If you are like me, the thought of having to draw reflections, be it in a mirror or water, makes you want to gnaw your arm off. Now, you might say, "Well, reflections are easy, just take a copy of the image, flip and rotate it, and there you go."
However, this technique does not take into account that a reflected image is not the same as the actual image; the reflection is a different view!
It is easiest to demonstrate the angle differences in the reflection of something simple, like a square. The same vanishing points are used for the object and the reflection. To find the height of the reflection is simple, measure the from the top of the object to the reflection line and then drop that measurement down from the reflection line. The blue line represents the waterline; see how the reflected square is at a different angle, yet with the same perspective line as the upper square?
Let's move on to a complex object. As you can see in the following photo, Mr. Duck looks completely different in his reflection, in part because the viewer is slightly above him. This is where drawing reflections can become daunting--all that perspective to figure out! This is also where having references comes in handy; if you are drawing a figure and need to depict a reflection, a long bathroom mirror and an assistant to take a photo of you in the pose you are looking to draw is invaluable. (Note: do not stand on the mirror!) Barring that, you can use a doll, stick model, or figurine for reference.
Different viewpoints of the object provide different reflections.
Sometimes a reflection is warped due to the shape of the item casting the reflection. In the next Mr. Duck picture the left image is when I made the mirror concave and the right is convex. Think of those fun-house mirrors that combine concave and convex surfaces to create weirdly distorted images, or the convex side mirror on a car labeled "Items In Mirror Are Closer Than They Appear".
As another example, you can see in the following picture of the slightly curved top of a car that, when I drop the vertical points of reference, the top of the house is warped. The house also does not conform to the rule of "take the height of the object and drop it down for the height of the reflection." In these cases, the closer to the reflection the object is, the truer to dimension it is.
Ripples in the water create a barred effect on the reflected image; this is due to the one side of the ripple showing the viewer the sky, and the other showing the image. In the Mr. Duck photos you can see where the ripple is displaying the white of the tub, causing a white band across the reflection.
Here is a photo that shows how the ripples reflect different parts of the trees, causing a zigzag effect.
A lot of the time the ripples or movement of the water obscure or distort the reflection.
When depicting objects that are further away from the water, drop the height from the base of the object; the base of the object will not show in the reflection. In this case the shore obscures the base of this particular tree.
To add a touch more realism to beach scenes, add in a damp patch of sand.
A great way to paint reflections is to drop the image of the object down into the water; then take a large soft brush and sweep it back and forth across the image to soften it. This works the best with oil or acrylic landscape pictures. You can see in the following photo that the movement of the water blurs the reflection.
When looking at an image in a mirror or other polished surface, remember that the view in question represents the total distance of the light from the object being viewed to the mirror and then to the eye. Thus, for example, a candle flame reflected in a mirror five feet away from both the candle and the viewer will be effectively a candle ten feet from the viewer, and thus much dimmer than one five feet away. This cumulative distance is not as critical to art, but is critically important in photography. In the example below left, for example, note that the duck image at left -- the real duck -- is slightly out of focus, whereas the reflected image to the right is in focus. This is a factor of depth of field; smaller lens apertures and slower shutter speeds produce wider depth of field (i.e. everything in focus), whereas wider lens apertures allow faster shutter speeds but shallower depth of field (one object in focus and things in front and behind out of focus).
Here is an ACEO that shows my process of figuring out a reflection. First, I figured out the water reflection line, there are a couple different water reflection lines. I changed the reflection of the arm to the right as I refigured the water line under it. A good way to figure the water line out is to think of where something dropping into the water from that point would hit the surface. I used a see thru ruler to mark my image heights and vertical lines. In a place you are not too sure of, the judicious use of ripples can help.
The best advice I have is to study photos of reflections, and take reference photos; also keep in mind that the human eye tends to skip over things, so while you might see that one little imperfection in your reflection, others are not likely to. I highly recommend the book "Perspective for Artists" by Rex Vicat Cole, which goes into depth about reflections, and angles of incidence, as well as providing many valuable drawings to illustrate the text. Another good book for perspective and the human body is "Dynamic Figure Drawing" By Burne Hogarth.