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
Conclusion • When light waves travel from a less dense (air) to a more dense medium (glass), the ray is bent towards the normal.
Why is light refracted? air glass The speed of light depends on the material through which the light is travelling. When light enters a different material (e.g. from air into glass), the speed of light changes. This causes the light to bend orrefract. 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
Fast Medium - low refractive index like air Slow Medium - higher refractiveindex like glass Refraction • Because the frequency of the wave is determined by the source, if the wave speed slows down, its wavelength must decrease. (And vice versa) Frequency stays the same
Effects of refraction image actual location Refraction causes visual effects such as this ruler appearing to be bent. This happens because light from the part of the ruler in the water is refracted as it travels from the water into the air. How does refraction make this stone look closer to the surface of the water than it really is? 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.
Try answering these questions • What is refraction of light? • Does the light bend towards or away from the normal when the light enters at an angle into a more dense medium? • What happens to the speed of the light when it enters a more dense medium? • What happens to the wavelength of the light when it enters a more dense medium? • What is the speed of light?
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: The incident ray, the normal to the incidence point and the refracted ray are all in one plane
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 medium Values 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?
What is total internal reflection? When a light ray hits the boundary between a material which is more dense (large n, slower speed) to a less dense medium d (e.g. glass and air), there is a critical angle θc at which the angle of refraction is 90o. If the angle of incidence of the light ray is greater than a specific value, called the critical angle, then the light ray is actually reflected. This is called total internal reflection. It makes the inner surface of glass act like a perfect mirror.
Splitting white light RichardOfYorkGaveBattleInVain Y A prism splits a ray of white light into the colours of the rainbow. This process is known asdispersion. The colours that make up white light are called the spectrum. The order of the colours in the spectrum is always the same. Use this phrase to remember the order of colours:
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.