How to add prefix to values in one column if not null using pandas

I have a data frame in which I need to add prefix to rows from one of the columns if it’s not null

   Name  marks
0   Tom     99
1  Jack     98
2  Nan     95
3  juli     90

how can I add the prefix= "/new/" to the column Name if not null?
so the New df will look like the following:

   Name       marks
0  /new/Tom    99
1  /new/ Jack  98
2  Nan         95
3  /new/juli   90

>Solution :

like this I think

df['Name']=np.where(df['marks'].notna(),df['Name']+'/new/',df['Name'])