# Sensitivity of Human Eye to Visible Light

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## Description

#### Sensitivity of Human Eye to Visible Light

###### TRY SOME QUICK QUESTIONS AND ANSWERS TO GET STARTED
Yes! The cornea is the transparent surface that covers the iris and pupil.
Yes! Things appear coloured to an observer because colour corresponds with a property of light that is visible to the human eye.
Photo-transduction takes place when rod and cone cells in the human eye convert their chemical response to light into electrical signals ready to input into the visual system.
No! Human vision relies on trichromacy, not the RGB colour model. However, the RGB colour model takes advantage of the way the human eye responds to red, green and blue light to reproduce and organise colours.
Researchers estimate the number of colours the human eye can distinguish is between one and seven million.

• In this context the term ‘sensitivity of the eye’ refers to the varying response of the rods and cones in the human eye to different wavelengths of light.
• The diagram illustrates the varying response of the eye to wavelengths between approximately 380 and 700 nanometres (nm). These are the wavelengths of the visible colour spectrum.
• The information is in the form of a graph. The vertical axis shows the variation in sensitivity in degrees on a scale between zero and 100%.
• The horizontal axis arranges the scale of wavelengths to reflect the commonly used sequence of ROYGBV. In this arrangement red (700 nm) is at the origin of the two axes on the left and violet (380 nanometres) is on the right.
• The diagram splits the spectrum into 150 different wavelengths from red to violet but there are 198 colours in total from infrared to ultraviolet which are both shown as black.
• Breaks in the cyan line along the bottom of the curve identify the position of ROYGBV. Below the cyan line is another line of colours which shows the range of wavelengths associated with each each spectral colour.
• To find out more about the way that the range of colours used in this diagram have been selected see (add link).

#### Some key terms

Colour is not a property of electromagnetic radiation, but a feature of visual perception by an observer.

• The human eye and so human visual perception are tuned to the visible spectrum and so to spectral colours between red and violet.
• There are no properties of electromagnetic radiation that distinguish visible light from other parts of the electromagnetic spectrum.
• Objects appear to be different colours to an observer depending on the wavelengths, frequencies and amplitude of visible light at the moment it strikes the retina at the back of the eye.

A human observer is a person who engages in observation by watching things.

• In the presence of visible light, an observer perceives colour because the retina at the back of the human eye is sensitive to wavelengths of light that fall within the visible part of the electromagnetic spectrum.
• The visual experience of colour is associated with words such as red, blue, yellow, etc.
• The retina’s response to visible light can be fully described in terms of wavelength, frequency and brightness.
• Other properties of the world around us must be inferred from patterns of light.

A photon is the basic building block of light. A photon is a single indivisible bundle (particle or wave) of energy within an electromagnetic field.

• In the field of optics, light is explained in terms of waves (wavelength, frequency and energy) but this description doesn’t always fit the evidence. It became clear during the 20th century that light sometimes exhibits wave-like behaviour, at others both waves and particles, or just particles.
• Contemporary physics considers that electromagnetic fields propagate through space configured as bundles of energy. These are bundles of photons.
• A photon is a type of elementary particle and represents a quantum of light (eg. visible light). Another way of putting this is that a photon is the smallest quantity (quantum, plural quanta) into which light can be divided.

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