Connect MacBook Pro to use iMac as monitor?

I have a MacBook Pro Retina, 13" Early 2015

that I would like to hook up to my

iMac 21.5" Mid 2011

to be able to use the larger screen of the imac for my work on the laptop when I am working from home.

I have a cable with thunderbolt for plugging into my iMac and HDMI on the other end to hook onto the laptop. I've turned the both on and logged in and tried pressing Command F2 and Command FN F2 but the monitor on the iMac does not show the laptop's desktop.

Can I use this cable for this purpose? If not, is there another cable that will allow me to do this?

Michelle