Senior 1 practice book · Unit 9 of 9

Rectilinear Propagation of Light

50 questions that complete the Senior 1 quiz for this unit: 22 core and 28 challenge. Easy and hard questions are mixed. Try each question, then tap “Show answer”.

Common misconceptions

We see things because light goes out from our eyes and hits them.We see an object when light from it (its own or reflected) enters our eyes; in a totally dark room we see nothing, even with our eyes open.
A shadow is a dark picture or reflection of the object.A shadow is only a region that light cannot reach because an opaque object blocks it; it shows the outline but none of the details or colours.
A bigger pinhole gives a bigger image.The image size depends only on the distances (image height ÷ object height = v ÷ u). A bigger hole makes the image brighter but more blurred.
To see more of yourself in a small mirror, you should step back.A plane mirror only needs to be half your height to show all of you, and stepping back does not change how much of you that mirror shows.

What this unit covers

Topics marked new are not tested much in the quiz, so this book gives them extra questions.

Sources of light: luminous and non-luminous objects
Transparent, translucent and opaque materials
Rays and beams: parallel, converging and diverging
Rectilinear propagation: meaning and evidence that light travels in straight lines
Shadows: umbra and penumbra, point and extended sources, size of a shadow
Eclipses of the Sun and of the Moon
Pinhole camera: image properties, effect of changes and magnification
Laws of reflection; measuring angles from the normal
Regular and diffuse reflection
Image in a plane mirror: characteristics, distances and lateral inversion
Images in inclined and parallel mirrors
Speed of light: travel times, light-years, light crossing a vacuum
Locating the image in a plane mirror: ray diagrams, no-parallax method, mirror size to see oneself
Applications: periscope, kaleidoscope, sundial, finding heights from shadows, mirrors in shops

Go to the questions

Questions (1–50)

Easier and harder questions are mixed, just like in a real exam. The 28 harder ones are marked ★ Challenge.

