if 'other' from drop down is selected, the respective details textbox is now validated using a reqiured validator

Hi, I'm developing a system with a number of drop down lists. One of the questions relates to the light conditions at the time of the incident (for example, it could be dark, poorly lit, dusk etc...)
One of the options on the drop down list is 'other'. (i argue with my client that if they're using dropdown lists, don't use other, but they disagree)

The next field is 'If other, please give details'. What i'm basically looking to acheive is to say that if you select 'other' from the dropdown list, the 'if other' field becomes required using a requiredfield validator. (and the reverse too, so if you choose a pre-poulated answer such as 'dusk', the 'if other' field is not required.)
I have all my other validations working fine and my drop down list is populated from an SQLDatasource (not ASP.net Items)

My dropdown list id= ddlLight
My 'other' field id= txtbxLightOther
...