Science notes, chapter 8
Light and Optics
Textbook Science Form 1 (DLP), pages 220–253
By the end of this chapter you can
- Tell real and virtual images apart, describe the images formed by plane, concave and convex mirrors, and explain how mirrors and optical instruments are used (8.1)
- Describe the properties of light — it travels very fast and in straight lines, forming shadows (8.2)
- State the characteristics of the image in a plane mirror and use the Law of Reflection to draw ray diagrams (8.3)
- Explain refraction when light moves between media of different densities, draw ray diagrams and give daily-life examples (8.4)
- Explain how white light is dispersed into a spectrum and how a rainbow forms (8.5)
- Explain scattering of light and why the sky is blue at midday and reddish at sunset (8.6)
- Identify primary and secondary colours, and explain addition and subtraction of light, including colour filters (8.7)
8.1The Use of Mirrors
Real image and virtual image
A real image is an image that can be formed on a screen.
A virtual image is an image that cannot be formed on a screen.
A virtual image is an image that cannot be formed on a screen.
In Activity 8.1, light from a candle passes through a tiny pinhole in black cardboard and makes an image of the candle on a white cardboard screen. Because the image appears on a screen, it is a real image. When you look into a mirror, your image seems to be behind the mirror. You cannot catch it on a screen, so the image in a plane mirror is a virtual image.
Three types of mirrors
| Mirror | Shape of shiny surface | Image of a nearby object | Uses |
|---|---|---|---|
| Plane mirror | Flat | Same size, upright, virtual; image distance = object distance | Dressing mirror, dance studio, making a room look spacious, lifts, periscope, kaleidoscope |
| Concave mirror | Curves inwards (like a cave) | Bigger (magnified), upright, virtual | Make-up mirror, dentist's mirror |
| Convex mirror | Bulges outwards | Smaller, upright, virtual; gives a wide view | Dangerous road corners, supermarkets (to prevent theft), bicycle and vehicle side mirrors |
Plane mirrorHelps a dancer correct his movements
Concave mirrorDentist sees teeth bigger and closer
Convex mirrorSafety at a dangerous road corner
Convex mirrorShopkeeper sees every corner of the shop
Photographs: Science Form 1 (DLP) textbook, Photograph 8.2, pp. 224–225.
Concave = closer and bigger (make-up, dentist). Convex = wider view, smaller image (road corners, shops, side mirrors).
Light reflects off a mirror
A plane mirror works by reflection of light: light rays that shine on the mirror bounce off it. Every image you see in a mirror is made by reflected light.
Optical instruments that use reflection
Scientists invented optical instruments to extend the ability of our senses, for example to see things that are hidden from view.
PeriscopeTwo plane mirrors, each fixed at 45°, facing each other. Light from the object hits the top mirror and is reflected down; the bottom mirror reflects it again into the observer's eye. Used in submarines to see the sea surface, or to see over a wall.
KaleidoscopeA toy made of three mirror strips joined into a triangle prism inside a tube, with colourful beads at one end. Light is reflected again and again, so you see many more images than there are beads — beautiful patterns.
Solving problems with mirrors (textbook p. 228)
Want to see behind a wall? Use a periscope.A path with a blind corner? Fix a convex mirror at the corner so people can see what is coming.
Cycling? A convex mirror on the bicycle lets you see the road behind you.
Why are plane mirrors often fixed on the walls of a lift?
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They make the small space look bigger (spacious) and more comfortable, and passengers can see everyone around them, which helps safety.
8.2Properties of Light
Light travels very fast: its speed is 3.0 × 108 m s−1 (300 000 km every second).
Light travels in straight lines.
Light travels in straight lines.
- Lightning before thunder. Lightning and thunder happen at the same time, but light travels much faster than sound, so we see the lightning before we hear the thunder.
- Shadows. Light travels in straight lines and cannot bend around an opaque object (an object light cannot pass through). The dark area behind the object, where light is blocked, is a shadow.
- Rainbows. Light can also be dispersed (split up) by water droplets in the sky to form a rainbow (see 8.5).
Your shadow is shortest at noon, when the Sun is directly above your head, and long in the morning and evening, when the Sun is low. The ancient sundial used the moving shadow of a stick to tell the time, and shadow puppets (wayang kulit) also use shadows made by blocking light.
