Short answer: The whole scene gets turned
upside-down from refraction through the curved surface of the glass. In addition
to that, the upper part reflects off of the flat, horizontal surface of the
liquid in the glass - that reflection also flips the image, turning that part
back right side up.
More details: The bottom half of the liquid-filled glass is
shaped a lot like a converging lens - the kind of lens in reading glasses,
magnifying lenses, and our eyes. Wait - I thought the bottom of the glass made
an upside-down image. Reading glasses, magnifiers, and eyes don't make
upside-down images, do they?! Well, yes and no. When you use reading glasses or
a magnifier, you are generally looking at something close to the lens. In that
case you get an upright image in front of the lens. But try looking at an object
far from the lens - it probably looks blurry. That's because light from a distant object
makes an image behind the lens, and if you hold the lens close to your eye, your
head is in the way of the light forming the image. So try this: Hold the glasses
or magnifier at arms length (or maybe further, depending how strong the lens is)
and see if the lens makes an image of a distant object that appears to hover
between you and the lens. (Or you could try to focus the image on a wall or piece of paper or something.) That image is upside-down, and that is the type of
image you see through the bottom of the glass above - an inverted image of distant
objects (clouds and grass).
What about your eye - your eye looks at distant
things, doesn't it? Yes, and the image it forms on the retina is upside-down,
just like the image of the clouds seen through the glass above. You don't notice
that the image is inverted, because your brain knows how to interpret it so you
"see" everything right-side-up!
The second effect in this photo is reflection.
Light from the grass and play equipment comes from below and is traveling
somewhat upward as it arrives at the glass. It enters the glass and liquid, but
moves upward and hits the underside of the liquid surface, reflecting off that
surface, before exiting the glass and liquid. The glass and liquid invert the
image just like they did to the clouds, but the reflection does another
inversion, bringing the image back right side up.
Wait! Mirror reflections flip
left and right, they don't turn images upside-down, do they? Yes and no.
Normally when we look at a mirror reflection, we are looking at a mirror hanging
on the wall - a vertical surface. That mirror flips left and right. But the
surface of the liquid is a horizontal surface, so it flips up and down. Take a
mirror (even the smooth surface of your cell phone) and hold it horizontally
like the surface of the liquid. Look at the reflection of objects in the mirror,
and see that the reflection is upside down!
Image and original question thanks to Atula Sharma.
