For this masterclass I'll be discussing the way light and water interact, and how you can capture that in a painting. My chosen medium for this is oil, but I won't be going too much into technique, so the ideas should work for any medium. Having said that, oil is particularly good for depicting water as it has the richness of hue that is so often needed and provides a shiny finish. As my example I'll be talking about this picture of Monkstone Beach (near Tenby) that I painted in summer 2018. It shows pretty much all the aspects of light and water in one picture.
It can be difficult to know where to start with such a complicated piece, but generally I start with the sky. In this case it was a fairly plain blue sky graduated towards the horizon due to haze and a little pollution (which typically shows up as a yellow or purple tinge). The sky is important as you'll find it affects the water. You can see that it partially reflects off the surface - something which is most obvious in the shadows on the left. What you may not notice though is that this depends on the angle it hits the water. The further away the water, the more light gets reflected. The nearer the water, the less you see - hence in the shadow you'll see that the reflection is brightest further away, and as you get closer it turns from full colour to just a variation of light and dark (which is what you can see in the extreme foreground). I had spotted this effect many decades ago as a five year old child playing in puddles, but at the time had no idea what caused it.
There is another reflection of sky taking place too. From behind the viewer the light reflects off the water and onto the rocks. This is a very subtle effect making the foreground rock appear well lit from the front, and also putting a little bluish light on the dark rock that helps prevent it dominating the picture. In this particular scene its not a big factor, but find a spot where the sun reflects off water onto a rock otherwise in shadow and you get some interesting patterns in constant flux.
Its not just the sky that gets reflected in this way of course - the rocks do too. The rocks furthest away are reflected in full colour, where as the foreground rock does little more than change the colour of reflected light. As we loose the reflected light on approaching the viewer, we get more transparency, so we start to see what is in the water instead. In this picture I've started to include that detail about halfway through the pool, steadily making it more prominent the nearer you get to the viewer (i.e. the base of the picture).
So that's light sorted, now for the shadows. The sun is clearly to the left, casting a blue shadow for each rock to the right - or this is how we perceive it. The reality is the shadow is a lack of contribution of direct sunlight, filled in with ambient light from the blue sky. So its the sky that determines the perceived colour of the shadows. In different conditions they would not be so blue. The gradation between the front and back of the image is due to the reflected sky-light I mentioned earlier. The water in 'double shadow' from both reflected sky and sun is particularly dark and needed a combination of prussian blue and vandyke brown to achieve.
Just a couple more things to highlight now. First the foreground water having a slight shimmer helps the viewer to perceive it as water. Painting the objects in the water with a bluish tinge I worked by adding in colours. I partially blended them and then used a larger brush to gently smudge until I got the shimmer effect. I helped the viewer further by varying the edge between the light and shadow where it sharply changes to suggest shimmer and also the changing texture of the bottom of the pool.
The other interesting part is where water meets sand. In most cases where this happens you get a patch of sand at the water line that is a sort of grey/brown tinge depending on the sand colour and the amount of reflected sky (so its more blue the further away it is on a sunny day). In this picture there is an added complication in that the close up sand is rippled so I've had to carefully balance pools of slightly blue-green water over the darker patches of sand, and put lighter patches of sand between to represent the drying out tops of the ripples. Its difficult to get this right, and I'm proud of got it this good this time round.
Finally the none wet parts of the picture that may be of interest. For the rocks (even the dark ones) I start with a little naples yellow spread sparsely. This helps give opacity and texture. For the darker rocks I've used vandyke brown and cobalt blue (or a darker blue), making it more blue that brown, and then knocked it back a little with titanium white, giving the subtle effect of the sky-light reflected from the water backlighting it that I've mentioned. For the lit rocks I've used the same combination but with more white and more naples yellow. I've also added a little tinge of crimson as the upper rocks had a slight reddish note, and a touch of sap green to vary the lower rocks.
For the sand, its mainly a mixture of titanium white and naples yellow, with just a little crimson (used very sparingly to give a tinge). There is also a slight touch of vandyke brown for variation, but that may be hard to spot. The wet sand is the same, but with a variation of that vandyke brown/cobalt blue combination to darken it.