Q: my pc is iMac  OS X Lion 10.7.5 .can I use air play to see everything from computer to tv by apple tv?

About AirPlay and Airplay Mirroring

AirPlay Mirroring requires a second-generation Apple TV or later, and is supported on the following Mac models: iMac (Mid 2011 or newer), Mac mini (Mid 2011 or newer), MacBook Air (Mid 2011 or newer), and MacBook Pro (Early 2011 or newer). For non-qualifying Macs you can try using Air Parrot.

Several Apple Articles Regarding AirPlay

Apple TV (2nd and 3rd gen)- How to use AirPlay Mirroring

How to set up and configure AirPort Express for AirPlay and iTunes

About AirPlay Mirroring in OS X Mountain Lion

iTunes 10- About playing music with AirPlay

Troubleshooting AirPlay and AirPlay Mirroring

Using AirPlay

Thanks to the $15 Beamer, AirPlay streaming is still possible on Macs  that do not support Airplay and mirroring.

Another solution is the Air Parrot.

Welcome to the Apple Support Communities

See > http://support.apple.com/kb/HT5404 AirPlay Mirroring is only compatible with Mid 2011 or newer iMacs, and OS X Mountain Lion.

If you have a Mid 2011 iMac or newer, you can upgrade to OS X Mountain Lion in order to get AirPlay Mirroring. To upgrade, make a backup of your data, open the App Store and purchase OS X Mountain Lion. When the download finishes, the OS X Mountain Lion installer will appear, so follow the steps to upgrade.

If your iMac isn't supported or you don't want to upgrade to OS X Mountain Lion, you can use a third-party app as AirParrot > http://www.airparrot.com

You are welcome