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Boardworks GCSE Separate Sciences 2009 Reflection and Refraction

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Presentation on theme: "Boardworks GCSE Separate Sciences 2009 Reflection and Refraction"— Presentation transcript:

1 Boardworks GCSE Separate Sciences 2009 Reflection and Refraction

2 Boardworks GCSE Separate Sciences 2009 Reflection and Refraction

3 Light moves in straight lines
Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Teacher notes This is an introductory slide to enforce the idea that light travels in straight lines. In order to further explain the concept of the umbra and penumbra, the shadow formation exercise could be linked to eclipses. If we see a total eclipse of the Sun then we are in the umbra of the Moon’s shadow. If we see a partial eclipse, only part of the Sun’s light can reach us and we are in the penumbra.

4 Boardworks GCSE Separate Sciences 2009 Reflection and Refraction
The law of reflection Boardworks GCSE Separate Sciences 2009 Reflection and Refraction When a light ray hits a mirror it changes direction: the ray is reflected. normal incident ray reflected ray i r point of incidence plane mirror angle of incidence (i) = angle of reflection (r) This is called the law of reflection and is true for any type of wave being reflected from a surface.

5 The law of reflection in action
Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Teacher notes This animation demonstrates a practical use of the law of reflection, and introduces the idea that mirrors form an image of an object. This image does not always have the same properties as the object, as demonstrated by viewing the image halfway down the periscope.

6 Describing images – size
Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Many images are enlarged or reduced versions of the object. The extent to which an image’s size differs from an object’s is known as the magnification. Many devices take advantage of the ability of mirrors and lenses to alter the size of an image. These include projectors, microscopes and binoculars.

7 Describing images – orientation
If the rays of light from the top and bottom of an object cross over before an image is formed, the image will appear upside-down. This is an inverted image. Inversion can also occur if rays from the right and left of an object cross over. This is known as lateral inversion and is seen most commonly in plane mirrors.

8 Describing images – real or virtual?
Boardworks GCSE Separate Sciences 2009 Reflection and Refraction When we look into a mirror we see an image. The image appears to be behind the mirror. If you look behind the mirror, the image is obviously not there, so we say it is a virtual image. A virtual image is one which cannot be formed on a screen. Teacher notes Ask students to think of examples of real and virtual images to test their understanding. A real image is one that can be formed on a screen, such as the real image from the projector, which you are reading now!

9 Describing images – quiz
Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Teacher notes This activity tests the students’ understanding of image formation and description. It could be used as a plenary or revision exercise, with students answering individually, or in teams.

10 Images in plane mirrors
Boardworks GCSE Separate Sciences 2009 Reflection and Refraction If we look into a mirror, we see an image. Teacher notes Students could be asked to create their own ray diagrams before the illustration is fully revealed. What kind of image is formed in the plane mirror? laterally inverted same size as the object virtual

11 Boardworks GCSE Separate Sciences 2009 Reflection and Refraction
Convex mirrors Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Convex mirrors are curved so that they bulge outwards. Convex mirrors are diverging mirrors. They reflect rays of light away from a focal point (F) which lies behind the mirror. F Rays parallel to the mirror’s central axis are reflected so that they appear to have come from this focal point. ƒ = focal length

12 Images in convex mirrors
Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Teacher notes This animation is designed to clearly illustrate how the law of reflection can be used to allow ray diagrams to be drawn for convex mirrors. Ensure all students are comfortable with the term tangent and can use the method shown in the animation to produce their own drawings. Q: Why is it simpler to draw the object on the mirror’s central axis? A: The central axis is the normal to the mirror. Drawing a ray to intersect this point simplifies the drawing process.

13 Boardworks GCSE Separate Sciences 2009 Reflection and Refraction
Concave mirrors Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Concave mirrors are converging mirrors, as they reflect rays of light towards a focal point (F). If a light source is placed at the focal point, the mirror will produce a beam of parallel light rays. F The distance between the mirror and the focal point is called the focal length (ƒ). ƒ becomes smaller as the mirror’s curve increases. ƒ

14 Finding f of a concave mirror
Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Choose a distant object to get parallel rays of light. ƒ Move the concave mirror back and forward to produce a clear image on a screen. Teacher notes This slide illustrates a simple method for finding the focal length of a concave mirror. This experiment could be carried out in the classroom. Use a ruler to measure the distance between the mirror and screen. This is the focal length (ƒ).

