7.2. Focusing devices: Unit 7.2. context. Lenses and curved mirrors. Lenses. The language of optics

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1 context 7.2 Unit 7.2 ocusing devices: Lenses and curved mirrors Light rays often need to be controlled and ed to produce s in optical instruments such as microscopes, cameras and binoculars, and to change the for people wearing contact lenses or glasses. We can control and light by using lenses and curved mirrors. Lenses use refraction to light. Curved mirrors use reflection. Orientation Images can be the same way up as the original. If so, they are referred to as being upright. Inverted s are those that have been turned upside-down. 226 ig Curved mirrors light into a point. Here an array of curved mirrors is being used to sunlight on a tower to boil water and generate electricity. The language of optics Lenses and curved mirrors light to produce a variety of different types of s. Special terms are used to describe the s formed. Real and virtual Images are either real or virtual. Virtual s do not have light rays actually passing through them. Instead, they are found by extending the light rays until they cross. Virtual s cannot be captured on a screen or directly on film. The seen in the bathroom mirror is virtual. Real s are formed wherever light rays cross. Real s are difficult to see as they float in space and need to be captured on a wall, a sheet of paper or a screen. Once captured, they are easily seen and can actually be touched. The light energy contained in a real will cause reactions in chemicals on photographic film which will permanently record the. This cannot be done with a virtual. A projector produces a real that cannot be seen until a screen is placed in its way. Magnification An is described as enlarged if it is bigger than the original. It is described as diminished if it is smaller. Magnification specifies exactly how enlarged or diminished the is. To calculate magnification, divide the size of the by the size of the. or example, if a 2 cm produces a 10 cm, then its magnification is times. If the is diminished, then magnification will be a fraction. or example, if an is 8 cm high and the is 4 cm, then its magnification is 4 8 ½ or 0.5. Lenses The two main types of lenses are: convex lenses these curve outwards and are fatter in the middle concave lenses these curve inwards (a little like a cave) and are thinner in the middle. Convex lens Concave lens parallel rays of light principal axis principal axis parallel rays of light focal length focal length ig The can be found by shining light rays directly onto the lens. The of a convex lens is very obvious. The of a concave lens is not as obvious but can be found by tracing back the refracted rays. Prac 1 p. 234

2 Convex lenses Convex lenses produce two different types of s, depending on where the is located. Real s if the is at a distance greater than the focal length of the lens, an inverted real is formed. A real can be projected onto a screen or even onto film, which will then permanently record the. Virtual s if the is at a distance less than the focal length of the lens, a magnified, upright virtual is formed. This can t be projected onto a screen. Prac 2 p. 235 Unit 7. 2 weak lens strong lens ig The shape of a lens can affect its focal length. long focal length short focal length Ray tracing diagram Convex lens What you see ig A real is formed by a convex lens when the is beyond the. screen real real a scale drawing that allows you to predict the size and location of the produced by the lens Ray tracing diagram Convex lens What you see ig A virtual is formed by a convex lens when the is inside the. This is how a simple magnifying glass works. virtual virtual eye traces rays back to form a virtual 227

3 ocusing devices Concave lenses Concave lenses produce only upright, diminished virtual s. Ray tracing diagram What you see virtual eye traces rays back to form a virtual virtual ig Virtual formation in a concave lens. inding the focal length Rays coming into a lens from a distant are almost parallel and form an very close to the. ocal length can be found by measuring the distance from the lens to the. distant almost parallel rays approximate focal length convex lens real, inverted ig An of a distant can be used to find the approximate focal length of a convex lens. Worksheet 7.3 Lenses ocusing devices Prac 3 p. 236 Science Clip What s in a name? The word lens means lentil in Latin. Lentil seeds have the same shape as small convex lenses. The eye ocusing in the eye is performed in two stages by two separate lenses. Most of the ing is performed when the light first enters the eye and passes into the cornea. The cornea is a curved transparent membrane that acts as half a convex lens. The cornea collects the light rays from the world around us and helps them to converge onto a second lens sitting just behind the cornea. The curvature of the second lens is adjustable and es the light rays on to the back of the eye where the retina detects the s and sends them to the brain. 228

