Post on 04-Jan-2016
© Boardworks Ltd 2003
Reflected light
How do light rays from a book and other such (non-luminous) objects reach our eye?
Light from the lamp or another light source strikes the book and some of the light is reflected into our eye.
© Boardworks Ltd 2003
Reflection : Good or Poor?
Objects which reflect light well have smooth, shiny surfaces and are usuallypale colours.
They give clear images because they reflect the light regularly.
Mirrors are excellent reflectors.
Objects which do not reflect light well have rough, matt surfaces and are usually darkcolours.
They give no or diffuse images because they reflect the light irregularly.
© Boardworks Ltd 2003
Reflection : Good or Poor?
Arrange these items along the arrow:
White Paper
Red
Roses
Polished Black Shoes
Al foil
Yellow
Banana
Blue Car
Tree Bark
Tangerine
Green Leaf
Tarmac Road
Best Reflectors
Worst Reflectors
© Boardworks Ltd 2003
What happens to the light which isn’t reflected?
• Some of this light may be absorbed (e.g. as heat).
• Some of this may also be transmitted (e.g glass reflects a small amount of light, absorbs some of the rest and allows most of it to pass through.
© Boardworks Ltd 2003
Reflection: Investigating Reflection.
These activities allow you to investigate the main laws of reflection.
You should summarise each investigation with a law you have written based on that exercise.
1) Reading in mirrors.
2) How far away is the image?
3) The maths of reflection.
4) Reflecting without mirrors.
© Boardworks Ltd 2003
Reflection: 1. 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
© Boardworks Ltd 2003
Reflection : 1. Words
Dog Man Ball
Bat Bike Ants
Park Fins Pink
Litter Sandy Shark
© Boardworks Ltd 2003
Reflection : 1. 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
]
© Boardworks Ltd 2003
Reflection : 1. Lateral inversion
The way plane mirrors reflect light regularly gives a clear image which is the same size as the object
- but what is different about the image ?
This is called lateral inversion.
© Boardworks Ltd 2003
Reflection : 2. How far away is the image?
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
© Boardworks Ltd 2003
Reflection : 3. The Maths of Reflection
Fix a plane mirror to a piece of A5 paper and draw around it.
Angle ofI ncidence [i]
Angle ofReflection [r]
Angle i
Angle r
Draw a normal line (at 90º) through the middle of the mirror outline.
Use a ray box to shine an incident ray at the mirror - plot the incident and reflected rays.
Measure the angles of incidence [i] and reflection [r] and write the answers in the table on the right.
Repeat for another 5 angles of incidence.
© Boardworks Ltd 2003
Reflection : 4. Reflecting without mirrors
Mirrors are good reflectors but not perfect - they give 2 reflections.
We use glass prisms instead of mirrors in good quality binoculars and other instruments.
© Boardworks Ltd 2003
Reflection : 4. Reflecting without mirrors
Shine rays of light into a prism as shown in the ray diagrams below.
Copy and complete the ray diagrams using a ruler and pencil.
Don’t forget to include arrows on your rays!
© Boardworks Ltd 2003
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
© Boardworks Ltd 2003
Reflection : Summary
You should now know that :
Pale and shiny surfaces are good reflectors, dark and rough surfaces are not.
The image in a plane mirror is laterally inverted.
The image is the same distance behind the mirror as the object is in front.
The image in a plane mirror is the same size as the object.
angle of incidence = angle of reflection
¡ = r