1True or false
A lunar eclipse can only happen at full Moon.
Show answer
Answer: True
The Earth must be between the Sun and the Moon, which is the position at full Moon.
Common mistakeLearners link eclipses with new Moon only; that is true for SOLAR eclipses, but lunar eclipses need full Moon.
2True or false · ★ Challenge
In a dark, dusty room the beam of a laser pointer can be seen as a straight line.
Show answer
Answer: True
Dust scatters a little of the light towards your eyes, showing that the beam follows a straight path.
Common mistakeSome learners think a beam can only be seen where it hits a wall; dust or smoke reveals its straight path.
3True or false · ★ Challenge
If the girl walks further away from the mirror, she needs a taller mirror to see her whole body.
Show answer
Answer: False
The half-height rule does not depend on her distance from the mirror; as she moves back, her image also moves back.
Common mistakeLearners expect distance to help or hurt; in a plane mirror the minimum length stays half her height at any distance.
4True or false
Light needs air or some other material to travel through.
Show answer
Answer: False
Sunlight crosses about 150 million km of empty space (vacuum) to reach us, so light needs no material.
Common mistakeLearners transfer the idea from sound, which does need a material; light can travel through a vacuum.
5True or false · ★ Challenge
In diffuse reflection, the rays do not obey the law of reflection.
Show answer
Answer: False
Each ray still obeys i = r; the surface is uneven, so the normals point in different directions and the rays scatter.
Common mistakeLearners think scattering means the law is broken; it is the surface that is irregular, not the law.
6True or false
A red-hot iron rod in a blacksmith's fire is a luminous object.
Show answer
Answer: True
It is so hot that it gives out its own light, so it is luminous while it glows.
Common mistakeLearners think only the Sun, flames and bulbs are luminous; any object hot enough to glow produces its own light.
7True or false · ★ Challenge
If an opaque object is lit by a larger light source (with the same positions), its umbra becomes smaller and its penumbra larger.
Show answer
Answer: True
With a wider source, fewer points behind the object are hidden from the WHOLE source, so the umbra shrinks while the partly lit penumbra spreads.
Common mistakeLearners expect a bigger lamp to give a bigger total shadow; it actually gives more partial shadow and less full shadow.
8True or false
A kaleidoscope uses inclined plane mirrors to make many images of a few coloured beads.
Show answer
Answer: True
Mirrors at an angle (often 60°) give several images, which form a symmetrical pattern.
Common mistakeLearners think the pattern comes from many beads; it comes from multiple images in inclined mirrors.
9Fill in the blank · ★ Challenge
A magnifying glass held in sunlight brings the light together to a bright spot. The beam leaving the glass is a ______ beam.
Show answer
Answer: converging
The rays come together (meet) at the bright spot, so the beam converges.
Common mistakeWriting 'parallel' describes the sunlight before the glass, not the beam that leaves it.
10Multiple choice · ★ Challenge
A pupil holds up her RIGHT hand in front of a plane mirror. Her image appears to hold up:
1. AIts right hand
2. BBoth of its hands
3. CNeither hand, as it is virtual
4. DIts left hand
Show answer
Answer: D. Its left hand
A plane mirror image is laterally inverted, so the right hand appears as the image's left hand.
Common mistakeChoosing 'right hand' assumes the image is like a photograph; in a mirror left and right are swapped.
11True or false
When the angle between two plane mirrors is made smaller, more images of an object between them are seen.
Show answer
Answer: True
n = 360° ÷ θ − 1 increases as θ decreases.
Common mistakeLearners expect a narrower gap to show fewer images; the smaller the angle, the more reflections are possible.
12Multiple choice · ★ Challenge
You can hear a friend talking behind a wall but you cannot see her. What does this suggest?
1. ALight travels in straight lines, but sound can bend round obstacles
2. BLight travels more slowly than sound around a wall
3. CSound travels in straight lines, but light bends round the edges of walls
4. DThe wall is transparent to sound but opaque to light
Show answer
Answer: A. Light travels in straight lines, but sound can bend round obstacles
Light from your friend travels in straight lines and is blocked by the wall; sound spreads round the edge of the wall.
Common mistakeChoosing 'light is slower' is wrong: light is about a million times faster than sound; the difference is in how they spread.
13Multiple choice
A light-year is:
1. AA time: how long one year lasts on another planet
2. BA speed: how fast light moves during a year
3. CA distance: how far light travels in one year
4. DA time: how long sunlight takes to reach the Earth
Show answer
Answer: C. A distance: how far light travels in one year
Despite the word 'year', a light-year measures distance.
Common mistakeChoosing a time is tempting because of the word 'year'; but year × speed of light gives a distance.
