© Boardworks Ltd 2003 KS4 Waves : Refraction. © Boardworks Ltd 2003 By the end of this lesson you should be able to: Define refraction Draw ray diagrams

$Page 1: © Boardworks Ltd 2003 KS4 Waves : Refraction. © Boardworks Ltd 2003 By the end of this lesson you should be able to: Define refraction Draw ray diagrams$
© Boardworks Ltd 2003

KS4 Waves : Refraction


By the end of this lesson you should be able to:

Define refraction

Draw ray diagrams showing refraction of light by a glass or Perspex block

List and explain everyday examples of refraction


Animation showing refraction at the air/glass boundary


Investigating refraction

Apparatus:

Power packRay boxSlitPerspex blockProtractor

The ray box

will get very

hot. Be careful

when handling it!


Exam tip

You will lose marks in an examination when you are drawing a light ray if you do not:

1. Use a ruler.2. Add an arrow to

show direction.


What to do……

Draw around the Perspex block on a piece of paper.


The normal

Mark on a line known as the NORMAL perpendicular to the surface of the block.

For the light ray entering the block and

the light ray leaving the block mark each

ray with two crosses.

Draw in the incident ray, emergent ray, remove the block and then join up the two rays.

Repeat several times with the light ray entering the block at different angles.


What happened……

As the light ray moved from air into perspex?

As the light ray moved from perspex into air?

If the angle of incidence = 0°? What do you notice about the incident

ray and the emergent ray?


Results

If the light ray entered the block parallel to the normal then it travels through undeviated.

If the incident ray enters the block at an angle to the normal then the direction of the ray changes as it enters and leaves the block, the light ray has been refracted.

Does the angle of incidence……

…affect the angle

of refraction?


Measure the angles of incidence and refraction and fill in the table below

Angle of incidence ( i) Angle of refraction ( r)


Air to Perspexangle of incidence > angle of refraction

i > r

As the light ray moved from air into perspex it moved towards the normal.

If light rays move from a less dense medium (air) to a more dense medium (perspex) they ‘bend’ towards the normal.

i > r


Perspex to Airangle of incidence < angle of refraction

i < r

As the light ray moved from perspex into air it moved away from the normal.

If light rays move from a more dense medium (perspex) to a less dense medium (air) they ‘bend’ away from the normal.

i < r


Angle of incidence = 0°

When the angle of incidence is 0 the light ray is not deviated from its path.

Un-deviated light ray


Animation to show what happens to a ray of light passing through a rectangular block of glass.


Revision tip

Remember the word:TAGAGA

Towards (normal) Air Glass Away (from normal) Glass Air


Fast and slow

If you were running along a beach and then ran into the water when would you be moving slower, in the water or on the beach?

In a similar way as light moves from one medium to another of different density the speed of light changes.

Do you think light moves faster or slower as the density of the medium it travels through increases?

In the waterLight moves more slowly through denser media.


The speed of light

Light travels at 300 000 km/s in a vacuum, as it enters denser media the speed of light decreases.

0306090

120150180210240270300

Vac

uu

m

Wat

er

Per

spex

Speed oflight(thousandskm/s)

Looking at the chart, which do you think is denser, Perspex or water?

Perspex must be denser because light travels slower through Perspex than water.


Refraction : effects of refraction

Many visual effects are caused by refraction.

This ruler appears bent because the light from one end of the ruler has been diffracted, but light from the other end has travelled in a straight line.

Would the ruler appear more or less bent if the water was replaced with glass?

More bent, because glass is more dense than water.


Refraction : magic coins

Place a coin in the bottom of a bowl and clamp an empty cardboard tube so that it points above the coin.

Gradually add water to the bowl and watch the coin through the tube float up - can you explain this?


Refraction : apparent depth

The rays of light from the coin get bent [refracted] as they leave the water.

Your eye assumes they have travelled in straight lines.

Your brain forms an image at the place where it thinks the rays have come from - the coin appears to be higher than it really is.


Animals and human hunters

Animals (including humans) allow for refraction when hunting fish in water.

image

actual location

The animals do not aim at the fish (it is just the refracted image), instead they aim at a location where they know from experience the fish actually is.


Refractive index

We can study refraction of light by comparing its speed in air to that in a medium.

A number called the refractive index is the ratio of these two speeds:

Refractive index = speed of light in air

speed of light in substance

Example:

The speed of light in air is 300 000 000 m/s, the speed of light in water is 225 000 000 m/s. What is the refractive index of water?

