Have you seen a number of plastic bottled placed in the streets or gardens?
These plastic bottles are placed to repel cats, it is said that light reflected on the bottle is diffused and surprises cats. This method was featured on TV about 20 years ago in Japan. Since then, it has been popular all over Japan. However, it seems to be ineffective against cats unfortunately.
There are plenty of other cat repelling goods such as flavoring agents which cats do not like.
It may be effective temporarily, but cats may get used to the smell or it may get washed by rain.
Some people may build fences around their houses for repelling cats, but it may be easy for cats to jump over fences with a height of 1 or 2 meters.
In recent days, the Japanese company developed a device which emits ultrasound waves in random order to repel cats. According to research, this device seems to be the most effective one for repelling cats!