14Multiple choice · ★ Challenge
A ball is moved closer to a small lamp, while the lamp and the screen stay where they are. The shadow of the ball on the screen:
1. ABecomes larger
2. BBecomes smaller
3. CStays the same size
4. DDisappears completely
Show answer
Answer: A. Becomes larger
Closer to the lamp, the ball blocks a wider cone of rays, so the shadow on the screen grows.
Common mistakeChoosing 'smaller' assumes the shadow shrinks like the ball moving away from the screen; it is the angle at the lamp that matters.
15Fill in the blank
Muddy river water after heavy rain lets some light through, but you cannot see the bottom clearly. Such water is ______.
Show answer
Answer: translucent
Some light passes, but it is scattered by the mud, so objects are not seen clearly.
Common mistakeWriting 'opaque' is wrong because some light does get through; opaque means no light at all.
16Multiple choice · ★ Challenge
A girl 1.6 m tall wants to see her whole body in a vertical plane mirror. What is the shortest mirror she can use?
1. A0.8 m
2. B1.6 m
3. C3.2 m
4. D0.4 m
Show answer
Answer: A. 0.8 m
Rays from her feet and head reach her eyes from points on the mirror halfway up each distance, so the mirror needs to be half her height: 1.6 ÷ 2 = 0.8 m.
Common mistakeChoosing 1.6 m assumes the mirror must be as tall as she is; it only has to cover half her height.
17Multiple choice · ★ Challenge
Sunlight reaching a school garden is usually treated as a parallel beam. Why?
1. AThe Sun sends out light in one direction only, straight towards the Earth
2. BThe air bends all the Sun's rays until they become exactly parallel
3. CThe Sun is so far away that rays reaching a small area are almost parallel
4. DSunlight is a converging beam that becomes parallel after midday
Show answer
Answer: C. The Sun is so far away that rays reaching a small area are almost parallel
Rays from a very distant source that reach a small region are spread by a tiny angle, so they are almost parallel.
Common mistakeChoosing 'one direction only' is wrong: the Sun sends light in all directions; we receive only a tiny, nearly parallel part of it.
18Multiple choice
A sundial tells the time because:
1. AIts pointer glows more brightly at certain hours
2. BIts shiny face reflects sunlight onto a small clock that is hidden inside it
3. CThe Sun's rays bend round the pointer at fixed times
4. DThe shadow of its pointer moves as the Sun crosses the sky
Show answer
Answer: D. The shadow of its pointer moves as the Sun crosses the sky
Light travels in straight lines, so the pointer's shadow falls in a direction set by the Sun's position, which changes through the day.
Common mistakeChoosing 'rays bend round the pointer' contradicts rectilinear propagation, which is exactly why a sharp shadow forms.
19Multiple choice · ★ Challenge
Why is there NOT a solar eclipse every month, when the Moon passes between the Sun and the Earth?
1. AThe Moon gives out its own light at that time and so it hides nothing
2. BThe Moon's orbit is tilted, so its shadow usually passes above or below the Earth
3. CThe Sun is far too big for the small Moon ever to cover any part of it as seen from the Earth
4. DThick clouds always hide the Sun at the time of each new Moon
Show answer
Answer: B. The Moon's orbit is tilted, so its shadow usually passes above or below the Earth
The Moon's orbit is tilted slightly to the Earth's orbit, so usually Sun, Moon and Earth are not exactly in line.
Common mistakeChoosing 'the Sun is too big' ignores that the Sun is also much further away, so it and the Moon look about the same size.
20Multiple choice
Which list contains ONLY luminous objects?
1. AThe Moon, a candle flame and a lit torch bulb at night
2. BThe Sun, a candle flame and a glowing firefly
3. CA mirror, a white wall and the Sun at noon
4. DA star, a switched-off TV screen and a fire
Show answer
Answer: B. The Sun, a candle flame and a glowing firefly
The Sun, a flame and a firefly all give out their own light.
Common mistakeChoosing the list with the Moon is tempting because the Moon looks bright, but it only reflects sunlight.
21Fill in the blank · ★ Challenge
A pinhole camera forms an image 2 cm tall of a tree 8 m tall. The magnification is ______.
Show answer
Answer: 0.0025 (1/400)
m = image height ÷ object height = 0.02 m ÷ 8 m = 0.0025.
Common mistakeWriting 0.25 divides 2 by 8 without changing cm into m; both heights must be in the same unit.
22Multiple choice
The speed of light in a vacuum (and almost the same in air) is about:
1. A340 m/s
2. B3 × 10⁵ m/s
3. C3 × 10⁸ km/s
4. D3 × 10⁸ m/s
Show answer
Answer: D. 3 × 10⁸ m/s
Light travels about 300 000 km (3 × 10⁸ m) every second.
Common mistakeChoosing 340 m/s gives the speed of SOUND in air, which is nearly a million times slower.
23Fill in the blank · ★ Challenge
Taking the speed of light as 3 × 10⁸ m/s and one year as about 3.15 × 10⁷ s, one light-year is about ______ m.
Show answer
Answer: 9.5 × 10¹⁵
distance = speed × time = 3 × 10⁸ × 3.15 × 10⁷ ≈ 9.45 × 10¹⁵ m ≈ 9.5 × 10¹⁵ m.
