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Mirrors, Lenses, and the Eye, Why are some surfaces mirrors?, In very dim…

- - - - The image is formed by the mirror looks like a photograph of the object except that the image is reversed left to right.
  - - - Light rays that are parallel to a line called the optical axis are reflected through one point-the focal point.
        
        The distance from the mirror to the focal point is called the focal length.
        
        The type of image formed depends on where the object is.
        
        If an object is more than one focal length from a concave mirror, the image will be upside down.
        
        If the object is closer than one focal length, the image will be right-side up.
        
        If an object is placed exactly at the focal point, no image forms.
  - - - The image is always right-side up and smaller than the object.
- - - - The more curved the lens is, the shorter the focal length.
- - - - In bright light, the iris relaxes and the pupil becomes smaller.
      - Then less light enters the eye. In dim light, the iris contracts and the pupil becomes larger.
  - - - There, chemical reactions produce nerve signals that the optic nerve sends to your brain.
        
        There are two types of light-sensitive cells in your retina-rod cells and cone cells.
        
        Rod cells
        
        Rod cells are sensitive to low-light levels.
        
        They enable people to see objects in dim light.
        
        Cone cells
        
        Cone cells enable a person to see colors.
        
        The retina has three types of cone cells. Each type of cone cell responds to a different range of wavelengths.
        
        Your brain interprets the different combinations of signals from the three types of cone cells as different colors.
  - - - The muscles surrounding the lens change the lens's shape.
        
        To focus on nearby objects, these muscles relax and the lens becomes more curved.
        
        To focus on distant objects, these muscles pull on the lens and make it flatter.
- - - - The wavelengths of light waves absorbed and reflected depend on the materials from which the object is made.
    - - Light waves have no color as they travel from an object to your eyes.
  - - - Light waves with different wavelengths spread out after passing through the prism and form a color spectrum.
  - - - When white light hits an object, it reflects the wavelengths you see as its color.