Light can make a scene appear stunning – this is a few pictures of Mt Cook and Mt Tasman reflected off Matterson lake in Southland, NZ. The colours of the deep blue sky, the red of the sunset the image in the water are all caused by light change direction due to the substances and surfaces it is interacting with. Without these common behaviours of light, such a scene would not be possible!
How does light behave? From the picture, what are some of the characteristics of light? What do the spot lights indicate about how light travels? What about the security people on the edge of the crowd, what helps us recognise them and what does that indicate about how light travels?
Handout – Diagrams for Light
Mirages – seeing things that aren’t there, the oasis in the desert, upside down trees, boats and surface water on the road. What are the usual weather conditions when a mirage occurs? How do you think the air changes as you move away from the surface upwards? How does the density change?
Sunsets – why does the sun appear larger in the sky? Is there any change in the medium that the light waves travel through? Change in density? What is different about the sun’s position in the sky? When the Sun is lower in the sky how is the angle the light waves enter the atmosphere different than whenever the Sun is overhead?
Mark on the angle of incidence and refraction for air to glass, then draw on the extra ‘normal line’ (from which all angles are measured) and mark on the angle of incidence and refraction for glass to air Allow time for the class to measure the angles and fill out the results table
Talk through the results
Why does the light change direction? It’s all to do with the change in speed… Another analogy is when a car’s wheels on one side of the car brake or lock causing the car to change direction or spin
The refraction of light Air gets thinner the higher you are from the surface of the earth or the sea. In normal conditions, the density of air decreases with increasing altitude. When light rays travel along the surface of the earth, the air below the light ray is therefore denser than the air above it. One of the typical properties of light is that it refracts towards the denser medium, and thus a ray moving along the surface of the earth is in fact constantly refracted slightly downward, following the earth's curve instead of escaping straight into space. Imagine if you will that the denser air exerts a kind of friction on the ray of light, drawing it towards itself. When we see something, we imagine that the object we are seeing is in the direction from which its light reaches our eyes. But when we look at the distant horizon, we see objects which are in fact partly below the horizon. The light from these objects is refracted in a low curve along the curved surface of land or sea, and reaches the observer's eye from what only appears to be the direction of the horizon. Many people must be familiar with the often repeated fact that when we are looking at the setting sun, it is in fact already below the horizon. In astronomy, this phenomenon is known as refraction: the refraction of light in the atmosphere raises objects in the sky at the horizon by about half an (angular) degree. This is all very well, but Nature has a way of surprising us: very often, the density of air does not change consistently with the altitude, and instead, colder, denser air and warmer air form layers of different temperatures at different altitudes. The movement of light in this kind of stratified ocean of air can be quite erratic, thus creating a strongly distorted image of the landscape — mirages, in other words. The stratification of the air, furthermore, is not necessarily clearly defined or stable, yet it may at times be strong enough to create at least a moment's amazing mirages. In Finland, the conditions for mirages are particularly favourable in spring, when the sea ice thaws. Sea water of 0° C combined with a spring 'heat wave' of 15° C in the air can create miraculous images.
Apparent depth dominoes Two beakers, one empty with a 2 pence piece blue-tacked to the bottom and white paper wrapped around it, the other filled to the top with water. Using the video camera set it up so it is aimed at the right angle so that the 2 pence piece is just hidden by the paper of the empty beaker. Fill the beaker up with water and the money should then be made visible by the light refracting as it comes out of the water. What the? Explain how this is visually possible. In your explanation use the words – refraction, light rays, speeds up, changes direction, change of optical density
Rainbows are one of the most beautiful spectacles nature has to offer -- so beautiful, in fact, that they've inspired countless fairy tales, songs and legends. 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.
Which colour is refracted the most? How do you remember the order of the colours?
We can see the relationship between colour, wavelength and amplitude using this animation.
