PHYSICS - UNIT 2: Light
Introductory Activities
Teacher Notes
These are a set of introductory activities. They are intended to stimulate the students' curiosity about what light does and to express that curiosity in questions that they would like to find answers to. The questions from the whole class and other physics classes are combined and submitted back to the students. At the end of the unit, as an assessment exercise, they are expected to submit written explanations to a specified number of the questions. The studnst choose which questions they answer.
The activities are designd to be short, taking about 5 - 10 minutes each and are done over two periods. The students can move freely from one to the other as activities become free.
The tasks are designed as a worksheet that the students complete and hand in. It does not need to be assessed, although some feedback to student answers and comments woudl be appropriate.
Student Worksheets (pages 2 - 5)
1. Reflection of Light
2. Large Curved Mirrors
3. Making Light bend
4. How deep is the pool?
5. How can a magnifying glass be flat?
6. Investigating a Slide Projector
7. Investigating Bifocal spectacles
8. Rainbow produced by a prism
9. Colour Mixing using Light boxes
10. Do you see more of yourself in a mirror if you go closer?
Other possible activities (page 6)
There are many possible activities that could be included, the only limits are availability of equipment, amount of bench space, and feasibility of the activity. Some grand ideas did not work in practice.
Context Questions (pages 7 & 8)
A compilation of the questions that previous students have produced is included. They are useful as a reserve to augment the questions a particular class or year generates.
Photonics (pages 9 - 11)
A small selection of the practical activities from the Photonics workshop for the Unit 4 Detailed Study that are relevant to Unit 2 have also be included.
Physics Unit 2 : Light
What is “The Introductory Activity?”
The Introductory Activity is a set of short practical tasks designed to stimulate your interest in the topic and to encourage you to come up with questions, called Context Questions, for which you would like answers.
These "Context Questions", will form an important part of the work of this topic.
Typical Context Questions might be:-
- How does a microscope work?,
- How do telescopes differ?,
- Why does light bend?,
- How is a rainbow formed?
After the short practical tasks, you are asked to come up with about three questions of your own that you would like answers to. You will then form a group of three with two others to compare your questions and come up with a list of about six questions that the group thinks are worthwhile.
When you have done this, your group will combine with another group and produce a list of about nine questions.
After this, the questions from these large groups will be combined for the whole class.
What's the purpose of the Context Questions?
The questions will guide our work during the topic. Answers to many of the questions will come up during class work, experiments, problem solving and general reading.
At the end of the topic, as part of your Assessment you will need to submit answers to a selection of the questions.
1. Reflection of Light. How do flat or “plane” mirrors work?
Equipment: Mirror strip, pin board, sheet of paper, 4 pins and a protractor
· Place a strip mirror in the middle of the page parallel to the short edge. Now draw a line along the back of the mirror.
· Place two pins in the page, so that they are in a line pointing towards the mirror at an angle between 30 and 60 degrees, and a few cms apart.
· With your eye down at mirror level, look into the mirror and move your head to locate the images of the two pins in the mirror.
· Move your head until the images of the pins in the mirror appear one behind the other.
· Now place two extra pins in the page between your eye and the mirror so that these two pins line up with the images of the first two pins.
· Now you move the pins and mirror away and draw lines through each of the pairs of pins towards the mirror line.
Where do your two lines cross?
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· Draw a line at right angles to mirror line where the two lines cross and measure the two angles between this line and your two pin lines.
How do the angles compare? ......
2. Large Curved Mirrors
There are two large mirrors. One curves inwards and is called “Concave”, the other curves outwards and is called “Convex”.
· For each mirror, walk up to it from several metres away. Describe how your appearance or “image” changes. (Refer to size, direction and distortion if any.)
Concave Mirror: ......
Convex Mirror: ......
Where have you seen Convex mirrors before?
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· Stand about a metre in front of the convex mirror and then in front of a plane mirror and compare how much of the room you can see in each of them. In which type of mirror can you see more?
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3. Making Light bend
Equipment: Glass block, pin board, sheet of paper and 4 pins.
· Place the glass block with the largest side down in the middle of the sheet of paper and draw a line along each of the four edges.
· Now place two pins in a line, a few cms apart, to meet a long edge at about 45 degrees.
· Looking through the glass block from the other side, place two more pins in the paper, about a few cms apart, so that all four pins appear to line up.
· Now draw lines through each pair of pins to meet the nearest edge.
· Connect up these two lines in the space where the glass block was by a straight line.
The line represents the path taken by the light as it travels from the head of the first pin through the glass block to the head of the last pin, and then to your eye.
Describe what happens to the light as it travels from air into the glass block and what happens as it leaves the glass block into the air.
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4. How deep is the pool? - Apparent Depth
Equipment: Glass block, 2 small sheets of graph paper, ruler and calculator
When you look into a swimming pool, the water does not appear as deep as it really is. This experiment explores this phenomenon.
