download k4.3_2.0a authored by keith ross, uni. of gloucestershire. accessed from date created may...

Download K4.3_2.0a <Lead lecture – Light> Authored by Keith Ross, Uni. of Gloucestershire. Accessed from http://www.ase.org.uk/sci-tutors/ date created May 05. [ This document can be freely copied and amended if used for educational purposes. It must not be used for commercial gain. The author(s) and web source must be acknowledged whether used as it stands or whether adapted in any way. ]

Light and colour

Keith Ross

University of Gloucestershire


Overview of lecture

• Key ideas and children’s views

• Perception is more than seeing

• Teaching Light and Colour


1: Seeing

Figure 4.4 How does the light and our eyes help us see? (From Guesne 1985)


The eyes as receptors of light


Using light to ‘see with’


Light and Sight


Alternative conceptions – firmly held

‘As they arrived next morning, three children told how they had (separately)

taken their cats into a dark cupboard in the evening following the lesson.

One was a striped ginger-and-white cat and its owner had expected to be able to see

only the white stripes. In fact, nothing at all had been visible.’


‘To her great surprise the girl was unable to see the cat’s eyes either;

but she surmised that this was because the cat was quite young (about a year old), and its

eyes were still “too weak” to be seen.’


Observation - naive or theory-based?

• no observation is theory-free:

• observation depends on comparing sense data arriving in our brains with ideas already in our heads


• When you know what this is put your hand up but say nothing. Look around to see how many know


It is a man on a horse


Prior knowledge helps us to sort out the image concealed in this picture


Tell each other what has happened.


Understanding takes place when we link sense data to

existing ideas

Try to remember what you are about to see and write it down – show each other what you wrote.


• There’s more to seeing than light entering your eye – the brain needs to de-code the information


The scientific method is ‘Purposeful observation’.

• We interpret light signals to our eyes as physical objects ‘out there’

The clear circles on agar plates - Flemming, a microbiologist, saw moulds killing bacteria.


The scientific method is ‘Purposeful observation’.

• Darwin had seen selective breeding amongst pigeons, and saw the same thing with Galapagos finches (or did he?)

• Mendel, a trained mathematician, saw huge significance in his 3:1 ratio of tall to short peas. (Is that why he obtained this perfect ratio?)


Scientists

• are biased in their observations - aren’t they supposed to be objective?

• objective observation cannot be done.

• When we observe, our minds are trying to make sense by comparing our existing ideas with our sense data.

• double blind tests?


Iodine going black

• a picture of photosynthesis

• or strange words on bottles, a series of processes with a couple of leaves, one of which goes black.

• “This shows that photosynthesis requires light”.


Theories are the creation of our minds.

• Distinguish between the creativity of Newton (Laws of Motion) and Shakespeare (Macbeth).

• Scientific ideas must be tested against reality


How do we see?

• How is the eye like a camera?

• How is the eye not like a camera?

• Why is the image formed upside down on the retina?

• The brain does the rest!


The pinhole camera

• Camera, chamber, comrade, room


Shadows (Shade)

• How are shadows formed?

• Why do some shadows become fuzzy, or poorly defined?


Shadows

• Children often confuse shadows and reflections


Reflections (Flexible)

How is a reflection (image) different from a shadow?

• Detail

• Colour

• Shape


Reflected light makes images

Light appears to come from behind the mirror

www.saburchill.com/ physics/chapters3/0002.html

http://www.saburchill.com/physics/chapters3/0002.html




3: Candle light

• Does it travel further at night?

• Why does a cinema look dark when we first enter, but then we see more easily?


Travelling light

Light travels in straight lines - and very fast

Thunder and lightning

Hitting a cricket ball

• 8 times round the world in a second

• To the moon and back in 3 seconds

• To the nearest star in 8 minutes,

• To Jupiter in three hours

• The next nearest star in 4.5 years


5: Rainbow• Which direction

should you look to see the rainbow?

Refraction and reflection of light in a water droplet

Blue bends more than red

http://my.unidata.ucar.edu/content/staff/blynds/rnbw.html













Refraction (KS3)

• Why do we see rainbow colours when white light is passed through a prism?


6: Colour

• What do these words mean:– Transparent

– Translucent

– Opaque

• What happens to light when it ‘hits’ opaque objects– Scattered

– Reflected

– Absorbed


Colour mixing – positive and negative

• Mix paint colours, e.g. Magenta (red), Cyan (blue), yellow, - more light is absorbed – darker colour –subtracting from white. Pigments and artists.

• Adding coloured lights, e.g. red (orange), blue (purple), green – more light is added – nearer to white light.


After images

• Your eyes have three receptors R G B

• If one gets blinded (tired) only the other two will work when white light (RGB) enters your eye.

• Stare at the cross and dot then relax your eyes into the white screen


Use the hand lens to identify which guns are firing for each of these colours: red, green, blue, cyan, magenta and yellow.


Adding lights to a dark TV

screen

download k4.3_2.0a authored by keith ross, uni. of gloucestershire. accessed from date created may...

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