Turnbull High School Physics Department CfE National 4 /National 5 Physics Unit 1: Waves and Radiation Section 3: Light Name: Class: 1
National 5 Unit 1: Section 3 I can state the law of reflection and describe the reversibility of light I can explain the term refraction. I can identify the normal, angle of incidence and angle of refraction in a refraction diagram. I can describe how light will behave travelling from one medium to another and explain it in terms of changes in wave speed. In addition: I can measure the focal length of a convex lens and calculate its power. I can explain the terms critical angle and total internal reflection. I can describe a use for fibre optics. I can carry out an experiment to find the critical angle for light in a material. I can draw ray diagrams for convex lenses and describe the image formed. I can identify eye defects and which type of lens should be used for sight correction. 2
Revision from S2: Light Reflection The diagram below shows the path of a ray of light when reflected off a mirror. The normal is a line drawn at 90 to the mirror. Angle of incidence = Angle of reflection The principle of reversibility of light states that a ray of light which travels along any particular path from some point A to another point B travels by the same path when going from B to A, e.g. in the above diagram the ray travels from A to O to B. If the direction was reversed then the ray would follow B to O to A. The Law of Reflection:- When light is reflected the angle of incidence (measured between the incident ray and the ) is always the angle of reflection (measured between the ray and the ). When a ray of light is shone back along one of the reflected rays, it travels back along the ray. This is called the reversibility of rays of light. 3
Curved Reflectors These can be used in transmitters and receivers of any waves, e.g. light, sound, infrared, microwaves, TV signals and satellite communication. Receivers and Curved Reflectors Fitting a c _ r dish to a receiver aerial can make the received signal s _. When incoming signals hit the c _ r dish, the dish f them all onto the r _ a _. The r _ a _ therefore receives a s _ signal than it would if the dish was not fitted to it. Show this by completing the diagram. 4
Refraction of Light Your teacher will show you a short power point on refraction. Light travels in straight lines called light r _. When light passes from one material into another of different density, its s changes, its w _ and so its d could also change (unless the light hits the material at 90 to its surface - along the normal). This change of speed of light when travelling from one material to another is known as r _. It may also cause a change in direction (bending) but not always. A normal is a dashed where a light ray hits line drawn at 90 to the surface of a material the material. 5
Experiment 1 Refraction of Light In a given material (called a medium) light travels in a straight line. When the light moves from one material to another it changes speed and as result it may bend as it enters the new material. This effect is called refraction. Set up a ray box to shine a single light ray through the following Perspex shapes. On blank paper trace round the shapes and trace the path of the ray(s) through, and after leaving the blocks. Complete the diagrams below (show normals and angles clearly). (a) Plane rectangular block Label the incident, refracted and emergent rays. Is the direction of the ray inside the block the same as outside? 6
(b) Plane rectangular block at an angle to the incident ray Label the incident, refracted and emergent rays. What happens to the ray when it: (a) Enters the block (b) Leaves the block What do you notice about the direction of the incident and emergent rays? 7
(c) Triangular prism Note: Note: 8
Experiment 2 Refraction in Lenses Set up a ray box to shine a three parallel light rays through the following lenses - trace the path of the rays through, and after leaving the lens. (a) (i) Convex lens (thin) Do the rays converge (come together) or diverge (spread out)? What is the point where the rays meet called? Mark in the focal length. 9
(ii) Convex lens (thick) Do the rays converge or diverge? What do you notice about the focus of the thick lens? Mark in the focal length. A (or ) lens brings rays of light to a focus. A thick lens has a focal length than a lens. 10
(b) Concave lens Do the rays converge or diverge (spread out)? Focal Length and Power of Lenses T lenses refract (b _) light more than t _ lenses - so t lenses are more p _. A powerful lens has a s focal length. Convex lenses have a p _ ( ) power. Concave lenses have a n _ ( ) power. 11
