Science notes, unit 5

Properties of light

Textbook Science Year 4 (DLP), pages 87–110

What this chapter covers

You can say that light travels in a straight line and show it with an investigation (5.1.1)
You can tell transparent, translucent and opaque objects apart by the shadows they form (5.1.2)
You can name the factors that change the size and the shape of a shadow (5.1.3, 5.1.4)
You can explain reflection of light and give examples of it in daily life (5.2.1, 5.2.2)
You can draw a ray diagram to show reflection of light (5.2.3, 5.2.4)
You can explain refraction of light and how a rainbow is formed (5.3.1, 5.3.2, 5.3.3)
You can explain why the properties of light are important in daily life (5.3.4)

Light helps us to see. In this unit you will learn three properties of light: light travels in a straight line, light can be reflected and light can be refracted. You will also find out how shadows are formed.

5.1Light travels in a straight line

Light moves from its source in a straight line. This is the first property of light.

We can see this in many places around us. Sunlight shines in straight rays between the tree trunks in a forest. The light from a car's headlights makes a straight beam on a dark road.

Straight rays of sunlight shining between tree trunks in a forest — Sunlight rays between the tree trunks.

Light beams from the headlights of a car on a dark road — Light from car headlights.

Textbook p.88.

Investigation: does light travel in a straight line?

Put a torch, three cardboards with a slit each, and a black screen in a row. Switch on the torch.

Torch, three cardboards with slits in a straight line and a screen; a ray of light reaches the screen — Slits in a straight line: light reaches the screen.

Torch and three cardboards; the middle slit is higher than the others and no light reaches the screen — Slits not in a line: no light on the screen.

Conclusion: light forms on the screen only when the slits are in a straight line. So light travels in a straight line.

Why must the three slits be in a straight line for the light to reach the screen?

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Light travels in a straight line, so it cannot bend round the card to go through a slit that is not in line.

5.2Shadows

A shadow is formed when an object blocks light. How clear the shadow is depends on the kind of object.

Kind of object	Light that passes through	Shadow	Example
Transparent	All the light	None	Clear plastic
Translucent	Some of the light	Less clear	Coloured plastic
Opaque	No light	Clear (dark)	Manila card

A way to remember: transparent lets all the light through, translucent lets some light through, and opaque lets none through.

A glass of plain water is put in front of a torch. Which kind of object is it, and what is the shadow like?

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It is transparent. It lets all the light pass through, so there is no shadow.

Size of a shadow

The size of a shadow depends on two distances:

the distance between the light source and the object;
the distance between the object and the screen.

A torch, an object and a screen with the shadow on it. Distance A is torch to object. Distance B is object to screen — Distance A: torch to object. Distance B: object to screen.

In the experiment, the manipulated variable is the distance between the torch and the object. The responding variable is the size of the shadow. The constant variable is the distance between the object and the screen.

Example results
Torch to object	Size of shadow
5 cm (near)	Large
10 cm (further)	Smaller

Conclusion: the further the object is from the light source, the smaller the shadow.

Do not mix up the two distances. If you move the object away from the torch, the shadow gets smaller. If you move the object nearer to the screen, the shadow also gets smaller.

Shape of a shadow

The shape of a shadow changes when we change:

the orientation of the object (how it is turned or tilted);
the position of the light source.

For example, a cylinder gives a circle shadow when the light shines on its end and a rectangle shadow when it lies on its side.

Name two factors that change the shape of a shadow.

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The orientation of the object and the position of the light source.

5.3Reflection of light

When light shines on an object and bounces back, we say the light is reflected. This is reflection of light.

We see our image in a mirror because the mirror reflects light from our face into our eyes. Smooth and shiny surfaces reflect light well, such as water, floor tiles and metal.

A girl looking at her image in a mirror — Light from the girl is reflected by the mirror to her eyes.

Textbook p.94.

A deer drinking and its image on the water surface — An image on the water surface.

Textbook p.94.

The moon is not a source of light. It looks bright at night because it reflects sunlight towards the Earth.

A torch has a shiny part behind the bulb. It reflects the light forward so the beam is brighter.

Uses of reflection in daily life

Device	How it uses reflection
Face mirror	Reflects light from our face to our eyes so we can see our image.
Side mirror of a car	Lets the driver see vehicles that are blocked from view.
Convex mirror at a bend	Lets a driver see a vehicle that is hidden round the bend, to avoid an accident.
Dental mirror	Lets the dentist see the teeth easily.
Periscope	Two mirrors reflect light from an object that is blocked from view to the observer's eyes.

A dentist using a small mirror to see inside a patient's mouth — A dental mirror.

A periscope with two mirrors on a submarine and a dashed line showing the path of light — A periscope uses two mirrors.

Textbook p.97.

Ray diagram

A ray diagram shows how light is reflected. It has the mirror, the incoming ray and the reflected ray, each with an arrow to show the way the light goes. Draw the rays with a ruler because light travels in a straight line.

Why do we use a ruler to draw a ray diagram?

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Because light travels in a straight line, so the rays must be straight.

5.4Refraction of light

When light travels from one medium (such as air) into a different medium (such as water), its direction changes. This is refraction of light.

A ray of light going from air into water and bending at the surface — The ray bends when it goes from air into water.

A spoon in a glass of water looks bent because the light from the spoon is refracted when it goes from water to air.
A hand lens magnifies things because it refracts light.

The spoon is not really bent. It only looks bent because light is refracted.

Rainbow

When the Sun shines on rain droplets, the light is refracted and a rainbow is formed. In the classroom we can model this with a mirror in a basin of water placed in sunlight: the water refracts the light and the mirror sends it onto paper on the wall.

Which property of light makes a rainbow?

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Refraction. The sunlight is refracted by the rain droplets.

5.5Why the properties of light matter

Property	What would happen without it
Travels in a straight line	A torch beam could not shine straight onto the thing we look for.
Reflected	We could not see our face in a mirror.
Refracted	A hand lens could not magnify an object.

Summary of the three properties

Travels in a straight line: torch beam, sunlight in a forest.
Reflected: mirror, periscope, dental mirror, convex mirror.
Refracted: bent spoon, rainbow, hand lens.