4 light rays cornea pupil lens Science Clip Eye for an eye The ciliary muscles stretch and relax the jelly-like lens in the human eye so it gets thinner or thicker. Its curvature therefore changes, allowing us to on s at different distances. Unit 7. 2 Prac 4 p. 237 ig Light from an is ed first by the cornea and then by the lens, which projects a real onto your retina. The projected onto the back of our eye is actually upside-down but our brain automatically inverts the. missing <<S3_2_7_02_10>> ig A magnifying glass is a simple microscope that helps you see small s. Magnifying glass A magnifying glass is a simple microscope consisting of a single convex lens allowing you to view small s. or the magnifying glass to work, the being viewed must be less than one focal length from the lens. However, this creates a virtual that cannot be projected onto a screen. Therefore you need the lenses in your eye to re the diverging light rays to form a real on the back of your eye allowing you to view the magnified. 229

5 ocusing devices virtual formed by eyepiece lens convex eyepiece lens (thick) telescope real formed by ive lens convex ive lens (thin) ig When you look through a telescope you see an of an. Telescopes Telescopes make small, distant s appear larger. By itself, a single lens will only produce smaller s of s a long way away. The stars and the Moon would appear even smaller! In order to produce a magnified of such s, two lenses are used. The ive lens in a telescope produces a real, inverted just inside the of a second lens, called the eyepiece lens. The produced by the first lens now acts as the for the second lens. Because the first is inside the of the second lens, the second (the one seen by the telescope user) is virtual and enlarged compared to the first one. The thinner the first lens (ive lens), the larger the first. But thin lenses have longer focal lengths this is why telescopes are long instruments. A telescope is ed by adjusting the distance between the two lenses. The produced by a simple telescope is upside down, but this is usually not important when viewing s such as planets and stars. Curved mirrors Curved mirrors and lenses have many similarities. Like lenses, curved mirrors have Prac 5 p. 237 a principal and a focal length. Curved mirrors can also form both real and virtual s. As a result, mirrors can also be used to magnify and project s. There are two main types of curved mirrors: convex mirrors these bulge out in the middle concave mirrors these are thinner in the middle. concave mirror parallel rays of light principal axis convex mirror parallel rays of light principal axis focal length focal length ig The of a curved mirror can be found by shining light rays directly onto it. The of a concave mirror is very obvious. The of a convex lens is not as obvious, but it can be found by tracing back the reflected rays. Concave mirrors Concave mirrors produce an enlarged (magnified) virtual of an placed close to the mirror. These enlarged close-up views make them useful when shaving or applying makeup, or when a dentist needs to look at some tooth decay. 230

6 Ray tracing diagram ig If an is held at a large distance from a concave mirror, a real, inverted is produced. Unit 7. 2 outside the real What you see light from distant of distant ig When an is a very large distance from a concave mirror but directly in front of it, a very small, real is produced at a point known as the. Convex mirrors Convex mirrors gather rays of light from a wide area to produce a smaller, virtual behind the mirror. Convex mirrors are useful when a wide view is needed. They are used in shops for security across the whole store, at dangerous intersections where vision is difficult, and in some car rear-vision mirrors to give a wider view of what is behind the car. Worksheet 7.4 Mirrors Prac 6 p. 238 Prac 7 p. 238 ig A convex mirror produces a wider view than a flat mirror. virtual Ray tracing diagram What you see ig A convex mirror produces only virtual, smaller s. 231

7 ocusing devices 7.2 QUESTIONS Remembering 1 Recall the two main types of lenses and curved mirrors, sketching and naming each type. 2 Draw a diagram to recall how to find the focal point of a concave: a mirror b lens 3 Specify the two parts of the human eye that light. Understanding 4 Define the terms: a focal length b principal axis c myopic d hyperopic L 5 At the movies you see real s, not virtual ones. Explain how you can tell. 6 Copy the following and modify any incorrect statements so that they become true. a Real s formed by convex lenses are always bigger than the original. b Virtual s formed by convex lenses are always bigger than the original. c Concave lenses can form only virtual s. d Images in a concave lens are always the right way up. e Real s in a concave lens are always the right way up. 7 Use the ray tracing diagrams (pages 227 and 228) to describe what happens to the when a distant is brought closer to: a a convex lens b a concave lens 8 Describe how a lens or mirror could be used to start a fire. 9 An must be formed on the retina for it to be seen clearly. Explain how convex and concave lenses are used in spectacles and contact lenses to correct each vision defect shown in igure Illustrate your answers with a diagram. a b short-sightedness distant long-sightedness ig Applying 10 Copy the lenses in igure and identify each as concave or convex. ig Copy and complete the ray tracing diagrams in igure to demonstrate the path taken by the light rays. a b ig close eye eye retina retina 232