Note the power is given in dioptres (D); the focal length is measured in metres. Converging (convex) lenses have positive powers (e.g. +10 D, +17 D). Diverging (concave) lenses have negative powers (e.g. 10 D, 17 D). Example: A spherical convex lens has a focal length of 10 cm. Find the power of the lens. Problems: 1. A convex lens has a power of +5 D. Calculate the length of it s focal length in metres. 2. A convex lens has a focal length of 20 cm. Calculate the power of the lens. 3. Calculate the focal length of a lens with a power of -8D. What kind of Lens is this? 4. Calculate the power of a concave lens with a focal length of 40 cm 12
Experiment 3 Measuring the Focal Length of a Lens (Parallel Rays) What you need: Ruler, convex lenses, white paper What to do: 1. Choose a convex lens and use it to produce a sharp image of a distant object (e.g. a window) on the white paper. 2. Measure the distance from the lens to the image - this is the approximate focal length. 3. Complete the table for the different lenses. Convex lens shape X most curved Y curved Z least curved Focal length (cm) Power of lens (D) 13
(a) In which way do lenses with a short focal length look different from those with a long focal length? (b) Does the focal length of the lens affect the image (what is seen on the screen) in any way? (c) How can you tell from looking at two convex lenses which one is the most powerful? 14
Experiment 4: Refraction What you need: Ray-box kit; sheet of white paper; pencil. What to do: 1. Take the semi-circular glass block and place it on a sheet of paper and draw round it. 10 normal 20 30 40 50 2. Send a single ray into the block as shown, then measure the angle i, (angle of incidence), and angle r, (angle of refraction). 3. Repeat for different angles shown to complete the table. Angle of incidence, i (degrees) Angle of refraction, r (degrees) 10 20 30 40 50 15
Note: (a) The normal is the line drawn at to the surface. (b) The angle of incidence is the angle between the and the. (c) The angle of refraction is the angle between the and the. (d) How does the angle of incidence compare with the angle of refraction? (e) How would the angles of incidence and refraction compare if no refraction took place? (f) How would the angles of incidence and refraction compare if the block was made from a material which caused more refraction? (g) When a ray of light is sent into the centre of the straight edge of a semicircular glass block (as above) there is no change in direction when exiting from the curved edge. Can you explain this? 16
INVESTIGATION Experiment 5: Total internal reflection Aim: To investigate total internal reflection and measure the critical angle for Perspex. Apparatus: Semicircular Perspex block, a protractor, a ray box and power supply. Collect an Investigation Booklet. Instructions: State the aim of the experiment. Set up the apparatus as shown. Slowly increase the angle of incidence. Describe what happens. By drawing a ray on your diagram when the angle of refraction is 90, estimate the critical angle for the Perspex block 17
Critical Angle and Total Internal Reflection As the angle of incidence inside the block increases the angle in air increases. Beyond a certain angle the ray is no longer refracted out into the air. When no light passes from glass to air we have what is called Total Internal Reflection. The smallest angle of incidence at which total internal reflection takes place is called the Critical Angle. Different transparent materials have different critical angles. At angles greater than the critical angle all the light is reflected back inside the block! The critical angle for a material can be found using the experimental procedure described in experiment 5. 18
Summary of Refraction (a) Refraction takes place when light goes from one material (medium) to another. This causes the speed of the light to change. (b) When light goes from air to glass it: slows down and is bent towards the normal. AIR GLASS Incident Ray Refracted Ray Normal (c) When light goes from glass to air it: speeds up and is bent away from the normal. GLASS AIR Refracted Ray Incident Ray 19
Optical fibres An optical fibre has a dense solid glass core surrounded by a less dense solid glass coating. Unlike a mirror, there is not a "silvered surface" in the optical fibre. When a light ray hits the boundary between the core and coating at an angle greater than the C _ A, all of the light ray is reflected back into the optical fibre - This is known as T I _ R _. Optical fibres are used in medicine and telecommunications. Complete the diagram below: Optical fibres are often used in preference to copper wire in communications systems because they are: Cheaper to produce More lightweight and flexible Able to carry more signals per fibre and the signals are free from electrical interference. There is little loss of energy due to the pure glass. 20
