KS4 Waves : Reflection

© Boardworks Ltd 2003

KS4 Waves : Reflection


Objectives

Recall that light travels in straight lines.

Draw accurate ray diagrams.

Describe how we see objects.

Define the words opaque and transparent.

Describe the images formed in a plane mirror.

Recall how the angle of incidence and angle of reflection are related for a plane mirror.

Describe the differences between convex and concave mirrors.


Light rays

A light ray is a narrow beam of light. When drawing light diagrams always…..

1. Use a ruler.

2. Add an arrow to show the direction the light ray is travelling in.


How does light travel?

1) Fill a clear glass trough or empty fish tank with smoke.

2) Shine rays of light through the tank and describe what you see.


Light travels in straight lines

Your experiment shows that light travels in straight lines.

What every day consequences do you think this fact has?

You can not see around corners.

Shadows are formed in the area behind an object where light can not reach.


Seeing objects

How do we see an object?

Do we see all objects in the same way?

There are two ways we see objects…..

We see some objects because they are light sources.

We see some objects by reflected light.


Seeing objects

Light sources Objects seen by reflected light

Sun

Sun

Jupiter

Jupiter

Firefly

Firefly

Moon

Moon

Book

Book

Light bulb

Light bulb


Light sources

How do light rays from the Sun and other light sources reach our eye?

They travel in a straight line directly into our eye.


Reflected light

How do light rays from a book and other such objects reach our eye?

Light from the Sun or another light source strikes the book and some of the light is reflected into our eye.


Opaque and transparent materials

Use a textbook or other source to find out what the words opaque and transparent mean.

Opaque materials do not let light though.

Transparent materials do let light through.


Opaque and transparent materials

Hold different materials between a lamp and a screen.

Material

More or less

opaque?

Coal

Water

Glass

Thin ice

CoffeeFill in the results table shown estimating if the materials are more or less opaque than vegetable oil.

more

less

less

less

more


Reflections

There are two types of m____. F__ mirrors and c____ mirrors.

Flat mirrors are known as p___ mirrors.

If a mirror curves in then it is a c_____ mirror, if it curves out it is a c____ mirror.

irrors laturved

lane

oncaveonvex


Plane mirrors

Write out the letters of the alphabet down one side of a piece of paper, then next to the letter draw what the letter would look like as seen in a mirror.

What do you predict the letter ‘L’ would look like in a mirror?

What do you predict the letter ‘P’ would look like in a mirror?


Right and left

What happens to writing in a mirror?

The writing in a mirror is laterally inverted or back to front.

If you are wearing a T-shirt with some writing on and wave your right hand what will your image appear like in a mirror?

The writing on your T-shirt will be back to front and your image will be waving it’s left hand.


Rear-view mirrors

Imagine you are driving along in your car……

…and then you look up and see the following in your mirror…..

…with your knowledge of mirrors how must the writing be written on the vehicle?


Reading in Mirrors

In small groups, take it in turns to read the list of words on the next slide with your back to the

screen using a mirror.

You can only move on to the next word when you read the first word correctly.

Put your results in a table like this:

Name Time taken to read

[s]

Natasha 46

Pashmina 56

David 85


Words

Dog Man Ball

Bat Bike Ants

Park Fins Pink

Litter Sandy Shark


Results

A) Who read the words in the quickest time?

B) Plot a bar chart of you r results like:

C) What was the average time taken in your group?

A graph showing the results of 'Reading in Mirrors'

0

50

100

Natasha Pashmina David

Name

Tim

e ta

ken

[s

]


Image distance

1. Fix a plane mirror along the centre of a piece of A4 paper and draw around it.

Place a pin as the object in front of the mirror.

2. Line up a ruler with the image of the pin and draw along the edge of the ruler on the paper. Repeat for 3 more positions of the ruler.

3. Remove the mirror and ruler. Where the lines cross is the image position. What are the distances between the mirror and object and its image?

I


Image size and distance

For a plane mirror how does the size of the image and the image distance from the mirror relate to the original object?

Object Image

Image size and object size are the same.

The image distance is the same as the object distance from the mirror.


Investigating reflection

Apparatus:

Power packRay boxSlitPlane mirrorProtractorRuler

The ray box

will get very

hot, be careful

when handling it.


Investigating reflection

1. Place the plane mirror on a piece of paper.

2. Draw a line across the front of the mirror.

3. Draw a normal to the mirror.

4. Shine rays of light at varying angles to the normal and record the position of the incident and reflected rays

5. Measure the incident angles and the reflected angles for each ray. Record your results in a table.

Incident angle i

Reflected angle r

Reflected ray

Incident ray


Results

i r

0°

30°

45°

70°

85°

0°

30°

45°70°

85°

What do you notice? Is there a pattern?

Angle of incidence = Angle of reflection


Using plane mirrors

By positioning two plane mirrors at 45° to each other at either end of a tube we can make a _________.periscope

Periscopes are used in __________.submarines


Reflection : Curved Mirrors

ƒ

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

Finding of a concave mirror.

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

Hold a plain white screen in one hand.

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


Reflection : How does curvature affect ƒ?

Concave mirrors reflect rays of light to a focal point.

The distance between the mirror and the focal point is called the focal length [ƒ].

How can ƒ be changed?

Concave mirrors produce real images because the rays of light meet [unless the object is close].


ƒ Take a piece of Al or stainless steel sheet and curve it slightly.

Shine parallel rays of light at the reflector and plot their positions.

Draw around the reflector.

Measure ƒ and record your results.

Carefully bend the reflector and repeat the process to see how ƒ changes with curvature.

Reflection : How does curvature affect ƒ?


Reflection : Convex mirrors

ƒ

Convex mirrors reflect rays of light away from a focal point.

The distance between the mirror and the focal point is called the focal length [ƒ]

Convex mirrors produce virtual images - the rays of light do not meet.


Reflection : Curved mirrors

Concave reflectors are used to focus signals from distant satellites.

Convex reflectors are used to widen the field of view.


Which of the following is not a light source?

A. The Sun

B. A star

C. Traffic lights

D. A book


Which of the following is not seen by reflected light?

A. Your hand

B. Jupiter

C. Light bulb

D. The Moon


Which of the following is the most opaque?

A. Milk

B. Sea water

C. Air

D. Drinking water


What do we call the area behind an object where light can not reach?

A. Transparent

B. Shadow

C. Opaque

D. Outline


Which statement below is untrue for the image in a plane mirror?

A. It is laterally inverted

B. It is real

C. It is same size as the object

D. It is the same distance from the mirror as

the object

Documents

KS4 Waves : Reflection