8 12 The terms diverging (moving apart) and converging (coming together) may be used to describe lenses. Identify which term applies to: a a convex lens b a concave lens 13 Convex lenses can form real s, virtual s and an uned blur. Identify where the would need to be to produce each type of. 14 A camper is using a magnifying glass to set a piece of paper on fire. Identify what type of lens is being used and what the hot spot on the paper is an of. 15 Demonstrate what the following terms mean by re-drawing the stick figure in igure : a inverted b magnified c diminished 19 A curved mirror produces a large upright when held close to an. Identify the type of mirror it is likely to be. Analysing 20 A convex lens can produce an enlarged of an insect. Analyse why it can t produce an enlarged of the Moon. Evaluating 21 You have two lenses one thick and one thin to build a telescope. Propose which one you should use for the eyepiece and which one for the ive lens. Creating 22 Construct a diagram that shows why no is formed of an placed at the of a convex lens. 23 igure shows the of a person as seen in a dessert spoon. a Identify whether the spoon is acting as a lens or a mirror and whether it is convex or concave. b Use the language of optics to describe the as fully as you can. c Construct a ray diagram showing how the was formed in the spoon. Unit 7. 2 ig You look into a magnifying glass at an ant. a State whether you can touch the ant or its. b Identify whether the is real or virtual. 17 Calculate the magnification in each case for s produced by various lenses. N Object height Image height 2 cm 6 cm 5 cm 20 cm 25 mm 5 mm 16 mm 4 mm 8 mm 160 mm 18 Identify which type of mirror would be best for use: a at a dangerous intersection b by a dentist ig

9 ocusing devices 7.2 INVESTIGATING Investigate your available resources (for example, textbooks, encyclopaedias, internet) to complete the following tasks. 1 Research the history of an optical instrument such as the telescope or camera. Include the following information: a who invented it and when b what improvements have been made over the years and by whom c a diagram of the first instrument developed and a diagram of a modern version of this instrument include a discussion of some of the differences or improvements between the original and modern versions of the instrument. Present your information in a written report that includes a timeline. L 2 Research one type of sight defect such as long-sightedness, short-sightedness, cataracts, night blindness or colour blindness. ind out the following information: a what causes the defect b the symptoms displayed (include diagrams if applicable) c any treatment(s) available to control or cure the defect. Present your research as an information leaflet that may be found in a doctor s surgery. L e xploring To explore how telescopes, microscopes, binoculars and cameras work, a list of web destinations can be found on Science ocus 3 Second Edition Student Lounge. 7.2 PRACTICAL ACTIVITIES 1 Water lenses To investigate how water droplets can be used as a lens eye dropper or pipette a printed A4 sheet of glossy paper with fonts of various sizes small beaker of water wire loop as shown in igure Part A 1 Use the eye dropper or pipette to place droplets of water on the printed A4 sheet. 2 Observe how the size of the droplet affects the appearance of the text below it. Part B Does it matter if a lens is hollow on the inside? Will curved surfaces with nothing (but air) in between have the same effect as a solid lens? Design your own experiment to examine these questions. 1 Identify if the water droplet is behaving as a convex or a concave lens. 2 Explain why text below a smaller droplet appears bigger.? DYO ig

10 2 Lenses and a light box To investigate the refraction of light through various lenses light box and multiple-slit slide 12 volt power supply light box lenses set sheet of paper 1 Adjust the light box (using the knob on top) to produce a wide beam of light with parallel edges on a piece of paper. 2 Direct a wide beam of light through a lens shape with no slide inserted in the light box. 3 Now use the slide with multiple slits to direct several parallel beams of light through the lens. Use a pencil to mark parts of the light paths. 4 Remove the lens and light box from the paper and rule the complete light paths. 5 Repeat steps 1 to 4 for several different lenses, including concave lenses. (Use a new piece of paper in each case.) ig ray box lens Unit Describe in words the effect of: a a convex lens b a concave lens 2 Compare the light path through a wide convex lens with that through a thin one. 3 Identify whether there are any individual light rays that are not bent by the lens in each case. 4 What were the focal lengths of the lenses you used? Construct a trace or sketch of each lens and write the focal length under each one. >> 235