1.33


Calculating refractive index

Material Speed of light in material

Refractive index

Air 300 000 000

Water 225 000 000

Diamond 120 000 000

Perspex 200 000 000

1.0

1.33

2.5

1.5


Using refraction : lenses summary

There are two main types of lens:

Convex Concave

Convex lenses work by bending [refracting] rays of light to a principal focus.

The distance from the centre of the lens to the principal focus [F] is called the focal length [ƒ].

The image formed by a convex lens is inverted [back-to-front and upside-down].

The thicker the lens, the shorter the focal length[ƒ].


A lens can be thought of as a series of prisms.

The lens refracts all the rays to a point called the principal focus [F].

The distance between the centre of the lens and F is called the focal length [].

Imagine parallel rays of light from a distant object hitting the lens.

Draw normal lines [at 90° to the surface] for each ray.

Use the first refraction rule to work out the ray direction.

Draw normal lines where the rays enter the air [at 90º to the surface].

Work out the direction of the refracted rays using the second refraction rule.

When light enters a less dense medium [e.g. air], it bends away from the normal.

Using Refraction : lenses

When light enters a more dense medium [e.g. glass], it bends towards the normal.

F

ƒ


What do you think happens when…

Parallel light rays strike a convex lens?They pass through the focal point of the lens.

Diverging light rays?Form a parallel beam if they pass though the focal point (F).

F


Use a ruler to measure the distance between the lens and the screen - this is the focal length [ƒ].

Using Refraction : lenses - finding

ƒ

Chose a distant object [to get parallel rays of light].

Hold a plain white screen in one hand.

Hold the lens in the other hand and move it closer to the screen until a clear image appears.


Refraction : lenses

1. Find the focal length [ƒ] of your lens.

2. Fix the lens to the centre of a metre rule and mark the distances F and 2F either side of the lens.

2F F F 2F

3. Place the candle >2F away from the lens and move the screen until an image appears and record observations.

4. Repeat for the candle at 2F, between 2F and F, at F and between F and the lens.


Results

Object position

Image Position

Real or virtual

Magnified or

diminished

Inverted or erect

>2F

at 2F

between 2F and F

at F

between F and lens


Refraction : lenses

Object >2F away

O

2F F F 2F

I

The image [ l ] is formed between F and 2F away from the lens, is inverted and diminished.


Object at 2F

O

2F F F 2F

I

The image [ l ] is formed at 2F away from the lens, is inverted and the same size.

Refraction : lenses


Object between 2Fand F away

O

2F F F 2F

IThe image [ l ] is formed further than 2F away from the lens, is inverted and magnified.

Refraction : lenses


Object at F away

O

2F F F 2F

The image [ l ] is formed at infinity - the rays never meet [we use this set-up for searchlights].

Refraction : lenses


Object between F and lens

O

I

The VIRTUAL image [ l ] is formed on the same side of the lens as the object, is the right way up and magnified.

2F F F 2F

Refraction : lenses


Results

Object position

Image Position

Real or virtual

Magnified or

diminished

Inverted or erect

>2F

at 2F

between 2F and F

at F

between F and lens

between F and 2F

at 2F

> 2F

at infinity

same side as object

virtual

real

real

real

magnified

magnified

same size

diminished

erect

inverted

inverted

inverted


2F F F 2F

Magnification = Distance from lens to image

Distance from object to lens

Refraction : lenses


Which of the following is the most dense?

A. Air

B. Water

C. Glass

D. Lead


When light changes direction as it moves from one medium to another we call this effect what?

A. Reflection

B. Refraction

C. Diffraction

D. Total internal reflection


What happens to the speed of light as it moves from air into glass?

A. Decreases

B. Increases

C. No effect

D. Decreases and increases


If a ray of light moves from air to glass parallel to the normal what happens?

A. No change in direction

B. It bends away from the normal

C. It bends towards the normal

D. It stops


If light travelling through a medium has a speed of 150 000 000 m/s. What is the refractive index of the medium?

A. 2.6

B. 0.5

C. 2.0

D. 1.5


Can you……

Explain what refraction is? Describe what happens to a light ray if it enters a

medium of different density at an angle? Describe what happens to a light ray if it enters a

medium of different density along the normal? Describe examples of refraction? Draw ray diagrams depicting the refraction of light? Calculate the refractive index for a medium?

Documents

© Boardworks Ltd 2003 KS4 Waves : Refraction. © Boardworks Ltd 2003 By the end of this lesson you should be able to: Define refraction Draw ray diagrams