Reflection from smartphone screen

[mainguy]

So here's a mystery one of you may be able to solve.

I noticed that the reflection of light from the fingerprints on my iPhone screen depend on it's orientation toward a light source.

For instance, use your finger to make two perpendicular smudges across the screen. You'll find that the smudge which is parallel to the light from the source (suppose its a bulb of light) will reflect light readily, while the smudge perpendicular to the rays from the source will just show up black.

Really unsure of what's going on here, thoughts?

 

[rumborak]

I suspect this is a similar effect to the one where, on a heavily scrubbed metal surface (with millions of tiny scratches), you always see circular reflection.

The reason being, on the microscopic level, you still need the surface to be at a certain angle to reflect the light towards you. Of all the scratches, only the ones that direct light towards you will stand out, and geometrically those form a circular pattern.
Similarly in your case, the walls of your smudges need to be orientated the right way to deflect light towards you,and only smudges in one direction will do so.

 

[jerromyjon]

You are essentially making a diffraction grating, the smears are many tightly bunched parallel lines which spreads the reflected light out much wider than the width of the smear. Polarized glasses use the same concept to dampen light reflected off of the ground to reduce glare.

 

[Tazerfish]

You are essentially making a diffraction grating, the smears are many tightly bunched parallel lines which spreads the reflected light out much wider than the width of the smear. Polarized glasses use the same concept to dampen light reflected off of the ground to reduce glare.

I am not convinced of that.
I would expect the smudges to be too large to actually cause significant diffraction patterns.
Additionally, polarising filters work by a different principle as far as I know.They don't absorb light along the lines of a grid spatially but absorb light of one specific polarisation.

rumboraks explanation, which can be fully understood using ray optics, seems much more reasonable to me.
Edit: A helpful analogy might be a lake with waves on it reflecting the sun.
The smudges in your case would be the waves.

 

[pixel]

For instance, use your finger to make two perpendicular smudges across the screen. You'll find that the smudge which is parallel to the light from the source (suppose its a bulb of light) will reflect light readily, while the smudge perpendicular to the rays from the source will just show up black.

I agree with Tazerfish, and maybe it's a matter of semantics but I am finding the opposite result from mainguy. If I place the screen on a table in front of a window, the smudge that is going toward and away from the window is dark, the other one is light. I think it's just a matter of reflection of light from a striated surface - the striations directed toward the window deflect light to the side while the other striations deflect light toward the observer.

 

[jerromyjon]

I noticed the same thing years ago when I smeared my finger across my phone screen. Then I used a tissue to clean the screen and tried in many different orientations of smears. When the light source was parallel to the smears I could see the light reflecting across the entire surface, but only when the angle was equal to see the light source directly. Rotating it 90 degrees I could see the light spread out across the surface without the source in view in the reflection, and as I tilted the screen away from the light source it would get dimmer but would still be visible. It is exactly like the streaks you see when you look at a bright light through a screen, you see the cross shape emanating from the source.