When the white light passes from air into the drop of water, the component colours of light slow down to different speeds depending on their frequency. The violet light bends at a relatively sharp angle when it enters the raindrop. At the right-hand side of the drop, some of the light passes back out into the air, and the rest is reflected backward. Some of the reflected light passes out of the left side of the drop, bending as it moves into the air again. In this way, each individual raindrop disperses white sunlight into its component colours. So why do we see wide bands of colour, as if different rainy areas were dispersing a different single colour? Because we only see one colour from each raindrop . You can see how this works in the diagram When raindrop A disperses light, only the red light exits at the correct angle to travel to the observer's eyes. The other colour beams exit at a lower angle, so the observer doesn't see them. The sunlight will hit all the surrounding raindrops in the same way, so they will all bounce red light onto the observer. Raindrop B is much lower in the sky, so it doesn't bounce red light to the observer. At its height, the violet light exits at the correct angle to travel to the observer's eye. All the drops surrounding raindrop B bounce light in the same way. The raindrops in between A and B all bounce different colours of light to the observer, so the observer sees the full colour spectrum. If you were up above the rain, you would see the rainbow as a full circle, because the light would bounce back from all around you. On the ground, we see the arc of the rainbow that is visible above the horizon. Sometimes you see a double rainbow -- a sharp rainbow with a fainter rainbow on top of it. The fainter rainbow is produced in the same way as the sharper rainbow, but instead of the light reflecting once inside the raindrop, it's reflected twice. As a result of this double reflection, the light exits the raindrop at a different angle, so we see it higher up. If you look carefully, you'll see that the colours in the second rainbow are in the reverse order of the primary rainbow.
Draw around the prism on a blank page and mark on a normal line at 90 degrees to the edge of the glass. Send a ray of light into the prism and find the best angle where dispersion occurs. Use the colours to mark on the correct order the white light splits up into. In addition to bending light as a whole, a prism separates white light into its component colours. Different colours of light have different frequencies , which causes them to travel at different speeds when they move through matter. A colour that travels more slowly in glass will bend more sharply when it passes from air to glass, because the speed difference is more severe. A colour that moves more quickly in glass won't slow down as much, so it will bend less sharply. In this way, the colours that make up white light are separated according to frequency when they pass through glass. If the glass bends the light twice, as in a prism, you can see the separated colours more easily. This is called dispersion .
White light split by a prism shows us the spectrum of colours that makes up the visible light spectrum. What characteristic of the colours of light account for the white light splitting? (Wavelength). Wavelengths of light higher or lower than what we can sense exist.
Waves- Light GCSE Physics
Learning Intentions <ul><li>State how light travels </li></ul><ul><li>Recall how objects can be seen by the eye </li></ul><ul><li>State and use the law of reflection for simple problems </li></ul>
Do you see what I mean? <ul><li>A ray of light can be represented as a simple straight line with an arrow pointing the direction the light is travelling. </li></ul>Luminous Non-Luminous Directly into our eye Reflected into our eye
The Facts of Light <ul><li>Light is a form of energy </li></ul><ul><li>We see an object when the light energy leaving it enters our eye. </li></ul><ul><li>Objects which give out their own light are called luminous </li></ul><ul><li>Those that do not give out light scatter the light that falls on them are called non-luminous </li></ul><ul><li>Light travels in straight lines in the form of a transverse wave </li></ul>
Light Reflecting <ul><li>Page 93 </li></ul><ul><li>Using the Method from page 93 find out how the angle of incidence is related to the angle of reflection </li></ul><ul><li>Watch the straight-forward demo and repeat for 3 different rays of light </li></ul>
Normal Incident Ray Reflected Ray Angle of incidence Angle of reflection Smooth Plane Surface Angle of incidence = Angle of reflection Light Reflection
Learning Intentions <ul><li>Investigate the path of a light ray through a glass block </li></ul><ul><li>Recall the term ‘ refraction ’ and predict how light is refracted in different media </li></ul>
Refraction Experiment <ul><li>Investigate how light travels as it passes from air to glass, then from glass to air </li></ul>
Results Table From Air to Glass Angle of Incidence Angle of Refraction 1. 2. 3. 4. From Glass to Air Angle of Incidence Angle of Refraction 1. 2. 3. 4.
What to do…… <ul><li>Draw around the Perspex block on a piece of paper. </li></ul>
The Normal <ul><li>Mark on a line known as the NORMAL perpendicular to the surface of the block. </li></ul>For the light ray entering the block and the light ray leaving the block mark each ray with two crosses. Draw in the incident ray , emergent ray , remove the block and then join up the two rays. Repeat two more times with the light ray entering the block at different angles.
Results If the light ray entered the block parallel to the normal then it travels through un-deviated. If the incident ray enters the block at an angle to the normal then the direction of the ray changes as it enters and leaves the block, the light ray has been refracted. Does the angle of incidence …… … affect the angle of refraction?
Air to Glass Angle of Incidence Angle of Refraction 1. 2. 3. Glass to Air Angle of Incidence Angle of Refraction 1. 2. 3.
