Light and reflection

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Apuntes de la lección

Big idea: how light lets us see

Big idea. Key concept: Systems. A light source, the objects it shines on and our eyes work together as one system that lets us see.
Related concept: Models. We draw light as straight lines called rays. A ray is a model: it is not a real thread, but it helps us predict where light goes.
Global context: Scientific and technical innovation. Mirrors, periscopes, cameras and telescopes all work by controlling light.
A luminous object gives out its own light, for example the Sun, a lamp or a candle flame. A non-luminous object, such as the Moon, a book or you, only reflects light from a luminous source.
We see an object when light from it enters our eyes. Our eyes do not send out beams. Light enters through the pupil, the lens focuses it onto the retina, and the retina sends signals to the brain.
Never look straight at the Sun or into a laser beam. Bright light can damage the retina for good.

Light travels in straight lines

Light travels in straight lines through a material such as air that is the same all the way through. A torch beam in a dusty room shows this.
Light travels extremely fast: about 300 000 km every second. Sunlight takes about 8 minutes to reach Earth. Light is far faster than sound, which is why you see lightning before you hear thunder.
A transparent material (clear glass, air) lets light pass straight through. A translucent material (frosted glass, tracing paper) lets some light through but scatters it. An opaque material (wood, metal, card) lets no light through.
A shadow forms when an opaque object blocks light. Because light travels in straight lines, the shadow has the same shape as the object.
Move the object closer to the light source and the shadow gets bigger. Move it closer to the screen and the shadow gets smaller and sharper.

Reflection and the law of reflection

Reflection is when light bounces off a surface. A flat, shiny surface such as a mirror reflects light in a very regular way.
The incident ray is the ray that arrives at the mirror. The reflected ray is the ray that bounces off.
The normal is a dashed line drawn at 90° to the mirror at the point where the ray hits it.
The angle of incidence (i) is the angle between the incident ray and the normal. The angle of reflection (r) is the angle between the reflected ray and the normal. Always measure from the normal, never from the mirror surface.
Law of reflection: angle of incidence = angle of reflection. If i = 40°, then r = 40°, and the angle between the two rays is 80°.
A ray that travels along the normal (i = 0°) is reflected straight back along itself.
In a school investigation you use a ray box (a low-voltage lamp that makes a thin beam), a plane mirror and a protractor. Never use the Sun or a laser pointer.

Investigating the law of reflection

Images in plane mirrors

A plane mirror is a flat mirror. When you look into it you see an image that seems to be behind the mirror.
The image is the same size as the object and the same distance behind the mirror as the object is in front of it.
The image is laterally reversed: left and right are swapped. That is why the word AMBULANCE is written back to front on the front of the vehicle, so drivers can read it in their mirrors.
The image is virtual. Light only seems to come from it, because your brain assumes light travels in straight lines. No light is really behind the mirror, so the image cannot be caught on a screen.
A smooth surface such as a mirror or calm water gives regular reflection: parallel rays stay parallel, so you see a clear image.
A rough surface such as paper or a wall gives diffuse reflection. Each ray still obeys the law of reflection, but the tiny bumps point the normal in different directions, so the light scatters and there is no clear image. Scattering is why you can see a wall from any part of a room.

How a plane mirror forms an image

Colour and white light

White light is a mixture of colours. A glass prism splits it into a spectrum: red, orange, yellow, green, blue, indigo and violet. Splitting light into colours is called dispersion. A rainbow forms when raindrops disperse sunlight.
Red, green and blue light mixed together make white light.
The colour of an object depends on which colours it reflects. A red apple reflects mostly red light and absorbs the others. White paper reflects all colours. Black cloth absorbs nearly all of them.
A colour filter lets through (transmits) its own colour and absorbs the rest. A red filter in white light gives red light. A blue light shone at a red filter is absorbed, so almost no light comes out.
In red light a blue shirt looks very dark, because the shirt reflects blue but there is no blue light to reflect.

The visible spectrum from red to violet

Think like a scientist: testing the law of reflection

Question: is the angle of reflection always equal to the angle of incidence?
The independent variable is the angle of incidence (for example 20°, 30°, 40°, 50° and 60°). The dependent variable is the angle of reflection, measured with a protractor.
Control variables include the same mirror, the same ray box, the same point on the mirror and always measuring from the normal.
In one class experiment the angles of reflection were 21°, 29°, 40°, 52° and 59°. They are close to the angles of incidence but not identical. The differences are measurement error: it is hard to read a protractor and to mark the middle of a thick beam.
To improve the investigation: use a narrow beam, draw the normal carefully, measure each angle more than once and take a mean.
Inquiry task: plan an investigation to find out whether the distance of an object from a plane mirror changes the distance of its image behind the mirror. List your variables, say how you will measure the distances and describe how you will make your results reliable.

