Disgust at robbery attempt on 80-year-old

An 80-year-old woman is shaken and bruised after being set upon by a man brandishing a knife in her western Sydney home.

The elderly woman, who was briefly in hospital, was left with bruises on her arm following the bungled armed robbery in Penrith on Wednesday night.

Police say the man threatened the woman with a knife and demanded cash but fled empty-handed after she screamed.

Detective Inspector Grant Healey has appealed to the public for assistance.

"Somebody will know this man, and if you do know him, it's a really despicable act and you should contact Penrith Police or Crime Stoppers," Det Healy said on Thursday.

"I hope they catch him and he gets punished like he deserves," a neighbour, named as Tom, told Sydney radio station 2GB.