Year 9 Science - Newtown High School of the Performing Arts
Rhythms, Ripples and Rays

Light and its Properties

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Explain the processes of absorption, reflection or refraction of light and discuss situations in which they each may occur

Light belongs to a special group of waves known as the Electromagnetic Spectrum.

It is a transverse wave with electric and magnetic fields oscillating at right angles to the direction the energy is travelling.
EM wave
Light, as well as all electromagnetic waves, have particular properties that make them unique when compared to other waves.

-  They do not require a medium to travel through and so are able to travel through the vast vacuum of space largely unhindered.

-  They all travel at the speed of light (300, 000 km/s) when travelling through the vacuum of space.

When light comes into contact with various mediums, however, several different processes can result.  The light could either be absorbed, reflect off a surface or may refract as it continues to travel through that medium.

 

 

 
AbsorptionT-shirt

When a light wave encounters a surface where it is unable to be reflected or to continue its movement through the material its energy is absorbed by the material.  This leads the substance to get hotter as a result since this energy is converted into heat.

This can easily be felt when wearing a black T-shirt during summer.  The white light that hits the T-shirt is made up of all colours.  Since the black material doesn't reflect any of the colours on it we see it as black.  As a result this causes a black T-shirt to become hotter than other colours.

The colours we see are as a result of them not being absorbed by the material.

For example a Red T-shirt appears red because the red light is reflected.  If white light hits a red T-shirt then all of the colours except the red are absorbed and the red is reflected back which allows that light to be seen.

 

Reflection

When light is incident upon some surfaces light can be reflected from it.  Law of reflection
The direction that the light reflects is so that it comes off the surface at the same angle as shown in the diagram.

This is known as the law of reflection.

i.e. Angle of Incidence = Angle of Reflection

Note that the angles are measured from a line that is at right angles to the surface.  This is known as the Normal.

Reflection is used in many devices or technologies from the simple wardrobe mirror, make-up compact mirror, wide angled mirrors used to see around corners on blind corners or in shopping centres through to periscopes and satellite dishes.

Refraction

refraction
When light is able to pass through some materials such as glass or perspex a process known as refraction occurs.

In this process the light is changing medium.  This results in the wave slowing down when moving to a medium of higher refractive index (kind of like a material with higher density)

Devices that have glass lenses have been developed to make use of the principle of refraction.

Glass lenses can be used to bend the light through them to magnify images in the case of telescopes or microscopes and can also be used in eye glasses to correct problems focusing at different distances.

Cameras also rely upon refraction to focus an image within the camera to produce a crisp and sharp image.