Diapositivas

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Preguntas de práctica

Vista previa gratis — 8 de 55 preguntas. Regístrate para verlas todas.

1.Which of these objects is luminous?
Easy
- AThe Moon
- BA mirror
- CA white wall
- DA candle flame
2.Why can you see a book on a desk?
Easy
- AYour eyes send out light that bounces off the book
- BThe book gives out its own light all the time
- CLight reflects off the book and enters your eyes
- DThe book absorbs all the light that hits it
3.Which material is opaque?
Easy
- AClear glass
- BThick card
- CTracing paper
- DClean water
4.Why does an opaque object make a shadow?
Easy
- AIt blocks light, which travels in straight lines
- BIt bends light round to the back of the object
- CIt makes new darkness that spreads out
- DIt reflects all of the light back to the source
5.A card is held between a small lamp and a wall, and casts a shadow. What happens to the shadow if the card is moved closer to the lamp?
Medium
- AThe shadow gets smaller
- BThe shadow stays the same size
- CThe shadow disappears
- DThe shadow gets bigger
6.What is the normal in a ray diagram?
Easy
- AThe ray that arrives at the mirror
- BThe ray that leaves the mirror
- CA line at 90° to the mirror where the ray hits
- DThe surface of the mirror itself
7.The angle of incidence is the angle between the incident ray and which line?
Easy
- AThe normal
- BThe mirror surface
- CThe reflected ray
- DThe edge of the paper
8.A ray of light hits a plane mirror with an angle of incidence of 25°. What is the angle of reflection?
Easy
- A65°
- B25°
- C50°
- D90°