Common mistakeDividing instead of multiplying gives about 10, which is far too small for the distance light covers in a year.
24Multiple choice
A simple periscope contains two plane mirrors. How are they arranged?
1. AParallel to each other, each at 45° to the tube
2. BAt 90° to each other, both facing the eye
3. CParallel to each other, each at 90° to the tube
4. DAt 60° to each other, as in a kaleidoscope
Show answer
Answer: A. Parallel to each other, each at 45° to the tube
Each mirror turns the light through 90°; with both at 45° and parallel, light entering at the top leaves the bottom in the original direction.
Common mistakeChoosing mirrors at 90° to the tube would send the light straight back, not along the tube.
25Multiple choice · ★ Challenge
At the same moment, a stick 1.5 m tall casts a shadow 2 m long and a eucalyptus tree casts a shadow 16 m long. How tall is the tree?
1. A21.3 m
2. B10.7 m
3. C24 m
4. D12 m
Show answer
Answer: D. 12 m
The Sun's rays are parallel, so height ÷ shadow is the same: h = 16 × 1.5 ÷ 2 = 12 m.
Common mistakeChoosing 21.3 m inverts the ratio (16 × 2 ÷ 1.5); a stick shorter than its shadow means the tree is shorter than its shadow too.
26Multiple choice · ★ Challenge
A ray strikes a plane mirror along the normal (the angle of incidence is 0°). What happens to the reflected ray?
1. AIt travels along the surface of the mirror
2. BIt goes straight back along the same path
3. CIt is reflected at 90° to the incident ray
4. DIt passes straight through the mirror
Show answer
Answer: B. It goes straight back along the same path
i = 0°, so r = 0°: the reflected ray leaves along the normal, straight back the way it came.
Common mistakeChoosing 'along the surface' mixes up angles measured from the normal with angles measured from the mirror.
27Multiple choice
In a ray diagram, where is the image of a point object in a plane mirror?
1. AOn the surface of the mirror itself, just opposite the object
2. BOn the normal through the object, as far behind the mirror as the object is in front
3. CBehind the mirror, but only half as far back as the object is in front
4. DIn front of the mirror, between the object and the mirror surface
Show answer
Answer: B. On the normal through the object, as far behind the mirror as the object is in front
The image lies on the line through the object perpendicular to the mirror, at an equal distance behind it.
Common mistakeChoosing 'on the surface of the mirror' is tempting because that is where the reflection happens, but the image appears behind it.
28Multiple choice
Light from a candle flame spreads out in all directions from it. A beam of this kind is called:
1. AParallel
2. BDiverging
3. CConverging
4. DOpaque
Show answer
Answer: B. Diverging
Rays that spread out from a point form a diverging beam.
Common mistakeChoosing 'converging' reverses the meaning: converging rays come together, diverging rays spread apart.
29Short answer · ★ Challenge
At noon in Kigali a person's shadow is very short, but late in the afternoon it is very long. Explain why.
Show answer
Model answer: At noon the Sun is almost overhead, so its rays come down steeply and the person blocks only a small patch of ground. In the late afternoon the Sun is low, the rays arrive at a shallow slant, and the region of ground they cannot reach behind the person is long.
Common mistakeSaying the person 'gets taller' or the Sun 'gets bigger' is wrong; only the angle of the rays changes.
30Short answer · ★ Challenge
Explain why the image in a plane mirror is called virtual, and why it cannot be caught on a screen placed behind the mirror.
Show answer
Model answer: The reflected rays spread out in front of the mirror; they only APPEAR to come from a point behind it, where they never actually meet. No light reaches the space behind the mirror, so a screen placed there receives nothing.
Common mistakeSaying the image is virtual because it is 'not real-looking' misses the physics: it is about rays not actually meeting.
31Multiple choice
Why are bathroom windows often made of frosted glass?
1. AIt stops all light, so the bathroom stays dark
2. BIt lets more light through than clear glass does
3. CIt reflects nearly all light like a mirror, so that nobody can see in
4. DIt lets light in, but people outside cannot see clearly inside
Show answer
Answer: D. It lets light in, but people outside cannot see clearly inside
Frosted glass is translucent: light passes through but is scattered, so shapes cannot be seen clearly.
Common mistakeChoosing 'stops all light' describes an opaque material; frosted glass still lets light through.
32Short answer · ★ Challenge
A pupil says the Moon must be luminous because it shines so brightly at night. Explain why she is wrong and give one piece of evidence.
Show answer
Model answer: The Moon does not produce light; it reflects light from the Sun. Evidence: during a lunar eclipse, when the Earth blocks the sunlight, the Moon becomes dark; and we only see the part of the Moon that faces the Sun (the phases).
Common mistakeJudging 'luminous' by brightness is wrong: a white wall in sunlight is bright too, but it only reflects light.
33Multiple choice
