Sensitivity of the Eye (AQA A Level Physics): Revision Note

Sensitivity of the eye

Rods and cones

• Light travels through the front of the retina to the rods and cones at the back

• Rods and cones are light-sensitive cells called photoreceptors

  • They detect light and convert it into an electrical signal which allows the brain to visualise the image

• When light falls on the chemical pigments in the rods and cones they are bleached

• This stimulates the cell to send signals to the brain via the optic nerve

• The cells are reset / unbleached by vitamin A from the blood

• There are about twenty times more rods than cones in the retina

  • Rods detect varying amounts of light

    • They work well with low-intensity light levels

    • Contribute little detail to the image

    • Are not sensitive to different colours / wavelengths of light

  • Cones can detect different colours 

    • Each cone is connected to the brain via one nerve fibre

    • They are sensitive to high-intensity light but do not function well in low-intensity light

    • They contribute greater detail to the image

    • They provide information to the brain on the colours in the image

• There is one type of rod

• There are three types of cone

  • Each type is sensitive to a different wavelength range of light:

    • red

    • green

    • blue

The graph shows the colours detected by the rods and different types of cone

• The eye responds to each colour differently:

  • It is less responsive to blue light than red or green

  • So blue light looks dimmer

• The brain processes signals from the three types of cone and interprets their weighted relative strengths as colour

The graph shows the relative absorption by the brain of each colour

• Any colour can be produced by combining different intensities of red, green and blue light

• For example, yellow light can be obtained by combining signals from the red and green cones