Más preguntas

9.A ray hits a plane mirror. The angle between the ray and the mirror surface is 20°. What is the angle of incidence?
Hard
- A20°
- B70°
- C40°
- D160°
10.A ray hits a plane mirror with an angle of incidence of 35°. What is the angle between the incident ray and the reflected ray?
Hard
- A35°
- B55°
- C70°
- D110°
11.A ray of light travels along the normal towards a plane mirror. What happens to it?
Medium
- AIt passes through the mirror
- BIt is reflected at 90° to its path
- CIt is absorbed and disappears
- DIt is reflected straight back along its own path
12.Which description of the image in a plane mirror is correct?
Easy
- AThe same size as the object and laterally reversed
- BLarger than the object and upside down
- CSmaller than the object and the right way round
- DThe same size as the object and a real image
13.Why can the image in a plane mirror not be caught on a screen?
Medium
- AThe image is too small to be seen on a screen
- BNo light really passes through the position of the image
- CMirrors absorb all the light that hits them
- DThe image is too bright for any screen
14.A girl stands 2 m in front of a plane mirror. How far is she from her image?
Medium
- A4 m
- B2 m
- C1 m
- D8 m
15.What does a smooth surface such as calm water produce?
Easy
- AScattering and no image at all
- BAbsorption of all the light that hits it
- CReflection that ignores the law of reflection
- DRegular reflection and a clear image
16.Why can everyone in a classroom see a white wall from different seats?
Medium
- AThe wall gives out its own light
- BThe wall acts like a plane mirror
- CThe rough surface scatters light in many directions
- DThe wall bends light round the corners of the room
17.What does a glass prism do to white light?
Easy
- ATurns it into a single red beam
- BMakes it travel faster
- CSplits it into a spectrum of colours
- DChanges it into invisible light
18.A red apple looks red in white light. Why?
Easy
- AIt reflects red light and absorbs the other colours
- BIt gives out red light of its own
- CIt absorbs red light and reflects the other colours
- DIt lets red light pass through it
19.White light is shone through a green filter. What light comes out?
Medium
- AWhite light that is dimmer
- BRed light and blue light
- CNo light at all
- DMostly green light
20.In a room lit only by red light, what colour does a blue shirt look?
Medium
- ABlue
- BVery dark, nearly black
- CRed
- DBright white
21.Light from the Sun reaches Earth across empty space. What does this tell us?
Medium
- ALight can travel through a vacuum
- BLight needs air in order to travel
- CSpace is full of a gas that carries light
- DSunlight is really carried by sound waves
22.A student looks at a candle through a straight tube, then through a bent tube. Which result shows light travels in straight lines?
Hard
- AThe candle can be seen better through the bent tube
- BThe candle can be seen equally well through both tubes
- CThe candle can be seen only when the tube is straight
- DThe candle cannot be seen through either tube
23.A class investigates reflection by changing the angle of incidence and measuring the angle of reflection. What is the independent variable?
Medium
- AThe angle of reflection
- BThe angle of incidence
- CThe type of protractor
- DThe colour of the paper
24.A student wants to test whether the colour of a card changes how much light it reflects. Which plan is a fair test?
Hard
- AUse a different lamp for each colour of card
- BUse a different distance for each colour of card
- CChange the card colour and the lamp brightness together
- DUse the same lamp, same distance and same light meter, and change only the card colour
25.
A shadow experiment uses a card 6 cm tall and a lamp. The lamp is 60 cm from the screen. The table shows the shadow height as the card is moved away from the lamp.
Distance from lamp to card (cm) Shadow height (cm)
10 36
20 18
30 12
40 9
What pattern do the results show?
Medium
- AThe further the card is from the lamp, the bigger the shadow
- BThe shadow height does not depend on the distance
- CThe shadow height is always six times the card height
- DThe further the card is from the lamp, the smaller the shadow
26.
In one experiment, these angles were measured.
Angle of incidence (°) Angle of reflection (°)
20 22
30 29
40 41
50 52
60 58
A student says the law of reflection must be wrong because the angles are not always exactly equal. What is the best reply?
Hard
- AThe student is right, because exact equality is needed
- BThe results prove the angle of reflection is always bigger
- CThe differences are small and are likely measurement error, so the results support the law
- DThe results cannot be used because there are only five rows
27.A student measures only one angle of incidence and says, 'The law of reflection is always true.' What is the main weakness?
Hard
- AOne reading is not enough to support a general rule
- BReflection only works with angles above 45°
- CThe angle of reflection cannot be measured
- DA mirror cannot be used for this test
28.Which of these are luminous? (select all that apply)
Easy
- AA lit candle
- BThe Moon
- CA switched-on lamp
- DThe Sun
- EA mirror
29.Which statements about the image in a plane mirror are true? (select all that apply)
Medium
- AIt is as far behind the mirror as the object is in front
- BIt is laterally reversed
- CIt can be caught on a screen
- DIt is upside down
- EIt is the same size as the object
30.Which of these are transparent? (select all that apply)
Medium
- AFrosted glass
- BClean water
- CAir
- DClear glass
- EWooden board
31.Which would make a reflection experiment more accurate? (select all that apply)
Hard
- AUse a thick, fuzzy beam
- BUse a narrow beam of light
- CRepeat each reading and take a mean
- DMeasure from the normal
- EMeasure from the mirror surface
32.Light travels faster than sound.
Easy
True or false?
33.We see a non-luminous object because it makes its own light.
Easy
True or false?
34.The angle of reflection is measured from the mirror surface.
Medium
True or false?
35.The image in a plane mirror is the same size as the object.
Easy
True or false?
36.White light is a mixture of different colours.
Easy
True or false?
37.A rough surface does not obey the law of reflection.
Hard
True or false?
38.Light needs a material such as air to travel through.
Medium
True or false?
39.Match each word to its meaning.
Easy
- Luminous
- Transparent
- Translucent
- Opaque
- Lets no light through
- Gives out its own light
- Lets light pass straight through
- Lets some light through but scatters it
40.Match each term to its meaning in a ray diagram.
Medium
- Incident ray
- Reflected ray
- Normal
- Angle of incidence
- Angle between incident ray and normal
- A line at 90° to the mirror
- The ray that bounces off the mirror
- The ray that arrives at the mirror
41.Match each object to what it does to white light.
Medium
- Red apple
- White paper
- Black cloth
- Green filter
- Reflects all the colours
- Reflects mostly red, absorbs the rest
- Absorbs nearly all the colours
- Transmits green, absorbs the rest
42.Put these steps in order to explain how you see a book lit by a lamp.
Medium
- The light travels to the book in straight lines
- The retina sends signals to the brain
- The lamp gives out light
- The book reflects some of the light
- The light enters the eye through the pupil
43.Put these steps for investigating the law of reflection in a sensible order.
Medium
- Measure the angles from the normal with a protractor
- Mark two points on the incident ray and two on the reflected ray
- Shine a thin ray at the point where the normal meets the mirror
- Draw a line for the mirror and a normal at 90° to it
- Remove the mirror and join the points with a ruler
44.Put these colours in the order they appear in the spectrum, starting with red.
Medium
- Green
- Violet
- Yellow
- Red
- Blue
45.Complete the sentence about the normal.
Easy
A line drawn at 90° to the mirror where the ray hits is called the ____.
46.Complete the law of reflection.
Easy
The angle of incidence is equal to the angle of ____.
47.Complete the sentence about splitting light.
Easy
A prism splits white light into a ____ of colours.
48.Complete the sentence about images.
Medium
The image in a plane mirror is ____ reversed, so left and right are swapped.
49.Describe the law of reflection and how you would measure the two angles.
Medium
50.Explain why a shadow has the same shape as the object that makes it.
Medium
51.Explain why you can see a clear image in a mirror but not in a sheet of white paper.
Hard
52.A ray of light hits a plane mirror with an angle of incidence of 35°. Calculate the angle of reflection in degrees.
Easy
53.A ray hits a plane mirror so that the angle between the ray and the mirror surface is 25°. Calculate the angle of incidence in degrees.
Medium
54.A boy stands 2.5 m in front of a plane mirror. Calculate the distance from the boy to his image, in metres.
Medium
55.Sunlight travels about 150 000 000 km to Earth at a speed of about 300 000 km/s. Use time = distance / speed to calculate the time in seconds.
Hard