11 ocusing devices 3 Images in a convex lens To investigate the formed by different convex lenses convex lens concave lens white card or screen plasticine or lens holder metre ruler candle or small globe with power supply 1 Set up your apparatus as shown in igure Determine the focal length of your lens by using it to form an of a window 5 metres or more away on your card/ screen. Measure the distance of the /screen from the lens this is the focal length. 3 Use your apparatus to obtain the sharpest possible on the screen with the candle or lamp more than two focal lengths from the lens. A darkened room will help. Copy the table below, and record your measurements. 5 Repeat for the other positions described in the table below. ig Attempt to repeat this experiment with a concave lens and record your results. 1 Describe what happened as the was brought closer to the lens. 2 Summarise the circumstances in which: a a real (on a screen) is obtained b a virtual (one that cannot be caught on a screen) is obtained c no is obtained 3 Assess whether it is possible to form a real (one that may be caught on a screen) using a concave lens. 4 Explain how the changes as the -to-lens distance is varied. Convex lens focal length: cm Object Image Diagram Description of position Distance from lens (cm) Distance from lens (cm) Description (e.g. larger/smaller, inverted/upright) Object more than two focal lengths from lens Object two focal lengths from lens Object between one and two focal lengths from lens Object less than one focal length from lens (i.e. inside the focal length) 236 Object exactly at the (one focal length from lens)

12 4 Simulating imperfect vision To investigate the s formed by convex and concave mirrors convex lens white card or screen extra piece of card plasticine or lens holder metre ruler candle or small globe with power supply 1 Set up your apparatus as shown in igure (Prac 3) so that you project a sharp, well-ed onto the screen. 2 Simulate an eye that is short-sighted by moving the screen away from the lens and describe what happens to the. 3 Record at what point the becomes unrecognisable. 4 Simulate an eye that is long-sighted by moving the screen towards the lens and describe what happens to the. 5 Record at what point the becomes unrecognisable. 6 Re the then cover ¹/ ³ of the lens with the extra piece of card and record what happens to the. 7 Increase the fraction of the lens that is covered by the card and record your observations. 1 Calculate the percentage difference between the focal point and the position where the becomes unrecognisable for long- and short-sighted simulations. 2 Describe what happens to the as more of the lens is blocked by the card. 3 Explain what this tells us about how the lens forms the and draw a ray diagram to demonstrate your explanation. Unit Telescopes and microscopes To investigate how telescopes and microscopes form s two convex lenses one thin (e.g. focal length 25 cm) and one thick (e.g. focal length 5 cm) cardboard scissors tracing paper or other translucent material (e.g. thin plastic from a shopping bag) lamp small to view Part A: The telescope 1 Construct and assemble the apparatus as shown in igure Place the a large distance (e.g. 1 metre) from the ive lens, and move the eyepiece lens and screen to obtain the sharpest possible looking through the eyepiece lens. Note the size of the compared with the. 3 While looking through the eyepiece lens and observing the, remove the screen. You should still see the! Think about why. translucent screen eyepiece lens ig Part B: The microscope 1 Now move the close to the lens (but not closer than the focal length). 2 Adjust the position of the lenses to obtain an that is larger than the. ive lens 1 Distinguish between a telescope and a microscope. 2 Describe how the removal of the screen changes the in Part A (step 3) above. 237

13 ocusing devices 6 Exploring curved surfaces: spoons To explore the properties of the s reflected in the curved surfaces of spoons metal soup spoon 1 Place the concave side of a metal soup spoon very close to your eye and describe what you see. 2 Move the spoon away from your eye and describe your observations. 3 Repeat this procedure for the convex side of the spoon. 1 Identify when the s you are viewing are real s and when you are viewing virtual s. 2 Explain why the in the concave side of the spoon inverts as you move the spoon further away. 7 orming s with curved mirrors To investigate the s formed by convex and concave mirrors convex mirror concave mirror candle screen screen concave mirror Part A: Concave mirror 1 Arrange the apparatus as shown in igure Move the screen until you obtain a clear of the candle. 3 Investigate the different s produced with the candle at different distances from the mirror. Is there a position where it is impossible to obtain an on the screen? Can you see a virtual in the mirror? Part B: Convex mirror 1 Hold the mirror at arm s length and look at your. 2 Gradually move the mirror towards you, noting any changes in the as you do so. 1 Explain what happens to the as an is brought closer to: a a concave mirror b a convex mirror 2 Identify which type(s) of are possible in each type of mirror. candle () plasticine ig