15 Images in concave mirrors
Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Teacher notes This experiment uses a different method for drawing ray diagrams, taking advantage of the focal point to simplify the diagram with multiple parallel light rays. Students may wish to draw their own ray diagrams for concave mirrors first, using the same method as shown for convex mirrors. Providing students with a worksheet of standard curved mirrors will help this exercise. The experiment can be used to demonstrate the importance of the objects’ position relative to the focal point; hopefully this will explain the different images formed in the students own diagrams. Emphasize the differences between the images when the object is in front of / behind the focal point.

16 Concave mirror summary
Boardworks GCSE Separate Sciences 2009 Reflection and Refraction When the object is beyond the focal point of the mirror the image is real and inverted. Its size varies depending on the object’s distance from F. When the object is between the focal point and the mirror the image is large, virtual and not inverted. Teacher notes There is no image when the object is at F as both refracted rays are parallel. The lines never intersect and the image forms at infinity. When the object is on the focal point the is no image. Can you explain why?

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What is refraction? Boardworks GCSE Separate Sciences 2009 Reflection and Refraction The straw appears to be bent in the liquid. What is causing this effect? As the light crosses the boundary between fluid and glass, it is bent, producing a distorted image. This known as refraction. Spear fishing has been used for centuries and is still practiced by subsistence communities. Photo credit: © 2009 Shutterstock, Roman Sigaev To accurately spear the fish, fishermen learn to aim a short distance behind the fishes’ image, in order to compensate for the effect of refraction.

19 Refraction in a glass block
Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Teacher notes This exercise should act as revision from KS3 Science. By activating the angle measurements in stages, students are given a chance to identify the various elements of the experiment. Initially they should try to define the normal, name the rays and angles shown and describe the relative angle size. Revealing the angles at the air to glass boundary will act to clarify these points to students. Students can then be challenged to identify i2 and r2, before revealing them to the class.

20 Refraction – labelling diagrams
Boardworks GCSE Separate Sciences 2009 Reflection and Refraction If an incident ray enters glass at an angle, then it is refracted, and bends towards the normal. The angle of incidence (i) is larger than the angle of refraction (r). normal incident ray refracted ray When the light leaves the glass, the opposite happens: it bends away from the normal. A material which light passes through, such as glass or air, is known as a medium.

21 Boardworks GCSE Separate Sciences 2009 Reflection and Refraction
Refraction summary Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Teacher notes This activity could be used as a plenary or revision exercise to check students’ understanding of refraction, and to test their ability to extract valuable information from a table.

22 Wavelength and speed effects
Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Teacher notes Draw the students attention to the way in which a change in direction of the wave coincides with a change in speed and wavelength. Ask students to identify the type of wave produced (transverse) and highlight that this is the same as a light wave. What other similarities can the students note between the behaviour of water waves and light waves?

23 Boardworks GCSE Separate Sciences 2009 Reflection and Refraction
A model for refraction Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Teacher notes This animation is designed to clearly explain the link between wave speed, direction and wavelength. At the end of stage one students could be challenged to predict what will happen to the column. Encourage them to explain why they have predicted this, and to describe the soldiers as a wave, using appropriate terms.

24 Boardworks GCSE Separate Sciences 2009 Reflection and Refraction
Speed of light Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Light travels at very high speeds. It reaches 300,000,000 m/s in a vacuum, and is marginally slower in air. This means that it takes light a mere eight minutes to reach the Earth from the Sun! In other materials the speed of light varies significantly: material speed of light (m/s) water 225,000,000 perspex 200,000,000 Teacher notes Students could be asked to research the speed of light in each material themselves. The idea of light taking time to reach its destination may also be discussed. The concept of light years could be introduced, and the students should realise that the light we see from the stars in the sky was actually emitted thousands of years ago. glass 200,000,000 diamond 120,000,000 As the speed of light varies depending on the medium, different materials refract light by different amounts.

