Visible Light Spectrum & Reflection of Light | Physics for Grade 10 PDF Download

Visible Light Spectrum

• Each colour within the visible light spectrum corresponds to a narrow band of wavelength and frequency
• The different colours of waves correspond to different wavelengths:
• Red has the longest wavelength (and the lowest frequency and energy)
• Violet has the shortest wavelength (and the highest frequency and energy)

Colours of the visible spectrum with increasing wavelength

• Wavelength and frequency are inversely proportional, this means that:
  • An increase in wavelength is a decrease in frequency (towards the red end of the spectrum)
  • A decrease in wavelength is an increase in frequency (towards the violet end of the spectrum)

Specular & Diffuse Reflection

Specular Reflection

• Specular reflection is defined as:
• Reflection from a smooth surface in a single direction
• When light reflects off a smooth surface, such as a mirror, specular reflection occurs
  • This is what gives a mirror its shiny appearance
• This is why a reflection can be seen clearly in a mirror
• In this case, the angle of reflection r is equal to the angle of incidence i

_{When reflecting off a mirror, the specular reflection occurs. The angle of incidence is equal to the angle of reflection}

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Diffuse Reflection

• Diffuse reflection is defined as:
  Reflection from a rough surface causes scattering
• When light reflects off a rough surface, which applies to the majority of surfaces, diffuse reflection occurs
• This is what gives objects a dull or matt appearance
• This is why a reflection cannot be seen clearly from a table surface, for example
• Even though a table's surface may look smooth from afar, it is actually made up of many tiny ridges which the light rays are scattered off
• When light scatters, it leaves the surface in all directions

When light is reflected from the majority of surfaces it is scattered – a process known as diffuse reflection

Colour Filters

• White light is a mixture of all the colours of the spectrum
• Each colour has a different wavelength (and frequency), making up a very narrow part of the electromagnetic spectrum
• White light may be separated into all its colours by passing it through a prism
  • This is done by refraction
  • Violet light is refracted the most, whilst red light is refracted the least
  • This splits up the colours to form a spectrum
• This process is similar to how a rainbow is created

White light may be separated into all its colours by passing it through a prism

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Colour Filters

• Colour filters work by absorbing certain wavelengths and transmitting other wavelengths
  • These certain wavelengths correspond to certain colours
• When white light passes through a coloured filter, some colours are absorbed whilst others are able to pass straight through
• For example, when white light passes through a red filter:
  • Red light is transmitted
  • All the other colours are absorbed
• The colour that is transmitted is the same colour as the filter

A red filter transmits only red light and absorbs all other colours of light

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Colour & Reflection of Light

• The colour of an opaque object is determined by which wavelengths of light are more strongly reflected
• Wavelengths that are not reflected are absorbed
  • Hence, this is why different objects appear to be different colours
• For example, white light upon a green surface will only have green light reflected and the others absorbed
  • This light is reflected into our eyes to see the surface in that colour

A green surface reflects green light and absorbs all other colours

• An object will appear white if:
  • All wavelengths are reflected equally
• An object will appear black if:
  • All wavelengths are absorbed
• An object will appear transparent if:
  • All the light is transmitted, and only a small amount is reflected or absorbed