Heat the sugar over a low heat in a small saucepan with 250 ml (8 fl oz) water, stirring occasionally until completely dissolved.
Add the orange rind and increase the heat. Without stirring, boil the syrup for about 12 minutes and then set aside to cool completely.
When it is cold, strain the sugar syrup over the orange juice and stir together. Refrigerate for about 2 hours until really cold.
Pour the chilled orange syrup into an ice cream machine and churn for about 10 minutes. When the sorbet is almost frozen, scrape it into a plastic container and put it in the freezer compartment for a further hour until completely frozen. Alternatively, pour the chilled orange syrup into a shallow metal container and put it in the freezer for 2 hours. Remove and whisk with a hand-held electric whisk or balloon whisk, breaking up all the ice crystals. Return it to the freezer and repeat this process every hour or so until frozen.
Serve scoops of sorbet with a splash of chilled Campari, if liked, and decorate with thin strips of orange rind.