Experiment 6: Optical Fibres Aim: To demonstrate light transmission through an optical fibre. Apparatus: A radio, an LED transmitter, a photodiode receiver, an amplifier, optical fibre and a 12 V light bulb with power supply. Instructions Hold one end of the optical fibre near a light bulb and view the other end. Set up the apparatus as shown above. Explain, using a diagram, how the light signal is carried through the fibre. 21
The Fibrescope (Endoscope) In one important development of this principle - fibre optics - flexible bundles of fine glass fibres transmit light round corners with ease. As each fibre is coated and transmits light independently of the others it is possible to 'see' through or take photographs through such a system. This principle is used to enable doctors to inspect inaccessible parts of the body - e.g. the stomach. Each individual fibre consists of a central core of high optical glass coated with a thin layer or cladding of another glass. This cladding prevents the light, which enters the end of the fibre, from escaping or passing through the sides to another fibre in the bundle. Light travels din the fibres using total internal reflection. The bundle used to carry the light from the external light source to shine on the object is called the light guide bundle. The bundle called the image guide is used to carry the image (picture) back to the eye. 22
The heat from the lamp does not pass down the fibres. This means that the other end of the guide is cold (called a cold light source). This is safer for the patient. Fibrescopes usually have a controllable bending section near the tip so the observer can direct the scope during insertion and be able to scan an area once inside. 1. Draw a simple diagram of an optical fibre to show total internal reflection. 2. Why is the end of the fibrescope 'cold' and explain why this is useful? 3. Which parts of the body do doctors study using an endoscope (fibrescope)? 4. Explain why two separate bundles of fibres are used in the fibrescope? 5. Why do you think that fibrescopes have a controllable, flexible bending section at the tip? 23
Tutorial 1 1. a) What is the law of reflection? b) Complete the diagram below to illustrate your answer to part (a). You must label the angle of incidence (i), the angle of reflection (r) and the normal on your completed diagram. mirror 2. a) i) Explain what is meant by the term refraction. ii) Name two examples of applications which make use of this refraction. b) Complete the ray diagram to show light entering and leaving the glass block. Label the diagram showing: 24
i) An angle of incidence (i) ii) An angle of refraction (r) iii) A normal 3. Copy and complete each of the following diagrams to show what happens to the ray of light as it passes from air into glass. 25
4. Light can be transmitted along an optical fibre through the process of total internal reflection. Complete the diagram below to show how the signal is transmitted along the optical fibre. 5. Doctors can use an endoscope to examine internal organs of a patient. The endoscope has two separate bundles of optical fibres that are flexible. a) Explain the purpose of each bundle of optical fibres in the endoscope. b) The tip of the endoscope that is inside the patient is designed to be very flexible. Suggest one reason for this. 26
Image Formation on the Retina Your teacher will show you a model eye. At the back of an eye, there is a layer of light-sensitive cells called the r _. When we look at an object, an image (picture) of the object is formed on the r _. The image is u _ d _ and l i _ ( b _ to f ). 27
Looking at Distant Objects Complete the diagram below: When we look at an object some distance from the eye, the light rays from the object which enter our eye are p _ to one another. The muscles around our eye lens are r, so the eye lens is t _. Looking at Close Objects Complete the diagram below: When we look at an object close to the eye, the light rays from the object which enter our eye are n p _ to one another. The muscles around our eye lens squash it, making the lens t so it can focus the light rays on the retina. 28
Ray Diagrams Image formation by a converging lens Images can be described as: real (light goes through the lens and image can be seen on screen) or virtual (we cannot get this image on a screen). inverted or upright magnified, same size or diminished (smaller). We can draw ray diagrams to determine the nature of the image formed by a lens for an object a particular distance away. Drawing Ray Diagrams Choose an appropriate scale (better done on graph paper). Draw ray 1 from the tip of the object parallel to the axis, passing through the focal point of the lens. Draw ray 2 from the tip of the object, passing through the centre of the lens. Where the two rays meet will be the tip of the image of the object. Object distance greater than twice the focal length 29
