# Liceo Scientifico “Belfiore” Mantova Physics course School year 2011-2012 Reflection and refraction of light.

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Liceo Scientifico “Belfiore” Mantova Physics course School year 2011-2012 Reflection and refraction of light

Light can travel travel through through a vacuum In the 17 th century, some properties of light were properties of light were already well known. For example: For example: Light has differentcolours Light can be reflected and refracted. These phenomena are described by the Laws of Reflection and the Laws of Refraction

Reflection 11 ’1’1  1 =  ’ 1 The phenomenon of reflection occurs when a beam of light changes its direction because of a mirror Specular reflection forms images

11 ’1’1  1 = angle of incidence  ’ 1 = angle of reflection The laws of reflection 1.The incident ray, the reflected ray and the normal to the reflection surface at the point of the incidence lie in the same plane 2.The angle which the incident ray makes with the normal is equal to the angle which the reflected ray makes to the same normal 3.The reflected ray and the incident ray are on the opposite sides of the normal

Refraction Light rays may bend as they cross a boundary from one material to another, for example from air to waterLight rays may bend as they cross a boundary from one material to another, for example from air to water This bending of light rays is known as refractionThis bending of light rays is known as refraction The light rays from the straw are bent (refracted) when they cross from water back into air before reaching the eyes of an observerThe light rays from the straw are bent (refracted) when they cross from water back into air before reaching the eyes of an observer

Refraction 11 ’1’1 A light ray passing from one transparent medium to another is partially reflected and partially transmitted or refracted A narrow beam of collimated and monochromatic light changes in direction due to its speed Refraction of light is the commonly observed phenomenon of the deviation of a monochromatic incident ray 22 medium 1 medium 2

Refraction 11 ’1’1 The relationship between the angle of incidence (formed by the incidence ray and the normal to the surface), the angle of refraction (formed by the refracted ray and the normal to the surface) and the index of refraction of the two mediums is described by Snell’s law 22 medium 1 medium 2

Snell’s law 11 ’1’1 22 medium 1 medium 2 Snell’s law states that for a given pair of media and a wave of a single frequency (monochromatic), the ratio of the sines of the angle of incidence θ 1 and the angle of refraction θ 2 is equivalent to the ratio of phase velocities (v 1 /v 2 ) in the two media, or equivalently, to the opposite ratio of the indices of refraction (n 2 /n 1 ): sin θ 1 / sin θ 2 = v 1 / v 2 = n 2 / n 1

Index of refraction

Dispersion of light and prism A ray of white light contains seven different frequencies: it contains the seven different colours of iris It was Sir Isaac Newton who discovered that sunlight, falling upon a prism, could split into its component colours This process in known as dispersion

Dispersion also occurs when a ray of white light is refracted, when it passes from a medium 1 to a medium 2 trough a transparent surface With a prism, the incident ray is dispersed when it is refracted passing from air to glass Every single ray is refracted a second time when it passes from glass to air and we can capture with a screen the very beautiful image of the rainbow Dispersion of light

This is the spirit of the experimental method of science: we reproduce the phenomena that naturally occur in our labs, through simple instruments The dispersion of colours in a prism occurs because of something called the refractive index of the glass When light enters a material, the difference in the refractive index of air and glass causes the light to bend Dispersion of light

The angle of bending is different for different wavelengths of light Different wavelengths of light will travel at different speeds and so the light will disperse into the colours of the visible spectrum, with longer wavelengths (red, yellow) being refracted less than shorter wavelengths (violet, blue) Dispersion of light

In a rainbow, raindrops in the air act as tiny prisms Light enters the raindrop, reflects off of the side of the drop and exits Dispersion and rainbow

In the process, light is broken into a spectrum just like it is in a triangular glass prism, like this

Dispersion and rainbow The angle between the ray of light coming in and the ray coming out of the drops is 42°for red and 40°for violet The angles cause different colours from different drops to reach your eye, forming a circular rim of colour in the sky: a rainbow

Double rainbow In a double rainbow, the second bow is produced because droplets can have two reflections internally and get the same effect The droplets have to be the right size to get two reflections to work

Rainbow The next time you spot a rainbow, you will see it in a whole new light

Rainbow and art A rainbow is one of the most beautiful events nature has to offer - so beautiful, in fact, that it has inspired countless fairy tales, songs and legends

Rainbow and art It's a good bet that most of the artists behind these tales were totally mystified by the rainbow phenomenon - just like most people are today « My heart leaps up when I behold A rainbow in the sky: So was it when my life began; So is it now I am a man; So be it when I shall grow old, Or let me die!… » William Wordsworth, 1802

Rainbow and science The science of rainbows is really very simple: it’s just basic optics

Light and colours It is amazing that a simple ray of white light contains seven different colours, for which seven different indexes of refraction exist

Experience and Physics If you will more deeply analyze the phenomena that we have explored in this lesson, you will discover other very important electromagnetic or quantum laws that are implied in an apparently simple optical experience