Hisyam's shadow is the shortest in the ______ when the Sun is ______ his head.
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afternoon (around noon); directly above his head.
8.3Reflection of Light
Image formed by a plane mirror
The image in a plane mirror is:
upright · laterally inverted (left and right swapped) · same size as the object · virtual · the same distance behind the mirror as the object is in front (image distance = object distance).
Worked example — distances
Mei stands 2 m in front of a plane mirror.Image distance = object distance = 2 m behind the mirror.
Distance between Mei and her image = 2 + 2 = 4 m.
The word "AMBULANCE" is painted laterally inverted on the front of an ambulance. When drivers in front look in their rear-view mirror, the mirror inverts it again, so they read it the right way round.
The Law of Reflection
The normal is an imaginary line drawn at 90° to the mirror where the ray hits it. Angles are always measured from the normal, not from the mirror.
Law of Reflection
1. The incident ray, the reflected ray and the normal all lie on the same plane.
2. The angle of incidence, i is equal to the angle of reflection, r (i = r).
1. The incident ray, the reflected ray and the normal all lie on the same plane.
2. The angle of incidence, i is equal to the angle of reflection, r (i = r).
Experiment 8.1 in short
Manipulated variable: angle of incidence, i (10°, 20°, 30°, 40°, 50°).Responding variable: angle of reflection, r. Constant variable: size of the slit.
Result: r is always equal to i — the hypothesis is accepted.
Measuring the angle from the mirror instead of from the normal. If a ray makes 30° with the mirror surface, the angle of incidence is 90° − 30° = 60°, so the angle of reflection is also 60°.
Uses of reflection: road signs, traffic cones and warning triangles have reflective surfaces that reflect car headlights back so drivers can see them at night.
A ray of light hits a plane mirror with an angle of incidence of 35°. What is the angle of reflection? What is the angle between the reflected ray and the mirror?
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Angle of reflection = 35° (i = r). Angle with the mirror = 90° − 35° = 55°.
8.4Refraction of Light
Refraction of light is the change in direction of light when it travels from one medium to another medium of a different density (for example from air into water or glass).
Refraction explains why a deep pond looks shallower than it really is, why a fish looks closer to the surface than it is, and why a pencil or spoon looks bent in a glass of water.
Ray diagrams for refraction
| Light travels from … | What happens | Angle |
|---|---|---|
| Less dense → more dense (air → water, air → glass) | Bends towards the normal | r smaller than i |
| More dense → less dense (water → air, glass → air) | Bends away from the normal | r bigger than i |
| Along the normal (at 90° to the surface) | Not refracted — goes straight through | i = 0° |
Denser → towards the normal. Going into a more dense medium (air → water), light bends towards the normal; going out into a less dense medium (water → air), it bends away. Density, not thickness, is what matters.
Experiment 8.2 in short (air → glass block)
Manipulated: angle of incidence, i. Responding: angle of refraction, r. Constant: size of slit, shape of glass block.Result: the greater the angle of incidence, the greater the angle of refraction — but r is always smaller than i, because the ray bends towards the normal in glass.
Catching fish: because the fish is really deeper than it looks, a fisherman with a spear must aim below the image he sees.
Why does the bottom of a deep swimming pool appear shallower than it really is?
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Light from the bottom of the pool is refracted away from the normal as it leaves the water (denser) into the air (less dense). Our eyes trace the light back in a straight line, so the image of the bottom appears higher, i.e. shallower than the real depth.
8.5Dispersion of Light
Dispersion of light is the splitting of white light into its seven colours. The band of colours is called a spectrum: red, orange, yellow, green, blue, indigo, violet.
- When white light enters the prism, each colour bends towards the normal by a different amount.
- When the colours leave the prism, they bend away from the normal and spread out further, forming a spectrum.
Each colour travels at a different speed in glass. Red light is the fastest, so it is refracted the least. Violet light is the slowest, so it is refracted the most.
Rainbow: when sunlight enters raindrops in the sky, the white light is refracted and dispersed into seven colours, forming a rainbow. You can also see a rainbow at a fountain or a waterfall, or make one with a mirror in a basin of water and a torch (Activity 8.7).