· Place the smallest face of the glass block on the sheet of graph paper. Now look down through the glass block at the graph paper.
What do you notice?
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· Now slide the other sheet of graph paper up the side of the block until the pattern inside the block and the pattern outside the block match up. The raised sheet of paper indicates that the glass made the bottom sheet appear closer to you. This is because light is slowed down in glass.
· You can calculate how much the light has slowed down by measuring the distance from the top of the block down to where you raised the graph paper and dividing this by the height of the block.
Make your measurements and calculate how much you think the light is slowed down by the glass.
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5. How can a magnifying glass be flat?
Equipment: Overhead projector lens, a convex and a concave lens and a page of text.
· Place the flat overhead projector lens over the printed page and slowly lift it.
What do you notice?
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· Now lift the convex lens (fatter in the middle) off the page, then the concave lens (thinner in the middle). Which type of lens is the flat lens like?
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· Look closely at the surface of the flat lens. what do you notice?
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Flat lenses are called Fresnel lenses. They are often found on the rear windows of vans. Why?
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6. Investigating a Slide Projector
Equipment: Slide projector, ruler and slide with lines 1 cm apart on it (Use the diffraction slide from the Diffraction kit, the small black box).
· Use the slide in the slide projector to measure and calculate how much the projector magnifies the slide
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How would you move the projector to get greater magnification?
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The image is now out of focus. To get back into focus do you have to now move the lens closer or further away from the slide. Which way do you move the lens?.
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7. Investigating Bifocal spectacles
Equipment: Light box, bifocal spectacles and a screen
· Arrange the light box to produce parallel rays of light.
· Shine the rays through different sections of the spectacles onto the screen.
Convex sections converge or bring the rays together, concave sections diverge or bend the rays apart.
Draw the outline of the spectacle and the different sections and label them convex or concave.
8. Rainbow produced by a prism.
Equipment: Lightbox, prism and a screen.
Draw the path of two different colours from the light source, through the prism and onto the screen, which colour is bent the most?
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9. Colour Mixing using Light boxes, colour slides and a screen.
Equipment: Three Light boxes, colour slides and a screen.
Use the light boxes and the colour slides to find the effect of mixing the following colours:
Red+ Blue, Red + Green, Blue + Green
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10. Do you see more of yourself in a mirror if you go closer?
Equipment: Long plane mirror place against a wall
· Stand in front off the large plane mirror and walk up and back from the mirror. Now answer the question.
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11. Telescope
If you have a large telescope bring it out and aim it at a distant object through the window.
Student Name : ......
Context Questions:
From doing the tasks and your other interests, write down 2 or 3 questions for which you would like answers.
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Some Context Questions
Why does a magnifying glass enlarge objects?
How does a telescope work?
What would be the effect of swapping the lenses in a telescope?
How does a Fresnel lens differ from a normal lens?
How does an optometrist make lenses
Other activities that were less successful.
12. Investigating Telescopes
There are many different types of telescopes using different types of lenses.
a) An early type was called the Keplerian telescope which consisted of two convex lenses. Look through the telescope at a distant object and describe the image.
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b) Galileo used a telescope using a concave lens as the eyepiece lens. Look through this telescope and compare the image with that of the Keplerian telescope
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13. Investigating the Microscope
The lenses have been selected and arranged to make a microscope.
- Look through the eyepiece lens at the far screen and describe what you see.
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- Move the eyepiece lens towards the other lens called the “objective” lens and describe what you see.
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These tend not to work because the diameter of the two lenses commonly found in schools is too small to give a large enough image to fill enough of teh visual field to be easier to see.
Other activities depending on your equipment
· Laser communication
· Optical fibres
Light Context Questions
These are the questions generated during the Introductory Activity. They have been grouped under different aspects of light. As you learn more about light you will be able to answer more and more of these questions.
By the the end of the unit you must submit answers to a selection of 12 questions. Your answer to each question should be no more than a few lines with a diagram if necessary. Your answers will be assessed and the grade will contribute to the overall assessment of your communication skills.
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Optical Instruments
How does the slide projector enlarge the slide?
How does a periscope work?
Why are concave mirrors so scary?
How does a microscope work?
How does a bicycle reflector work?
How is a telescope different in construction from a microscope?
If the telescope image is upside down, how do you make it right side up?
How does the magnification produced by a projector vary with the distance between the object and the screen?
What’s the difference between a magnifying glass and a telescope?
What makes a flat lens convex or concave?
What is the highest power a telescope can magnify?
How do the grooves in a Fresnel lens work?
Why do convex mirrors reflect more of their surroundings and make them smaller?
Why does a concave mirror show both upright and inverted images?
How does a convex lens make objects larger?
How does a mirror work?
Why is bullet proof glass so strong?
Why does my image appear upside down when looking into a concave mirror?