavelength blues and greens are now scattered. Only the longer wavelengths are left in the direct beam that reaches your eyes.