What is Light? – Part 2

Intro

When we first learn about light, we see very simply the world is either light or dark. There is day and night and there are little switches on walls that turn indoor light on or off. Soon, we learn that the Sun is the source of all natural light and things quickly become more complicated. Light from the Sun travels to our planet after a speedy trip of 149 million km. Light travels at 300,000 km per second. We can detect the light originating from the Sun in approximately 8 minutes. In other words, it takes a little over 8 minutes for light energy from the sun to travel to us. What is light and how do we see light with our eyes? What is this light energy, visible light or spectrum that is detected by our eyes?

Sample Problem

Background
The Sun is an enormous fireball shooting energy in all directions. When light travels between two places (from the Sun to the Earth or from a flashlight to the wall in a dark room), light energy makes a journey from the light source to a receiving object in the form of waves (light waves or electromagnetic waves). These are similar to waves traveling in the ocean but millions of times smaller in a vibrating pattern: electromagnetic energy.

The Problem

Colour is one of the hardest concepts of understanding light. Sunlight is captured and reflected off objects to make them visible and unique in colour. If we are able to see things because sunlight is reflected off them, why are objects of different colours? Wouldn’t sunlight remain the same light when it is reflected?

Theoretical Answer About Light

Sunlight isn’t light of just one colour — it’s what’s called white light or a spectrum of light that is made up of many different colours. Evidence of this is seen in a Rainbow. Rainbows are colourful curves of light that appear in the sky when droplets of water split sunlight into different component colors by refracting (bending) sunlight by differing amounts. Components of light are bent into different wavelengths producing different colours.

Theoretical Answer About Colour

Why does a leaf look green? When sunlight shines on a leaf, the green light is the portion of the sunlight that is captured and reflected, while all the other colours of lights are absorbed by the leaf. These light waves that are not captured and reflected are instead ‘soaked up’ or unseen to the eye. The same is true of a red flower. It is only the red light that is captured and reflected and the other lights in the sunlight spectrum are ‘absorbed’.

When sunlight hits an object, the incoming light energy excites tiny particles or atoms in an object, like the leaf. Electrons in the leaf are promoted by sun’s light energy at a specific wavelength/energy amount or ‘colour’. Corresponding energy particles from the sun’s light are captured into the leaf and then reflected. Excited particles fall back down, only the specific light that is unique to the leaf was excitable/captured and then released/reflected. More specifically, green light energy from the sunlight was ‘captured’ producing electromagnetic energy released from the leaf as green light.

What happens if you shine only blue light on a green leaf? Because the green leaf can only capture and reflect the green light of white light, when only blue light is shone onto a green leaf, it no longer looks green but appears almost black. No light is captured nor reflected and no light waves are reflected for our eyes to detect light or colour from the leaf.

Solution

Light Experiment to Introduce Colour

The purpose of this light experiment is to investigate ‘white’ light or sunlight and its colour components or different wavelengths of electromagnetic energy.

Effects of White Light and Coloured Light on Different Objects

Select 5 objects and shine a white light flashlight on the objects and observe the color that is visible to you. Record your findings in Table 1 below.

Table 1 – Colour Observations of Shining White Light on Objects (Record Colours)
White Light Source:
Item #1 Description:
Colour:
Item #2 Description:
Colour:
Item #3 Description:
Colour:
Item #4 Description:
Colour:
Item #5 Description:
Colour:

Now, select a colour (red, blue, or green). Cover your flashlight with clear tape and paint the glass lens of the flashlight using your selected colour paint or nail polish. Now you have a new coloured light source. Shine your coloured light on the same objects and observe the colour that is visible to you. Record your findings in Table 2 below.

Table 2 – Colour Observations of Shining Coloured Light on Objects (Record Colours)
Colour of Light Source:
Item #1 Description:
Colour:
Item #2 Description:
Colour:
Item #3 Description:
Colour:
Item #4 Description:
Colour:
Item #5 Description:
Colour:

Discussion & Conclusion
What did you observe when you changed your light source color? Explain your observations based on the sample answer provided above.



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