What happened…… <ul><li>As the light ray moved from air into perspex? </li></ul><ul><li>As the light ray moved from perspex into air? </li></ul><ul><li>If the angle of incidence = 0 °? </li></ul><ul><li>What do you notice about the incident ray and the emergent ray? </li></ul>
Dense Material Less Dense Material Normal Incident Light Refracted Light Angle of Incidence Angle of Refraction
Air to Perspex angle of incidence > angle of refraction <ul><li>As the light ray moved from air into perspex it moved towards the normal. </li></ul><ul><li>If light rays move from a less dense material (air) to a more dense material (perspex) they ‘bend’ towards the normal. </li></ul> i > r
Perspex to Air angle of incidence < angle of refraction <ul><li>As the light ray moved from perspex into air it moved away from the normal. </li></ul><ul><li>If light rays move from a more dense material (perspex) to a less dense material (air) they ‘bend’ away from the normal. </li></ul> i < r
Angle of incidence = 0 ° <ul><li>When the angle of incidence is 0 the light ray is not deviated from its path. </li></ul><ul><li>Un-deviated light ray </li></ul>
Refraction- summary table Angle of Incidence greater than / less than angle of refraction Angle of Incidence greater than / less than angle of refraction Angles Towards / Away from the normal line Towards / Away from the normal line Change of direction Increase / Decrease Increase / Decrease - From less dense to dense (air to water) - From dense to less dense (water to air) Change of speed Change of optical density
Refraction <ul><li>Many surfaces also refract light: rather than bouncing off the surface, some of the incident ray travels through the surface, but at a new angle. </li></ul>
Learning Intentions <ul><li>Recall the term ‘ refraction ’ and predict how light is refracted in different media </li></ul><ul><li>Draw light ray diagrams to illustrate ‘ apparent depth ’ </li></ul>
Apparent Depth <ul><li>When light travels from a dense to a less dense material it bends away from the normal (speeds up) </li></ul><ul><li>Our eye senses that light travels in a straight line </li></ul><ul><li>The object’s image ‘appears’ to be closer to the surface of the water. </li></ul>Image Water Air
Learning Intentions <ul><li>Investigate the path of a light ray of white light as it passes through a glass prism </li></ul><ul><li>Recall the mean of the terms ‘ dispersion ’ and ‘ spectrum ’ </li></ul><ul><li>Relate the wavelength of light to refraction </li></ul>
Dispersion Red light is refracted least. Violet light is refracted the most. R O Y G. B I V ed range ellow reen lue ndigo iolet White light
Dispersion (page 95) The white light ray is split into a spectrum of colours. This is known as dispersion . The different colours of light have different wavelengths . Different wavelengths are refracted different amounts. White light Glass prism
Dispersion Summary Red/violet Refracted more/less by glass Longer/shorter wavelength Red/violet Refracted more/less by glass Longer/shorter wavelength
Learning Intentions <ul><li>recall that visible light is one of many types of electromagnetic waves </li></ul><ul><li>state all types of EM waves </li></ul><ul><li>state the similarities and differences between EM waves </li></ul>
Learning Intentions <ul><li>Recall how an image is viewed in a mirror </li></ul><ul><li>State the differences between the object and its mirror image </li></ul><ul><li>Use ray diagrams to demonstrate how an image appears in a mirror </li></ul>
Uses of Reflection <ul><li>The most common use of reflection is to view objects using a mirror (shiny flat surface). </li></ul><ul><li>In science we refer to the reflection as an ‘image’ in the mirror. </li></ul>
The Image in a Plane Mirror <ul><li>The characteristics of images in a plane mirror are: </li></ul><ul><li>upright </li></ul><ul><li>virtual (behind the mirror and cannot be projected on a screen) </li></ul><ul><li>laterally inverted (right side of object is left side of image) </li></ul>R R Object Image Mirror
Image size and distance For a plane mirror the size of the image is the same as the object’s and the image distance from the mirror is the same as the object’s distance from the mirror. See page 94 for extended notes on an image in a plane mirror mirror Object Image
Shadows <ul><li>Light can travel through materials which are transparent, e.g. air, glass or water. </li></ul><ul><li>If light cannot travel through a material it is called opaque , e.g. metal, wood etc. </li></ul><ul><ul><ul><li>- If an opaque object is placed in the way of a beam of light a shadow is created. </li></ul></ul></ul><ul><li>- The kind of shadow that is formed depends on the size of the light source. </li></ul>