25 Boardworks GCSE Separate Sciences 2009 Reflection and Refraction
Refractive index Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Refractive index is a measure of how much a substance slows down light. The higher its value, the more a medium slows light. The more the light is slowed, the more it bends towards the normal. Refractive index is calculated by comparing speed of light in a vacuum to that in a given medium: refractive index = speed of light in vacuum speed of light in medium Teacher notes There are no units for refractive index. The speed of light in a vacuum is 300,000,000 m/s, and the speed of light in water is 225,000,000 m/s. What is the refractive index of water? refractive index = 300,000, ,000,000 = 1.33

26 Boardworks GCSE Separate Sciences 2009 Reflection and Refraction
Snell’s Law Boardworks GCSE Separate Sciences 2009 Reflection and Refraction The refractive index can also be calculated using Snell’s Law, which uses the angle of incidence (i) and angle of refraction (r) to establish how much a medium slows light. refractive index (n) = sin i sin r normal Use the information in the diagram to find the refractive index of glass. Teacher notes Students should note that the equation can be easily rearranged to find both the angles of incidence and refraction if required. Students could measure the angle of incidence and refraction in a range of different materials and calculate the refractive index of these themselves. refractive index = sin 45° sin 28° refractive index = 1.5

27 Boardworks GCSE Separate Sciences 2009 Reflection and Refraction
Using Snell’s Law Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Use Snell’s Law to answer the following: Diamond has a refractive index of 2.4. If light passes into a diamond crystal at an angle of 15°, find the angle of refraction. sin r = sin i refractive index sin r = sin 15° = 0.1 2.4 r = sin-1 0.1 r = 6.2°

28 Using Snell’s Law Use Snell’s Law to answer the following:
Perspex has a refractive index of 1.5. If a ray of light passing into a perspex block has an angle of refraction of 24°, find the angle of incidence. sin i = sin r × refractive index sin i = (sin 24°) × 1.5 = 0.61 i = sin i = 37.6°

29 Using Snell’s Law Use Snell’s Law to answer the following:
If a ray of light enters water at an angle of 15° and has an angle of refraction of 11.2°, find its refractive index. refractive index = sin 15 sin 11.2 refractive index = 1.33

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31 Total internal reflection
Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Total internal reflection is when a light ray hits the boundary between two materials of different densities, and is reflected rather than refracted. There are two conditions for total internal reflection: The angle of incidence must be greater than the critical angle. The light must be passing from a high refractive index to a low one. Sometimes only part of a light ray will be reflected, while the rest crosses the boundary and is refracted.

32 Total internal reflection – a recap
Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Teacher notes This exercise is designed to test the students’ knowledge of total internal reflection covered in GCSE core science. Students could vote on the correct position of each label. There is a possible ambiguity if ‘refraction’ appears before ‘refraction along the boundary’, so allow students to adjust their answers appropriately if this causes confusion.

33 Boardworks GCSE Separate Sciences 2009 Reflection and Refraction
Optical fibres Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Optical fibres are thin strands of solid glass which are widely used in communication, medicine, lighting and as sensors. They exploit total internal reflection in order to carry beams of light over long distances and along winding paths. The glass core is often encased in a layer of cladding, which prevents light escaping the core. A protective plastic jacket surrounds the whole fibre. Teacher notes The students should identify the need for the cladding to have a lower refractive index than the core in order for total internal reflection to occur. Why are the materials used to make the core and cladding of an optical fibre important?

34 Critical angle in different materials
Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Different materials have different critical angles. How does the refractive index (n) of different materials affect the critical angle (c) at a boundary with air? medium n c ice 1.31 50° water 1.33 49° glass 1.5 42° quartz 1.54 40° Teacher notes Students could be asked to produce their own graph to establish the relationship between the refractive index and the critical angle. diamond 2.4 24° The greater the refractive index, the smaller the critical angle.