Why can every pupil in the classroom see the words on a page of a book, wherever they sit?
1. AIt reflects light regularly in one direction, just as a polished mirror does
2. BThe paper produces its own light, like a lamp
3. CIts rough surface scatters light in all directions (diffuse reflection)
4. DThe paper is transparent, so the light passes through it
Show answer
Answer: C. Its rough surface scatters light in all directions (diffuse reflection)
Paper is rough on a tiny scale, so it sends reflected light in all directions; some reaches every pupil's eyes.
Common mistakeChoosing regular reflection is wrong: a mirror-like surface would send light in only one direction, so most pupils would see glare or nothing.
34Short answer · ★ Challenge
Explain how a periscope lets a pupil at the back of the crowd at Amahoro Stadium see the football over people's heads. Is the final image upright?
Show answer
Model answer: Light from the pitch enters the top of the tube and hits the top mirror at 45°, which reflects it straight down the tube. The bottom mirror, also at 45°, reflects it horizontally into the pupil's eye. Because light travels in straight lines it could not reach the eye over the crowd otherwise. The final image is upright (and the right way round, since the two reflections cancel the lateral inversion).
Common mistakeSaying the image is upside down confuses the periscope with the pinhole camera; two reflections at 45° keep it upright.
35Short answer · ★ Challenge
Explain why everyone on the night side of the Earth can see a total lunar eclipse, while a total solar eclipse is seen only along a narrow strip of the Earth.
Show answer
Model answer: In a lunar eclipse the Moon moves into the Earth's large shadow, so the Moon itself goes dark and anyone who can see the Moon sees it. In a solar eclipse the Moon's umbra on the Earth is small, so only the places inside that small umbra see the Sun totally covered; others in the penumbra see a partial eclipse.
Common mistakeSaying 'the Moon is bigger than the Earth' is wrong; it is the size of the shadow and who can see the dark body that matters.
36Short answer
Small shops and hair salons in Kigali often cover one wall with a large plane mirror. Give two reasons, using the properties of plane-mirror images.
Show answer
Model answer: The image of the room appears as far behind the mirror as the room is in front, so the room looks about twice as big. Customers (or a barber) can also see the back of a head or the shop behind them, and the mirror reflects light to make the room brighter.
Common mistakeSaying the mirror 'makes the room larger' is incomplete; it only LOOKS larger because the virtual image lies behind the mirror.
37Multiple choice · ★ Challenge
A pinhole camera is 20 cm long from the pinhole to the screen. A pupil 1.6 m tall stands 4 m in front of the pinhole. How tall is her image?
1. A8 cm
2. B0.8 cm
3. C80 cm
4. D32 cm
Show answer
Answer: A. 8 cm
h_i = h_o × v ÷ u = 1.6 m × 0.20 m ÷ 4 m = 0.08 m = 8 cm.
Common mistakeChoosing 80 cm comes from a slip when changing 0.08 m into centimetres (× 1000 instead of × 100); 0.08 m = 8 cm.
38Fill in the blank
In a ray diagram for a plane mirror, the lines behind the mirror that lead to the image are drawn ______ because no light really travels there.
Show answer
Answer: dotted (broken)
Light does not pass behind the mirror; the backward extensions only show where the rays appear to come from.
Common mistakeDrawing them as solid lines with arrows suggests real light behind the mirror, which is wrong.
39Short answer · ★ Challenge
Astronomers say that when we look at a star 4 light-years away, we see it as it was 4 years ago. Explain what this means.
Show answer
Model answer: A light-year is the distance light travels in one year. Light from that star takes 4 years to reach us, so the light entering our eyes tonight left the star 4 years ago. We see the star as it was then; if it changed since, we would not yet know.
Common mistakeTreating the light-year as a unit of time is the usual error: it is a DISTANCE, which is why it tells us how long the light has been travelling.
40Fill in the blank
The line drawn at 90° to a mirror at the point where a ray strikes it is called the ______.
Show answer
Answer: normal
All angles in reflection are measured from the normal.
Common mistakeWriting 'the incident ray' confuses the arriving ray with the reference line the angles are measured from.
41Multiple choice · ★ Challenge
Two plane mirrors face each other, parallel and 2 m apart. A girl stands 0.5 m from mirror A. Mirror B forms an image of her; mirror A then forms an image of that image. How far behind mirror A is this second image?
1. A2.5 m
2. B4.0 m
3. C3.5 m
4. D1.5 m
Show answer
Answer: C. 3.5 m
She is 1.5 m from B, so B's image is 1.5 m behind B, i.e. 2 + 1.5 = 3.5 m from A. Mirror A forms its image 3.5 m behind A.
Common mistakeChoosing 1.5 m stops after the first reflection in B; the image formed in A must use its distance from A.
42Short answer
Using two rays, explain why the image in a pinhole camera is upside down.
Show answer
