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Homework Help: Understanding colors (Lights And Optics)

  1. Jan 19, 2012 #1
    Well, This isn't a homework , but today in class we talked about how lights shining on a object produce different color. But I do not know what color it produces??

    I know that Primary colors of light are red green and blue. Secondary are magenta and cyan and yellow.

    If we have a red cherry on the desk and we shine yellow light, Why does it produce or reflect red?? Teacher said it reflects green but how?

    And vice versa if we shine a red light into a yellow banana, we get red
    and if we shine blue, we get black.. why??

    Can someone help me understand why these colors are produced and how i can tell??
  2. jcsd
  3. Jan 19, 2012 #2


    Staff: Mentor

    light itself isn't organized as primary and secondary colors that is a result of your eyes ability to see color. You have three types of receptors:


    light itself is characterized by its frequency red light has a lower frequency than green or blue light.

    the banana is yellow because it reflects that frequency and absorbs the other visible ones. as bananas age they go from green to yellow to brown (reddish) so its seems it would have a varying ability to reflect these colors but not blue and thats why you see black.
  4. Jan 19, 2012 #3
    But what if you don't know the frequency. But just the color...
  5. Jan 19, 2012 #4
    White light is composed of all of the frequencies in the visible portion of the electromagnetic spectrum. This means that all of the colors have a unique and corresponding frequency. Darkness is just the absence of light.

    Whenever you "see" a color, this is the result of light frequencies being added together starting with darkness. This is called additive color.

    Mixing light is not the same process as mixing pigments such as paint, crayons, or other dyes. Those "colors" are produced in a process called subtractive color in which the frequencies of light are subtracted from each other starting with white light.

    The components in our body responsible for the perception of color vision are the cone cells located in the retina of our eyes and different cells respond to different frequencies of light.

    So as jedishrfu has stated, when you perceive a that an object has color, it's because that the object absorbs all of the frequencies except for the ones that your photoreceptors respond to. Each color has a corresponding frequency; Theoretically, there are an infinite amount frequencies, therefore just as many colors. This is why the collection of colors is called a visible spectrum; It is a continuous band of frequencies. You can't separate a color from its frequency; The color is represented by it.
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