Remember the order with the name ROY G BIV — Red, Orange, Yellow, Green, Blue, Indigo, Violet.
Which colour is refracted the most by a glass prism? Explain why.
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Violet — it travels the slowest in glass, so it is refracted (bent) the most.
8.6Scattering of Light
Scattering of light happens when light is reflected in all directions by clouds or tiny particles (air particles and dust) in the atmosphere.
Midday: blue skyThe Sun is overhead. Blue light is scattered the most in all directions by the tiny particles, so blue light reaches our eyes from every part of the sky. The sky looks blue.
Sunset: reddish skyThe Sun is at the horizon, so its light passes through much more atmosphere. Blue light is scattered away; red and orange light are scattered less and reach our eyes. The sky looks reddish.Diagrams: Science Form 1 (DLP) textbook, Figures 8.21 and 8.22, pp. 239–240.
In Activity 8.8, milk powder stirred into water acts like the particles in the air. From the side, the beam looks bluish (blue light is scattered sideways); the light reaching the screen looks orange-red (the blue has been scattered out).
Complete: During the day, blue colour is scattered ______ compared to red colour. At sunset, red colour is scattered ______ compared to blue colour.
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more; less.
8.7Addition and Subtraction of Light
Primary colours of light: red, green, blue — they cannot be made by mixing other colours.
Secondary colours: yellow, magenta, cyan — made by mixing two primary colours.
Secondary colours: yellow, magenta, cyan — made by mixing two primary colours.
Addition of light
| Primary colour | + Primary colour | = Secondary colour |
|---|---|---|
| Red | Blue | Magenta |
| Red | Green | Yellow |
| Blue | Green | Cyan |
| Red + Green + Blue = White | ||
Mixing light is not the same as mixing paint. Red light + green light gives yellow light, not brown.
Subtraction of light
Subtraction of light: an opaque object reflects light of its own colour into our eyes and absorbs the other colours. That is why a banana looks yellow, a strawberry red and a leaf green.
| Object (in white light) | Reflects | Absorbs |
|---|---|---|
| Primary colour, e.g. green | Green only | All other colours |
| Secondary colour, e.g. yellow | Yellow, plus red and green (the primaries that make yellow) | Other colours |
| White | All colours | None |
| Black | None | All colours |
Colour filters
- A primary colour filter lets only light of its own colour pass through. A red filter lets only red light through.
- A secondary colour filter lets through its own colour and the two primary colours that make it. A yellow filter lets through yellow, red and green light.
Worked example — two filters (Activity 8.10)
White light → red filter → only red passes → yellow filter (lets red and green through) → red passes.Colour on the screen: red.
White light → red filter → cyan filter (lets only green and blue through) → red is absorbed → screen is dark (black).
Addition = mixing coloured lights to make new colours.
Subtraction = an object or filter absorbs some colours and reflects or lets through the rest.
Subtraction = an object or filter absorbs some colours and reflects or lets through the rest.
In daily life: colour televisions, coloured stage lights, stadium lights and the coloured lights in the fountain in front of KLCC all mix red, green and blue light to make many colours.
What colour is seen on a screen when white light passes through a magenta filter?
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Magenta — the filter lets through red and blue (and magenta) light, and red + blue = magenta.
Summary
| Remember | Key fact |
|---|---|
| Real vs virtual image | Real can be formed on a screen; virtual cannot |
| Mirrors | Plane: same size; concave: bigger (make-up, dentist); convex: smaller, wide view (road corners, shops) |
| Properties of light | Very fast (3.0 × 108 m s−1); travels in straight lines → shadows |
| Plane mirror image | Upright, laterally inverted, same size, virtual, image distance = object distance |
| Law of Reflection | i = r; measured from the normal |
| Refraction | Less dense → more dense: towards normal; more dense → less dense: away from normal |
| Dispersion | White light → spectrum ROYGBIV; red refracted least, violet most; rainbow |
| Scattering | Blue scattered most → blue sky at midday; red scattered least → reddish sunset |
| Addition of light | R + G = yellow; R + B = magenta; B + G = cyan; R + G + B = white |
| Subtraction of light | Objects and filters absorb some colours; white reflects all, black absorbs all |