• Rods and cones are required for sight.eye image by Pali A from Fotolia.com
  
  Two of the most important structures in the eye are rod cells and cone cells. These photoreceptor cells are specialized neurons that detect light and send the information to the brain almost instantly, forming sight. Although they are similar, rods and cones have vast differences, some of which we require to see properly.
  
  

Function

• Rods become active in low-light and are used to see in the dark. This is called scotopic vision. Cones, on the other hand, are active at higher light levels and are used for normal, or photopic, vision.
  
  

Shape

• Cones, as the name implies, are short, rounded pyramid shaped cells. Rods are longer that cones and have a cylindrical shape with a flat top.
  
  

Color

• Cones are the only photoreceptor cell that can detect color. Rods create gray-scale images.
  
  

Number

• There are 120 million rods in the human retina, according to Tutor Vista. However, there are only 6 million cones.
  
  

Location

• Rods are spread around the outside of the retina, while cones are mostly in the center of the retina.
  
  

Types

• There are three types of cones that are sensitive to different wavelengths, or colors, of light. Short-wavelength cones see blue, middle-wavelength cones see green and long-wavelength cones see from yellow to red. There is only one type of rod.
  
  

Problems

• People with missing, damaged or defective rods will have trouble seeing at night and in the dark. Problems with cones will result in colorblindness.