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mastery learning centre

State the properties of light

Draw ray diagrams

Explain the meaning of reflection and refraction

Explain how we see colours

Infer the colour of an object under coloured lights

LEARNING OBJECTIVES

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Light is a form of energy that is needed to support life. It can

travel through vacuum at a very high speed of 300 million

metres per second(3 x 108 ms-1). Light slows down when it

travels through solids, liquids and gases . Light can only travel

in straight lines .

ABOUT LIGHT

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Objects that give out light on their own are called l ight

sources or luminous objects . Objects that do not give out light

are called non-luminous objects .

ABOUT LIGHT

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When light hits an object, any one of the four dif ferent things

below can happen:

it can pass through the object

it can be absorbed by the object

it can be reflected by the object

it can be refracted by the object

ABOUT LIGHT

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A transparentmaterial allows almost all lightto pass through

it. Glass is a transparent material. Objects behind a

transparent material can be seen clearly through the material.

TRANSPARENCY

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A translucentmaterial allows some light to pass through it.

Plastic and frosted glass are translucent materials. Objects

behind a translucent material cannot be seen clearly through

the material. Opaque materials do not allow light to pass

through it at all. Objects behind an opaque material cannot beseen at all through the material.

TRANSPARENCY

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Light can be absorbed or reflected by an object. When light is

blocked , a shadow is formed behind the object.

REFLECTION

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When light is reflected, it bounces off the surface of the

object. In regular reflection of light, the reflected light can

form a sharp image . Smooth and polished surfaces produce

regular reflections.

REFLECTION

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In an irregularor diffuse reflection of light, the reflected light

cannot form a sharp image . A diffuse reflection is produced

when the surface is irregular.

REFLECTION

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A light raythat hits a surface is called the incidentray. The

reflectedlight is called reflected ray. When the incident ray

travels in a path that is at right angle to a smooth surface,

the light will be reflected back along the same path . This path

is called the normal .

REFLECTION

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When the incident ray strikes the surface at an angle to the

normal, it will be reflected at the same angle on the opposite

side of the normal.

REFLECTION

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A mirrorgives a regular reflection with a sharp and clear

image . The image produced is/appears

virtual (cannot be projected on a screen)

upright

laterally inverted (right becomes left and left becomes right)

equal in size to the object

as far behind the mirror as the object is in front of it

REFLECTION

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the incident ray, the normal and the reflected ray all lie on the

same plane

angle of incidence (i) is equal to the angle of reflection (r)

LAWS OF REFLECTION

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IMAGE FORMATION (PLANE MIRROR)

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Light travels at different speeds through different mediums .

Light travels fastestthrough gases and slowest through solids .

When light passes from one medium to another, its speed

changes and the ray of light bends . This is called refraction

of light.

REFRACTION

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When light passes from a less dense to a denser medium , the

refracted ray slows down and bends towards(nearer) the

normal .

REFRACTION

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When light passes from a denser to a less dense medium , the

refracted ray speeds up and bends away from the normal .

REFRACTION

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The bending of light causes an object to appear nearer than it

actually is when a transparent material is on top.

EFFECTS OF REFRACTION

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In the diagram, refraction causes point A to appear nearer tothe surface at B . So to the eyes, the straw appears to bendtowards the surface of the water.

EFFECTS OF REFRACTION

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Light can be dispersed by a prism. When a beam of light is

passed through a prism onto a screen, the prism refracts the

light into a spectrum of seven colours : red, orange, yellow,

green, blue, indigo and violet.

REFRACTION

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Thus, white light is made up of a spectrum of colours . When

light falls on an object, some of the colours in the spectrum

are absorbed while others are reflected . This allows us to see

the object as a coloured object. For example, a red object

reflects red lightand absorbs all other colours. A black objectabsorbs all the colours . A white object reflects all the colours .

COLOURS

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The three primarycolours are red , blue and green . All other

colours, called secondary colours, are results of combinations

of these three colours .

COLOURS

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OBJECTS IN WHITE LIGHT

Whi te ob jects appear whi te

as they re f lect a l l co lours

and absorb none .

Red ob jects re f lect red

l i ght and absorb a l l the

o t h e r c o l o u r s .

B lack ob jects absorb a l l

the l i ght . S ince no l i ght i s

re f lected , the ob jects

appear b lack due to an

absence o f l i ght .

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OBJECTS IN COLOURED LIGHT

The colour of an object is the colour of the light that is

reflected from it into our eyes .

In g reen l i ght , a g reen leaf

appears green because i t

re f lects g reen l i ght .

In red l i ght , a g reen leaf

appears b lack because i t

absorbs the red l i ght and

does not re f lect any l i ght .

red lightgreen light

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