Model answer: Light travels in straight lines. A ray from the top of the object passes through the pinhole and carries on downwards to the bottom of the screen; a ray from the bottom passes through the pinhole and reaches the top of the screen. The rays cross at the pinhole, so the image is inverted.
Common mistakeSaying the screen 'turns the picture over' is wrong; the inversion comes from the rays crossing at the hole.
43Multiple choice · ★ Challenge
In a barber's shop a clock hangs on the wall 3 m opposite a large plane mirror. A customer sits 1 m in front of the mirror, between the mirror and the clock. How far from the customer is the image of the clock?
1. A3 m
2. B2 m
3. C4 m
4. D6 m
Show answer
Answer: C. 4 m
The image is as far behind the mirror as the clock is in front: 3 m. Distance from customer = 1 + 3 = 4 m.
Common mistakeChoosing 6 m doubles the clock's distance; that would be the clock-to-image distance, not the customer-to-image distance.
44Short answer · ★ Challenge
In a ray diagram, two rays from a point object O are reflected by a plane mirror into an eye. Explain how the diagram is used to find the image, and how it shows that the image is as far behind the mirror as O is in front.
Show answer
Model answer: Draw each reflected ray obeying i = r, then extend the reflected rays backwards behind the mirror as dotted lines. The point where the dotted lines meet is the image I. Measuring shows that I lies on the normal through O and that the distance of I behind the mirror equals the distance of O in front.
Common mistakeDrawing the reflected rays meeting in front of the mirror gives a real image, which a plane mirror never forms; they must be extended backwards.
45Short answer
Describe how to locate the image of a pin in a plane mirror by the no-parallax method.
Show answer
Model answer: Stand the mirror upright on paper and place an object pin in front of it. Look at the image and place a second (search) pin behind the mirror, sticking up above it. Move your head from side to side and move the search pin until the image and the search pin stay together (no parallax). The search pin is then at the position of the image; measure both distances from the mirror.
Common mistakePlacing the search pin where the image 'seems' to be without moving the head often puts it in the wrong place; no parallax is the test.
46Multiple choice · ★ Challenge
Light from the Sun travels about 1.5 × 10¹¹ m to reach the Earth (speed of light 3 × 10⁸ m/s). How long does the journey take?
1. A500 s
2. B50 s
3. C0.002 s
4. D4.5 × 10¹⁹ s
Show answer
Answer: A. 500 s
t = distance ÷ speed = 1.5 × 10¹¹ ÷ 3 × 10⁸ = 500 s (about 8 minutes).
Common mistakeChoosing 0.002 s divides speed by distance; time = distance ÷ speed.
47Short answer
Explain how a builder can check by eye that three fence poles are in a straight line, and which property of light this uses.
Show answer
Model answer: He looks along the line from behind the first pole. If the first pole hides the other two completely, all three are in a straight line. This works because light travels in straight lines (rectilinear propagation).
Common mistakeSaying he 'measures the distances' misses the point: equal gaps do not show a straight line; the line of sight does.
48Short answer · ★ Challenge
Use n = 360° ÷ θ − 1 to find the number of images for θ = 120°, 90°, 72° and 60°. Describe the pattern and predict what happens as θ becomes very small.
Show answer
Model answer: θ = 120°: 2 images; 90°: 3; 72°: 4; 60°: 5. As the angle gets smaller the number of images increases, and as θ approaches 0° (mirrors almost parallel) the number becomes very large, like the endless images between parallel mirrors.
Common mistakeForgetting the '− 1' gives 3, 4, 5, 6: the object itself is one of the 360° ÷ θ 'copies', not an image.
49Short answer
Describe how you would use a ray box, a plane mirror, a protractor and a sheet of paper to test the second law of reflection.
Show answer
Model answer: Stand the mirror upright on a line drawn on the paper and draw the normal at one point. Shine a ray along a line at a chosen angle of incidence to the normal, mark the reflected ray with two dots, then remove the mirror and measure the angle of reflection from the normal. Repeat for several angles; i and r should be equal each time.
Common mistakeMeasuring the angles from the mirror line instead of from the normal still gives equal angles but is not the angle of incidence used in the law.
50Short answer · ★ Challenge
Plan a simple test, using a torch and a white screen, to sort these materials into transparent, translucent and opaque: clear plastic, tracing paper, cardboard, aluminium foil and frosted glass. Give the result you expect for each.
Show answer
Model answer: Hold each material between the torch and the screen in a dark room. A sharp, bright patch with a clear outline of the torch means transparent (clear plastic); a dimmer, blurred patch means translucent (tracing paper, frosted glass); no light on the screen (a dark shadow) means opaque (cardboard, foil).
Common mistakeTesting by looking at the material in daylight only is unreliable; a fixed source and screen shows how much light really passes.