Object distance less than the focal length These two rays do not meet on the right hand side, there is no real image. However, we can extend the lines to the left to make them meet. The image is drawn where these two lines meet. This is a virtual image. (dashed lines show this). The light does not go backwards to the left of the lens! If we look through the lens from the right we will see this virtual image floating in space. We cannot get this image on a screen. 30
Experiment 7: Investigation of Image Formation Aim: To see how the position, nature and size of an image depends on the object distance from a converging lens. Apparatus: Converging lens of known focal length, lens holder, illuminated object, metre stick and a white screen, Graph paper. Instructions Set up the apparatus as shown above. Position the object distance at greater than twice the focal length of the lens. Move the screen until a clear image is formed on it. Note the type of image formed and the image distance from the lens. Repeat the procedure for the following object distances and complete the table: (a) Exactly two focal lengths (b) Between one and two focal lengths (c) Less than one focal length. Object position from lens More than 2xFocal length Exactly 2x focal length Between one and two focal lengths Less than one focal length Type of image Check your answers by constructing ray diagrams for each case. 31
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A person who is l _ s can see c objects which are f a _ - This is because the eye c focus the p _ light rays coming from the object on the r _. However, the person cannot see c objects which are c to them - This is because the eye c _ focus the n - p _ light rays coming from the object on the r _. Complete this diagram to show how a long-sighted eye focuses light rays from a close object To correct long-sight a c _ l _ is placed in front of the eye.complete this diagram to show the affect the lens has on light rays from a close object. 33
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A person who is s s can see c objects which are c - This is because the eye c focus the n - p _ light rays coming from the object on the r _. However, the person cannot see c objects which are d (f a _ ) - This is because the eye c _ focus the p _ light rays coming from the object on the r _. Complete this diagram to show how a short sighted eye focuses light rays from a distant object To correct short sight a c L _is placed in front of the eye. Complete the diagram to show the affect the lens has on light rays coming from a distant object. 35
Experiment 8: The model eye (Teacher Demonstration) Aim: To demonstrate how short and long sightedness can be remedied using lenses. Apparatus: Flask filled with fluoroscene Ray box Model of Retina Instructions With the rays focusing short of the retina, place a various lenses in front of the flask until they focus on the retina. Repeat with the rays focusing behind the retina. Short sightedness is corrected by wearing glasses with lenses. Long sightedness is corrected by wearing glasses with lenses. 36
Tutorial 2 1. Complete the diagram below to show how a healthy eye would focus parallel rays of light. retina 2. Copy and complete the table below to give information about short and long sight. Eye Defect Description Lens Used Short-sightedness Long-sightedness 3. Draw a convex lens, and show how it affects parallel rays of light. 4. Draw a concave lens, and show how it affects parallel rays of light. 37
5. Figure 1 shows light rays entering the eye of a longsighted pupil. a) Copy and complete the diagram below to show how the light rays reach the retina of this long-sighted eye. retina Figure 1 b) Complete the diagram below and, in the dotted box in Figure 2, draw the shape of the lens that would be used to correct this eye defect. retina Figure 2 c) On your diagram of Figure 2 complete the path of the rays from this lens until they reach the retina. d) When this sight defect has been corrected, the student looks at a picture in a text book. How does the image on the retina of the student s eye compare to the actual picture? 38
6. A girl wants to find the focal length of a convex lens she has found in the classroom. (a) Describe how the focal length of the lens can be found experimentally. Your description should include: (i) A list of apparatus used (ii) A description of the procedure b) Why does the girl use light from the window rather than from the classroom lights? c) The girl finds the focal length of the lens to be 18 cm. Calculate the power of the lens. d) The girl finds a second lens which is labelled with a power of 20 D. Name this type of lens. 39
7. A coin which is 1 5 cm high is held near to a lens whose focal length is 3 cm. 3 cm 1 5 cm axis Describe the image produced when the coin is held: (a) 4 cm from the lens. (b) 6 cm from the lens. (c) 8 cm from the lens. (Hint: You must draw separate ray diagrams on graph paper for each problem!) 40
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