Apuntes de repaso

## Seeing - A **luminous** object gives out light; a **non-luminous** object reflects it. We see when light enters the eye. ## Straight lines and shadows - Light travels in straight lines at about 300 000 km/s. - **Opaque** objects block light and make shadows; **transparent** and **translucent** materials let light through. ## Law of reflection - **Angle of incidence = angle of reflection**, both measured from the **normal**. ## Plane mirrors - The image is the same size, the same distance behind the mirror, **laterally reversed** and **virtual**. - Smooth surfaces give regular reflection; rough surfaces **scatter** light. ## Colour - White light splits into a **spectrum**. Objects reflect some colours and absorb others; filters transmit one colour. ## Inquiry - Identify the independent, dependent and control variables, repeat readings and think about measurement error.

Luminous: Giving out its own light, like the Sun or a lamp.
Non-luminous: Not giving out light; seen only because it reflects light.
Ray: A straight line with an arrow that shows the path of light.
Opaque: Letting no light pass through.
Transparent: Letting light pass straight through.
Translucent: Letting some light through but scattering it.
Normal: A line drawn at 90° to a mirror at the point where a ray hits it.
Angle of incidence: The angle between the incident ray and the normal.
Angle of reflection: The angle between the reflected ray and the normal.
Plane mirror: A flat mirror.
Virtual image: An image that light only seems to come from, so it cannot be caught on a screen.
Lateral inversion: The left-right swap seen in a plane mirror image.
Diffuse reflection: Reflection from a rough surface that scatters light in many directions.
Spectrum: The band of colours, red to violet, that white light splits into.

Tarjetas

What is a luminous object?

An object that gives out its own light, such as the Sun or a lamp.

Why can we see a non-luminous object?

Light from a luminous source reflects off it and enters our eyes.

How fast does light travel?

About 300 000 km every second.

What is an opaque material?

A material that lets no light through.

Why does a shadow form?

An opaque object blocks light, which travels in straight lines.

How can you make a shadow bigger?

Move the object closer to the light source.

What is the normal?

A line at 90° to the mirror where the ray hits.

What is the law of reflection?

Angle of incidence = angle of reflection.

From what are the angles of incidence and reflection measured?

From the normal.

A ray hits a mirror at i = 30°. What is r?

30°.

What is the image in a plane mirror like?

Same size, same distance behind, laterally reversed and virtual.

What does virtual image mean?

Light only seems to come from it, so it cannot be caught on a screen.

Why does a rough surface give no clear image?

Its bumps scatter the rays in many directions.

What does a prism do to white light?

It splits it into a spectrum of colours.

Why is a red apple red in white light?

It reflects red light and absorbs the other colours.

Why do measurements of reflection angles differ slightly?

Measurement error: it is hard to read a protractor and mark the centre of a beam.

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Distance from lamp to card (cm)	Shadow height (cm)
10	36
20	18
30	12
40	9