Simply use a thick stroke as opposed to a shape, and apply a gradient to the stroke.
You can expand the gradient stroke afterwards, resulting in a gradient mesh object.
You can also stack multiple strokes via the Appearance Panel to add additional gradients. Here I've added a second stroke with a gradient to indicate shadowing on the lower portion...
One easy receipe for Photoshop:
Fill a new layer with solid 50% grey. Have so big canvas size in pixels that you can fit the needed photo rectangle even in case you must rotate the image to change the direction of the forthcoming lines.
Apply Filter> Sketch > Halftone Pattern > Lines to the grey layer. My ancient Photohop makes only horizontal lines, but I ...
If you want to tinker you can try Illustrator's Blending:
The next cartoon shows how the arc is made:
Start with the edge curves
Make a three step blend to get 3 intermediate curves, expand the blend and ungroup the parts
Convert the strokes to outlines to be able to make width variations with the direct selection tool. You can as well use path width tool, ...