How does the light from a laser reflect off objects with same color?

  1. Hey guys, I'm wondering how come if we're in a completely dark room and shine a laser beam at an object, we see the color of the laser beam (say it's red). How is this consistent with the the theory that objects appear a certain color since that is the frequency of light that they reflect? For example only "red" objects reflect red light and thus look red but a blue object does NOT reflect red, thus it is blue. But in the case of the red laser light, if we shine it at any object of any normal color in a dark room, it still reflects some of the red light. I'm quite puzzled even though there's probably a very simple answer. Someone please enlighten me :)
  2. jcsd
  3. Drakkith

    Staff: Mentor

    Objects do not perfectly absorb light, even if they are that color. Some is always reflected. Lasers typically shine a lot of light onto a very small spot, and enough is reflected to easily see.
  4. Some time ago I grew a large copper sulphate crystal, which looks like a piece of blue-coloured glass, about 3-4 mm thick. You can easily see things through the crystal (i.e. light passes easily, although it is tinted blue). However, no light from a red laser pointer can get through. The beam of light reflected from the surface of the crystal is still red.

    These observations show that there is a key difference between light reflection and transmission.
  5. Oh yeah that's true,
    So would there be some materials that would not reflect red at all because the frequency of the red laser beam that I'm using would not be one of the allowed frequencies in the object's electron's orbitals and so most of all the time would get absorbed?
  6. Drakkith

    Staff: Mentor

    There are materials that are very very close to perfect reflectors for small ranges of wavelengths, but I don't know of anything that is 100% perfect.
  7. sophiecentaur

    sophiecentaur 14,727
    Science Advisor
    Gold Member

    A pretty good 'black', for use when lining up TV cameras (a regular need in the not too distant past). It was used as part of a printed grey scale chart (with grey stripes of different densities and consisted of a small rectangular hole with a small box behind it. The box was lined with a black velvet fabric. Most of the light that entered the box was absorbed in the fabric but the vast majority of any that was reflected (random directions) would hit the inside surface of the box and be absorbed there (and so on and so on). The result was that the hole reflected such a small fraction of the incident light, it looked like a very black patch and was treated as a reference black under most lighting conditions. For laser light, you can bet there would be a finite amount of 'speckle' emerging from the fabric which, no doubt, under very low ambient conditions, would be visible. Loads better than black paint though!

    Could the same be done for a reference green, yellow, red? Possibly.
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