Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: why are wet things darker?

Learn more physics!

Most recent answer: 11/30/2013
Q:
Why is it that when things get wet they get darker, even though water is clear?
- Savyasachi Jha (age 15)
New Delhi, India
A:
That's a great question. I just tried an experiment, putting a little water on a painted white window sill, a black cloth grocery bag, and the light blue shirt I'm wearing. The water barely changed the appearance of the glossy white paint. It  made the black cloth a little shinier but not especially darker or lighter. The light blue cloth darkened a lot.

Here's what I think is going on. The light blue cloth reflects a lot of the light that hits it. That's caused by the high index of refraction of the cloth. The reflected light of course can't get absorbed by the dye in the cloth. Water has an intermediate index of refraction, between air and many solid materials. That reduces the reflection, allowing more light to get to the dye where it's absorbed. The material looks darker. This seems particularly effective on porous materials, perhaps because much of the light is reflected at various angles several times before bouncing out. Water reduces the reflection at all the surfaces.

On my glossy white paint, the water doesn't matter since the paint surface reflects the light anyway. I think the black cloth has so many dye molecules within a wavelength or so of the surface that it absorbs almost all the light that hits it even without a water layer to help. (The boundary between the outside and inside of the material is in effect a bit fuzzy, just as the images of things seen in a light microscope are fuzzy by about a wavelength of light.)

Mike W.

p.s. It's great to wonder about these things. An old German man once told me about sitting as a young student in a Greek class wondering why a greasy spot on his textbook was sort of transparent. He was thinking maybe that was connected with why smooth glass is transparent but rough glass isn't. When the teacher called on him, he had no idea what to say. Many years later, when he won the Nobel prize for work on the interaction of light and matter, he received a letter from the Greek teacher asking "Is it possible that you are the same James Franck who was so stupid in my class?"

(published on 12/21/2012)

Follow-Up #1: wet cloth is darker

Q:
Hello!! i have a question !!please answer it as quickly as you can!! we have a shirt!! then we lave it with the water!!i mean make it wet!! then if we see it again we will find it dark!! i mean for example if the shirt is lightblue before,it comes darkblue!!why??? thanks a lot!
- peyman (age 16)
iran
A:

Peyman- I think your question is answered in the thread above.

Mike W.


(published on 11/30/2013)

Follow-up on this answer.