35 Calculating the critical angle
Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Each medium has a different critical angle. We can calculate the critical angle if we know the refractive index: sin c = nr ni What do you notice about this equation? The critical angle varies depending on the refractive index (n) of both materials at a boundary. Calculate the critical angle of perspex at a perspex to air boundary. perspex: n = 1.5 air: n = 1 sin c = 1 = 0.67 1.5 c = sin c = 42°

36 Calculating the critical angle – examples
Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Calculate the critical angle for this glass to water boundary. water n = 1.33 sin c = 1.33 = 0.89 1.5 glass n = 1.5 c = sin c = 63° Now repeat your calculation for an air to glass boundary. When light hits a medium with a higher refractive index: ni nr > 1 Teacher notes Ask the students to explain their answer in the repeat calculation. This should return an ‘error’ message on students’ calculators. This is because sin (x) has a maximum value of one, so the first material must have a higher refractive index than the second. As sin (x) has a maximum value of one, total internal reflection is impossible.

37 Total internal reflection – true or false?
Boardworks GCSE Separate Sciences 2009 Reflection and Refraction

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39 Boardworks GCSE Separate Sciences 2009 Reflection and Refraction
Prisms Boardworks GCSE Separate Sciences 2009 Reflection and Refraction A ray of white light can be split into a spectrum of colours. This is known as dispersion. The different colours of light have different wavelengths. The different wavelengths are refracted different amounts. Richard – red of – orange gave – green battle – blue in – indigo vain – violet York – yellow Which colour is refracted the most? violet

40 Speed of light in materials
Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Teacher notes Q: Why doesn’t a glass block disperse light? A: It does by a small amount inside the block, but as the light leaves, it is refracted back into a white ray.

41 Boardworks GCSE Separate Sciences 2009 Reflection and Refraction
Dispersion – summary Boardworks GCSE Separate Sciences 2009 Reflection and Refraction

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43 Boardworks GCSE Separate Sciences 2009 Reflection and Refraction
Glossary Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Glossary angle of incidence – The angle at which a light ray meets a mirror or boundary, measured from the normal. angle of reflection – The angle at which a light ray is reflected by a mirror, measured from the normal. angle of refraction – The angle, measured from the normal, at which a light ray is refracted at a boundary between one medium and another. concave mirror – A mirror that curves in at the centre. convex mirror – A mirror that curves out at the centre. critical angle – If the light hits a boundary between two materials at an angle greater than this, total internal reflection occurs. dispersion – The splitting of a light ray into its component colours by refraction. focal point – The point to which light rays parallel to the central axis are reflected by a concave mirror, or the point from which such light rays appear to have originated in a convex mirror. focal length – The distance between a curved mirror and its focal point. inversion – The reversal of an image so that it appears upside-down. lateral inversion – The reversal of an image so that the left-hand side appears to be on the right and the right-hand side appears to be on the left.# law of reflection – When light is reflected, the angle of incidence equals the angle of reflection. light – Electromagnetic waves that are detected by the eye. medium – A material through which light travels. normal – A line on a ray diagram drawn at right angles to the surface being hit by the light ray. real – An image which can be formed on a screen. refraction – The bending of light when it enters a different medium. refractive index – A measure of a medium’s ability to slow down light passing through it. Snell’s Law – A formula describing the relationship between the refractive index and the angles of incidence and refraction. total internal reflection – Reflection that occurs when light hits a boundary with a medium with a lower refractive index than the medium it is travelling through, at an angle greater than the critical angle. virtual – An image which cannot be formed on a screen.

44 Boardworks GCSE Separate Sciences 2009 Reflection and Refraction
Anagrams Boardworks GCSE Separate Sciences 2009 Reflection and Refraction

45 Reflection and refraction quiz
Boardworks GCSE Separate Sciences 2009 Reflection and Refraction Teacher notes This multiple-choice quiz could be used as a plenary activity to assess students’ understanding of reflection and refraction. The questions can be skipped through without answering by clicking “next”. Students could be asked to complete the questions in their books and the activity could be concluded by completion on the IWB.


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