Question

Differentiate between alkenes and alkynes, cis isomers and trans isomers. Describe how cis-trans isomers are used for night vision

Answer 1

Alkenes contain only c-c double and c-c single bonds but no c-c triple bonds.
Alkynes contain only c-c triple and c-c single bonds but no c-c double bonds.

As for cis and trans here's a horribly drawn picture of a cis isomer:


And now here's a horribly drawn picture of a trans isomer:


As you can see for a cis isomer the two longest chains are on the same side of the double bond while in the trans isomer the two longest chains are on the opposite sides of the double bond.

Now as for how cis-trans isomers are used for night vision look here: http://www.brookscole.com/chemistry_d/templates/student_resources/0030012910_kotz/oscience/ch11/carbon4.html